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Project Specific Discussions => Preamplifiers => Topic started by: gary o on March 03, 2008, 10:53:15 PM

Title: RCA Ba2A Pre amp qestions & answers
Post by: gary o on March 03, 2008, 10:53:15 PM
looking at old RCA BA 2C schematic capacitors marked as MFD ...microfarad, but ones is marked as 330MMFD ??????Pico ?? Nano ???

I can post link to schematic later ...thanks for reading
Title: RCA Ba2A Pre amp qestions & answers
Post by: gary o on March 03, 2008, 11:03:31 PM
Hey that was quick ....thanks Patrick :grin:

Heres link schematic at bottom of page for anybody who is interested

http://www.coutant.org/ba2c/index.html

I long while back I DIYed a BA1A pre ...it doesnt get much use ..very low level useable but.., I checked and re checked dont think I did it wrong...anyway tomorow im gonna turn it into one of these to see how it sounds can always change it back

thanks once again.
Title: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on March 03, 2008, 11:06:21 PM
You'll probably find the BA-2 more useful, given it has gain control and 10 db more gain officially.    I think the max out level is quoted much lower, but I've not had a problem.
Title: RCA Ba2A Pre amp qestions & answers
Post by: gary o on March 04, 2008, 10:40:41 AM
Hi Doug yeah thanks thats what Im thinking...Iv soldered it up now just gonna check it and power up
Title: RCA Ba2A Pre amp qestions & answers
Post by: gary o on March 04, 2008, 07:38:30 PM
She works !!! a quick vocal verse, sounds quite sweet..got to tidy it up now & put it in the case with the other 3 pre amps, its a bit of a messi there.

Found mistake in the BA1A just before I dismanteled it....Goin back to the original subject of this post when I asked about MMFD answer was Picofarad but when i did my BA1A I thought it was MFD so I put MFD value cant remember 300 MFDish that should a bin 300PF ish.

So heres a litle question ...the capacitor in question was part of the feedback circuit... what would happen in this case ?? would it give more feedback than needed and result in lower output or would it allow more low freq feedback & thus messing it up a bit.

I had a switch for no feedback option & I prefered its sound with no feedback & as expected had bit more level without the feedback.
Title: RCA Ba2A Pre amp qestions & answers
Post by: JohnRoberts on March 04, 2008, 08:50:06 PM
Quote from: "gary o"
She works !!! a quick vocal verse, sounds quite sweet..got to tidy it up now & put it in the case with the other 3 pre amps, its a bit of a messi there.

Found mistake in the BA1A just before I dismanteled it....Goin back to the original subject of this post when I asked about MMFD answer was Picofarad but when i did my BA1A I thought it was MFD so I put MFD value cant remember 300 MFDish that should a bin 300PF ish.

So heres a litle question ...the capacitor in question was part of the feedback circuit... what would happen in this case ?? would it give more feedback than needed and result in lower output or would it allow more low freq feedback & thus messing it up a bit.

I had a switch for no feedback option & I prefered its sound with no feedback & as expected had bit more level without the feedback.


Using a XX microfarad cap in place of a xx picofarad cap in a feedback circuit very much depends on the topology and other values, but a general answer is the wrong value larger cap will be doing what the smaller one was at a much much much lower frequency.

JR
Title: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on March 04, 2008, 10:09:42 PM
Quote from: "gary o"
Found mistake in the BA1A just before I dismanteled it....Goin back to the original subject of this post when I asked about MMFD answer was Picofarad but when i did my BA1A I thought it was MFD so I put MFD value cant remember 300 MFDish that should a bin 300PF ish.

So heres a litle question ...the capacitor in question was part of the feedback circuit... what would happen in this case ?? would it give more feedback than needed and result in lower output or would it allow more low freq feedback & thus messing it up a bit.

I had a switch for no feedback option & I prefered its sound with no feedback & as expected had bit more level without the feedback.


Ahah, yeah that's 100% feedback (no gain at all) for most frequencies.  Amazing it didn't oscillate obviously.   So you probably had close to only the input transformer gain minus the output transformer loss.    The correct value is meant to roll off extreme high frequency; maybe to correct for transformer high peaking resonance or limit out-of-band interference.   It bypasses the feedback resistor, allowing only very high frequencies to have 100% feedback.  

I've not disconnected and measured feedback in a BA-1A, but if similar to the BA-11A or BA-21A there's probably 20-30 db of feedback present.  A smart guy here could chime in with the theoretical based on the circuit.  Any smart guys about?  

Similar note; many old schematics use M instead of K for resistors.  You'll see 10M instead of 10K and think something's wrong.
Title: RCA Ba2A Pre amp qestions & answers
Post by: gary o on March 05, 2008, 06:49:33 AM
Thanks for that explanation made easy for me to understand...yeah it was a 33Uf cap not 330Uf as i said & yes its parallell to a resistor so that must be feedback resistor then and in series with 0.10 uf cap from OP tran, I had a switch to disconnect that whole network for no feedback at all, which had higher level but wasnt much tho ??  had thinner cleaner sound, didnt sound as nice for vocals as the fuzzyer none feedback...overall I think the BA2A has nicer sound.

The BA2A has no feedback at all ...does it ??

Tubes are wire different way ? so work in different mode ??

Triode ??? maybe .... In treading in the dark here as you gathered..

Both amps are single ended....?

Th e other thing i notice about the amp is the huge OP tran I have {was expensive} it has to take full HT thru primary...is that why it is big

Before I saw the BA2A schematic I was wondering what to do with this transformer as it was gettin no use in the BA1A

Anyway I ramble on { in the words of Nat King Cole} thanks for your knowledge Im learnig lots its very interesting
Title: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on March 05, 2008, 11:45:27 AM
Quote from: "gary o"
Thanks for that explanation made easy for me to understand...yeah it was a 33Uf cap not 330Uf as i said & yes its parallell to a resistor so that must be feedback resistor then and in series with 0.10 uf cap from OP tran, I had a switch to disconnect that whole network for no feedback at all, which had higher level but wasnt much tho ??  had thinner cleaner sound, didnt sound as nice for vocals as the fuzzyer none feedback...overall I think the BA2A has nicer sound.

The BA2A has no feedback at all ...does it ??


no overall feedback loop, which you can't do if you use volume control; it defeats the feedback and actually makes things worse.  You can do local feedback around stages, and have a volume control in between, but not a standard pot in the middle of a loop.  Higher gain 3 and 4 stage program amps tend to use no feedback around the input stage, then volume control followed by a 2 or 3 stage amp with overall feedback loop.  Fixed gain preamps tend to have overall feedback loops, unless they are pre-1950 then all bets are off.   BA-1A is the first RCA preamp using overall feedback, and it dates 1945-1950, being replaced with BA-11A in 1950.

Quote from: "gary o"
Tubes are wire different way ? so work in different mode ??

Triode ??? maybe .... In treading in the dark here as you gathered..


yes, triode connection on both

Quote from: "gary o"
Both amps are single ended....?


right

Quote from: "gary o"
The other thing i notice about the amp is the huge OP tran I have {was expensive} it has to take full HT thru primary...is that why it is big


yes

Quote from: "gary o"
Before I saw the BA2A schematic I was wondering what to do with this transformer as it was gettin no use in the BA1A


I suspect if you had a real BA-1A you'd think differently; it's quite nice.  But, volume control trumps all in the modern age, given the way we're used to working.  Fixed gain doesn't bother me at all, as I've gotten used to it.  I marvel at folks complaining of low gain with 40db fixed gain preamps; you must be recording moderate to quiet sources at all times.  I usually have 20 db pads in front of 40db preamps when tracking bands, and still get plenty to work with.
Title: RCA Ba2A Pre amp qestions & answers
Post by: gary o on March 05, 2008, 08:29:14 PM
Wow I love hearing about the history of this old kit, Im not too worried about fixed gain amps now Iv sorted a level control out on my DIY BA6A i get the level i want from its input pot I use threashhold and pad the output...a level on the pre can be a bonus...The sound of my BA1A was good but not as nice as the other pres Iv made, I would love to get my ears on an original tho, same goes for the real versions of what i DIYed .. IMHO the BA2A sounds better than the my BA1A did , tho I cant A & B them now ha..Thanks for answering my questions i have learnt lots.

heres pic of the BA2A under test hanging out of the case ...its bit of a mess in there  hehe i have 4 pres in the one small case BA21A Urie M610 , V72 & soon the BA2A

 (http://aycu04.webshots.com/image/44323/2005828803002520153_rs.jpg) (http://allyoucanupload.webshots.com/v/2005828803002520153)
Title: RCA Ba2A Pre amp qestions & answers
Post by: gary o on March 06, 2008, 06:58:06 AM
I would chew on that spagghetti tho lol its a mess i know...thats prob the tidyest box iv made too, started out as 1 pre then 2 then a working experiment, I will re house & tidy all my stuff one day Im just try to make my dream vocal recording channe, Im getting there.

Doug I just read some more great info from you here

http://www.groupdiy.com/index.php?topic=25752&highlight=ba11a

Very interesting stuff

Are the larger value capacitors in these old units paper oil ?? I have a box them some where dont think they are correct values but I might be able to use them....would they effect the sound ?


Thanks
Title: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on March 06, 2008, 10:34:00 AM
The metal cased caps are paper/oil types, and the 0.047 and 0.1 types probably are also.  I've never been entirely sure about that type, but cap info in RDH4 suggests it as a primary manufacturing method.

On exact values, older types are usually +100%/-10% versus new types which seem to be +/-10 or 20%.  Older types are seldom low, and frequently much higher than specified, 100% high not being abnormal in an original cap.  New types seem to always be slightly low to dead on, seldom higher than quoted.  Sometimes you have to take this into account in restorations, and buy higher than quoted to match what you are actually replacing.  This of course takes some intelligent 'reading between the lines' on the original design.   It's not at all uncommon to recap an old piece and have less low end that you had before the cap job, simply because you went by the book.
Title: RCA Ba2A Pre amp qestions & answers
Post by: gary o on March 06, 2008, 07:03:35 PM
Wow that mad the +100% ....I have used modern 47UFs 200V {hope thats enough V} instead of old 40UFs and a Nos 10UF 350V ...but can these be low voltage, I see in other pres the caps on the cathodes are lower voltage...mind you C7 the 10UF is near the HT...I cooked a 2W 100K res R10 I have big one there now...was wondering about the carbon resistor mojo or not as maybe, I have modern ceramics there at mo..I keep saying one day il build 2 identical pres one with carbons other ceramics & see or hear for my self..

Doug are you saying yr not sure about the sound of that type of capacitor ?

Am I right in saying that C2 0.5UF & C8 330PF are the only caps that will effect the sound in this circuit ?

I just did a quick A B test between BA2A & my fav pre so far the Redd 47 using same Sowter 9045 input tran & the BA sounds sweet I really like it
Title: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on March 06, 2008, 08:35:35 PM
Something's wrong if you cooked a 2W R10; it's 1/2W in the original.  

I'm quite sure about the sound of the capacitor, I don't know for sure what it's construction is, other than the hermetically sealed bathtub types.  

C2 and C8 the most, but C1 and C3 also.  You can argue they all affect it.
Title: RCA Ba2A Pre amp qestions & answers
Post by: gary o on March 07, 2008, 04:33:07 AM
Ah yes I since found id put C7 wrong polarity what a will I have bit more level maybe I can put half watt R10 back now then....I dont have manual or parts list just schematic I didnt suspect anything was wrong as the amp seemed to work fine.

was wondering about the voltages for caps .....C1/7 seem to be in same can... & they usually hi ish voltage ...maybe C3 40UF could be lower voltage.

Are C1 & 3 bypass capacitors ?....C 7 is.....

As experiment I tried different makes & values for C2 the 0.5 could really hear any difference in quick test

Also my schematic isnt very clear....cant see values for R1 R2 looks like 820 & 660 I take it that junction between them goes to12 on P1 is for meter...I put a 1K & a 1K pot there seems to work ok & sound the same between 1 & 2K so will prob put back a fixed 1K8 I had there at first

Maybe I could re name this thread for anyone with future BA2A questions...Im not if & I can do that.

Cheers.
Title: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on March 07, 2008, 12:00:34 PM
C1 and C3 are cathode bypass, and 25V rating is more than plenty.  

C7 is power filter.

C2 is already overkill large.  Nothing to change there.  Nornal to see 0.1 or 0.25 in that impedance scenario.

R1-R2 affect bias point, and will make your distortion point move if you are into running up into distortion with test equipment and fiddling with values for most headroom.  They picked that spot for a reason; you may find you have a different reason to pick a different spot.  At normal operating levels (with plenty of headroom) you may not hear any difference between any reasonable chosen point.   Not uncommon to see 2.2K or 2.7K biasing that tube in triode in low level preamp stages.

As a general statement, it's not uncommon to find that two stage preamps of this vintage have essentially the same overload point in each stage, meaning there's great headroom in the 1st and nowhere left to go in the 2nd if the 1st is going full blast already.
Title: RCA Ba2A Pre amp qestions & answers
Post by: gary o on March 07, 2008, 07:29:39 PM
Great info Doug thanks its running sweetly now R10 is a 0.6W running cool level is higher too while its still apart im gonna try some input transformers from an old EMI tapr recorder just to see if I like there sound
Title: RCA Ba2A Pre amp qestions & answers
Post by: tardishead on May 11, 2008, 07:28:49 PM
I have a nice big Trimax output transformer

20k primary
600 secondary
+24dbm level
5 ma unbalanced DC

Is this ok to use for a RCA BA2 output transformer???
What is the current going through the transformer primary approx on the RCA BA2???
Title: RCA Ba2A Pre amp qestions & answers
Post by: tardishead on May 12, 2008, 05:05:42 AM
bump
Title: RCA Ba2A Pre amp qestions & answers
Post by: gary o on May 12, 2008, 06:36:26 AM
Hiya tardis I wpoud guess yes for your transformer but better ask one a the guys who really knows have a look at my sowter transformer specs that works great in my BA2a ba1a ba11a the transformer I cant find the model number at moment might be back in this thread....il look

so your building then ? keep me posted am interested in yr project.
Title: RCA Ba2A Pre amp qestions & answers
Post by: gary o on May 12, 2008, 06:45:41 AM
Not there...but found, its a 9040  5:1 ratio thats all I can find at moment...il try again later
Title: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on May 12, 2008, 11:23:42 AM
should work fine, but is somewhat overkill.  I'd save an output with that sort of level handling for a better suited project.
Title: RCA Ba2A Pre amp qestions & answers
Post by: rafafredd on May 12, 2008, 02:36:22 PM
Quote
You'll probably find the BA-2 more useful, given it has gain control and 10 db more gain officially. I think the max out level is quoted much lower, but I've not had a problem.


Hey, where can we find a schem for this?
Title: RCA Ba2A Pre amp qestions & answers
Post by: tardishead on May 12, 2008, 07:34:12 PM
Quote
should work fine, but is somewhat overkill. I'd save an output with that sort of level handling for a better suited project.

Like what? The transformer I have is for SE only - no centre tap.

I have a few old output transformers - they all seem to handle round about 20dbm. I thought it would be better to overate the output transformer so that the valves distort on peaks and not the transformer saturating.

What would the original transformer in RCA-2a etc be rated at???

I am a bit in the dark about "unbalanced DC". Surely there is more than 5ma DC going through the primary of the output transformer on the RCA bA2??
Title: RCA Ba2A Pre amp qestions & answers
Post by: gary o on May 12, 2008, 10:45:52 PM
Rafa

http://www.coutant.org/ba2c/index.html
Title: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on May 12, 2008, 11:59:35 PM
Quote from: "tardishead"
Quote
should work fine, but is somewhat overkill. I'd save an output with that sort of level handling for a better suited project.

Like what? The transformer I have is for SE only - no centre tap.

something that can actually put out that sort of level.  Lots of things.  It's hard to find a SE transformer that will put out that sort of level; they are expensive, and wasted on the BA-2.   Think more like BA-11A for that iron.  
Quote

I have a few old output transformers - they all seem to handle round about 20dbm. I thought it would be better to overate the output transformer so that the valves distort on peaks and not the transformer saturating.


 but that's a moving target; dependent on many things.  and for transformers, remember rating is at lowest frequency.  a +24dbm type with 30 Hz bottom rating will do closer to +34-35dbm at 100 Hz, increasing with frequency.  So unless you are tracking pipe organs or whale voices, you needn't worry with +24 dbm at 30 Hz response.  Many vintage tube pieces use a transformer that can be overloaded at the lowest frequencies about the same time the tube breaks up.  And it is a part of 'the sound' we are trying to recreate.  

Quote

What would the original transformer in RCA-2a etc be rated at???


the BA-2 is rated for an output level of -2 dbm, so you would be wasting a lot of expensive iron headroom using it in a BA-2.  The original iron is rated for about +12dbm.   UTC A-25 or similar.

Quote

I am a bit in the dark about "unbalanced DC". Surely there is more than 5ma DC going through the primary of the output transformer on the RCA bA2??


No, probably not.  I'm going to guess 3-4 mA.    And 5 mA will be rating for full low frequency response; lessening low end slightly past 5mA, melt down somewhere on (far, probably) down the line.

have a read here; I already forget why:

http://www.groupdiy.com/index.php?topic=21504&start=0
Title: RCA Ba2A Pre amp qestions & answers
Post by: tardishead on May 13, 2008, 10:41:25 PM
Thanks Doug - that great!

I will try the RCA BA-11A

I was going to use 6SJ7 instead of 6J7. I understand it has different pin out.
I dont think there would be any problems with this. Let me know if there is.

22mmf does that mean 22 picofarads??
Title: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on May 14, 2008, 02:25:49 AM
6SJ7 should work fine.  I recall 6C5 close if you want a pure triode in the triode section; save a few bucks and some wire.  

mmfd=pfd
Title: RCA Ba2A Pre amp qestions & answers
Post by: tardishead on May 16, 2008, 05:02:23 AM
BA11A prototyped. Running it on a 230VDC PSU - just to try it out.
Sounds nice but missing a bit of bottom end. I suspect the transformers a little - they came from an old military 4 microphone tube mixer which I gutted for parts. I bet the input transformer is not up to spec. I will check them out. It was probably meant for voice applications.

Also left out the 22mmfd cap in the negative feedback path coz because I had nothing available at the right voltage. This would make the unit brighter I presume.

I dont have any spec sheets on this preamp. Has anyone got any more info?

How much db is the negative feedback path. If I disconnect feedback what is the overall gain of the preamp?
Title: RCA Ba2A Pre amp qestions & answers
Post by: gary o on May 16, 2008, 07:21:33 AM
Hi Tardis heres more Ba11 info from doug and nyd...thought you might like to read

http://www.groupdiy.com/index.php?topic=25752&highlight=ba11
Title: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on May 16, 2008, 01:15:41 PM
feedback #s mentioned in the link I posted.  more than enough room there to tailor the low end as needed.  the boost cap doesn't do much, from what I've seen in practice; also should be tailored for the iron.

If you take the feedback out you will certainly have even less low end.  

Somewhere you will find a post from me listing all the feedback resistor values (for an original unit) to give variable gain in steps.  I did that by ear, and have not revisited with proper frequency testing.   I'd probably avoid the extremes of range I measured, especially lowering as much as I did.
Title: RCA Ba2A Pre amp qestions & answers
Post by: tardishead on May 16, 2008, 08:47:31 PM
OK took some readings of voltage drop across cathode resistors of the first stage and second stage.
First stage draws 0.5ma at idle.
Second stage draws 6.3ma

which means that I am really pushing the specs of the output transformer and accounts for a lot of the loss of the bottom end. The output transformer is rated at 5ma.

So I could increase the value of the cathode bias resistor to reduce the current.

Is 6.3 ma typical for the second stage of the BA-11A or perhaps there is something wrong.

Oh yeh - this is with a supply of about 250vdc so when I connect the correct 285vdc supply there will be even more current drawn.
Title: RCA Ba2A Pre amp qestions & answers
Post by: tardishead on May 16, 2008, 10:09:19 PM
All my components measure up to spec as in the schematic so I conclude that my output transformer is underated. 7-8 ma is probaly more like the proper rating.
Title: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on May 17, 2008, 02:11:55 AM
285VDC @ 6.75 mA is normal B+ input.  So 6.3mA sounds reasonable.  I think the primary Z is slightly lower than spec, which would push current up barely.
Title: RCA Ba2A Pre amp qestions & answers
Post by: tardishead on May 17, 2008, 05:29:22 AM
The output transformer I am using is 20k:600. 5maDC

I am trying to source the loss of bottom end. Would pushing the rating of the transformer 1.3ma higher than rated have a noticeable effect.
I put an oscillator thru the preamp and sure enough there seems to be a bottom end roll off from a certain point in the midrange down.
Title: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on May 17, 2008, 10:54:53 AM
how much roll-off starting where?

vary the B+ downwards and see if the low end changes.  that's the easy test.  

then check the transformer response out of circuit, or with an alternate parallel feed circuit with blocking cap.  

then consider modifying the feedback tailoring to get it flat, if the loss seems slight.  either because the iron isn't flat in the first place, or there's additional loss from DC current, or both.
Title: RCA Ba2A Pre amp qestions & answers
Post by: tardishead on May 17, 2008, 07:02:13 PM
Looking at datasheets it looks like the 6SJ7 draws slightly more current than 1620/6J7 at the same circuit conditions. Which explains why I am getting higher than expected current readings on the output stage.

I will try to bias the output stage a little colder (more like the BA-2c) and see if my bottom end comes back.

Its a good education all this.
Title: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on May 18, 2008, 09:08:52 AM
running the B+ down is easier that re-biasing the cathode, no?  Answer in a few seconds.  keep us posted.
Title: RCA Ba2A Pre amp qestions & answers
Post by: tardishead on May 18, 2008, 07:22:34 PM
Tried this. Unfortunately does not help the frequency response.

I will have to check the transformers - must be - unless a capacitor is seriously out of spec which would be strange because I did measure them before putting them in.

Doug - what levels are BA-1A, BA-2c and BA-11A rated at? How much gain do they have?
Title: RCA Ba2A Pre amp qestions & answers
Post by: tardishead on May 18, 2008, 09:50:33 PM
GOTCHA!

Input transformer I was using has a stupidly bad frequency response. It seems to peak at about 2khz and roll off quite steeply in each direction. I assume they were only meant for voice applications.

The output transformer however is fine and has a great response.
Having swapped the input transformer I now have a nice flat response from about 50hz to 15khz.

So basically I have biased the cathode of the output stage colder because the 6SJ7GT I am using draws a little more current at the same DC.
When I get the correct PSU DC voltage I will test again.
I should try some recording it seems to sound pretty nice.

I would still like to know the overall gain values for BA1A, BA2c and BA11A if anyone has them.
Title: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on May 19, 2008, 01:58:16 AM
ba1a and ba11a are standard 40 db types.   ba2 is 50 db.    a few db fluctuation is normal to see.  

ba1 rated for +10 dbm max output, ba11 for +18dbm.
Title: RCA Ba2A Pre amp qestions & answers
Post by: tardishead on May 19, 2008, 10:21:07 PM
On a different note - I am going to build a BA-2c. The DC resistance for the primary of the output transformer is said to be about 2500 ohms.
If the transformer had a primary DC res of 470 ohms would this have an effect on gain and bandwidth in this circuit.
Is it essential to get this DCR in the right ballpark??
Title: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on May 19, 2008, 11:28:40 PM
It generally suggests a lower AC impedance, and you would usually get something in a similar ballpark, say 1500-3500 ohms depending on particulars of manufacturers approach.   470 would certainly seem too low.   The real answer is to put a 600 ohm load on the sec (assuming you know it to be designed for 600 ohms), take some measurements, do some math, and arrive at the primary impedance.
Title: RCA Ba2A Pre amp qestions & answers
Post by: tardishead on May 20, 2008, 03:14:46 AM
Transformer is 20k to 600ohms.

But the primary is about 470 ohms DCR.
Title: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on May 20, 2008, 11:42:46 AM
okay, now I'm on the same page.  The RCA stuff tends to have a higher than normal DCR, when compared to other brands.  Langevin is the same way.  Not conversant enough to say specifically what's up, other than it would seem to be wound with much larger gauge wire, and have a simpler winding scheme.   That is a larger than expected difference, so I'd say it's outside of the norm when compared to a similar UTC, etc.  But no reason I can specifically raise for it to be problematic.
Title: RCA Ba2A Pre amp qestions & answers
Post by: tardishead on May 22, 2008, 12:10:23 AM
Ok now onto the next stage. After getting a good frequency response from a RCA bA11A that I built I decided to use the transformers to build a BA-2c.

Preamp works. I am using 6SJ7 tubes again and 250vDC. And to my horror I found a bass rolloff that starts about 1khz -
(100hz is about -7db lower than 1khz.)

I checked all the components a few times and everything is up to spec.
After verifying that the input and output transformers were good enough response for the BA-11A I am now totally bemused. I biased the cathode of the second stage colder to see if that would effect it (pulling less current through the output transformer just in case it was being overloaded) but there is no change in frequency response.

Really confused. Do I need to load the transformers with resistors? There is something strange going on - I am wondering whether it is another difference due to the use of 6SJ7 instead of 6J7.
Title: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on May 22, 2008, 12:44:55 AM
no, tubes don't have frequency response differences on that order.  I had a BA-2A that had EF86's stuck in it with no circuit changes, and it worked fine.   That's a fairly large difference compared to what you're talking about.  

do you get flat response out of the transformers by themselves, out of circuit?   I suspect the iron the most.  Resistive loading won't help a curve like that, other than possibly being sure you have 680 ohms or so on the output side.   That becomes more critical with low/no loop feedback circuits.    But that still sounds like too great a roll-off.

It's possible the 27 dB of feedback in the BA-11A circuit was flattening out the response of the transformers, if you haven't tested them.    The feedback curve may have happened to match a transformer curve.
Title: RCA Ba2A Pre amp qestions & answers
Post by: tardishead on May 22, 2008, 12:45:56 AM
A few more observations.

When I turn the unit off and put a tone accross the primary of the output transformer I get an almost perfect frequency response through it.

I put a 600R resistor across the output and it improved a little with the mic pre turned on.

With the unit powered up I put a tone onto the grid of the second stage - I still get exactly the same roll off - so it has to be something to do with the second stage and output transformer. Why would the transformer work well on the BA-11A and not on the BA-2c????
Title: RCA Ba2A Pre amp qestions & answers
Post by: gary o on May 22, 2008, 03:31:13 AM
Interestins stuff  wish I understood all thats goin on....my ba1a ba11 ba2a all pre all worked fine with my one Sowter transformer, tho it is different to yours..I made a AWA pre with 6SJ7s & had fun & games so I would try 6J7 anyway bit of pain with different pin out tho, keep goin its gonna be right soon  :grin:
Title: RCA Ba2A Pre amp qestions & answers
Post by: tardishead on May 22, 2008, 08:04:06 AM
OK - I put another transformer on the output and it works as it should.
50hz to over 10KHZ is perfect 15khz is about 1db down. So I have 2 working preamps.

I was just trying to understand why the transformer would cope with the situation of the BA-11A but drop bass on the BA-2c circuit - the output stages are very similar. Maybe its something to do with the B+ wiring differences of my BA--11a and BA-2c.

On the BA11a I connected a regulated 250vdc supply to the first dropper resistor on the B+ line which I think from memory is 2k7. When it hits the plate it was about 230vdc.

On the BA-2c I connected the 250vdc supply straight to the output transformer (there is no dropper resistor after the power supply). When it hits the plate on this circuit it is 248v.

Is it possible that I am working at the limits of this transformer and too much B+ would compromise the performance. It works in my BA-11a circuit but I am just trying to understand the differences.
Title: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on May 22, 2008, 10:08:45 AM
When testing with tone you should consider the source impedance.   You are likely driving the output directly with a much lower source impedance than the tube provides (which improves transformer response), so you have to simulate the tube source with series resistance when testing if you want a reasonably accurate comparison.    An accurate test of a 20K:600 transformer driven by a low-Z test source would involve 20K series resistance between source and primary, and a 600 load resistance on the secondary.   I think that is the discrepancy you are seeing.  
 
Transformer specs are not always as they seem.  Consider many modern pieces that specify something like 10K:600, but then also specify a maximum recommended source impedance much lower than the impedance ratio.  Many pieces require a lower Z drive, but do not tell you so, only assume you will do so.  I've found 10K:10K iron from the 1980's that only approximated the stated frequency spec when driven with something around 100 ohms source Z.    The assumption is for SS drive.  OTOH, many vintage pieces specs mean exactly what they state, because the assumption is they would be driven by tubes having higher output Z.  

The feedback loop in the BA-11A lowers the drive impedance of the output tube.  The BA-2A has no such mechanism.
Title: RCA Ba2A Pre amp qestions & answers
Post by: tardishead on May 22, 2008, 06:35:15 PM
Thanks Doug that explains it well.

Can you recommend any good books on tube theory that runs through basic amplifiers?

I just got a bucket of old Trimax transformers which sound awesome and I wanted to run them through their paces see what was good for which circuits.
Title: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on May 22, 2008, 07:14:54 PM
the easiest to digest and follow IMO is the classic 1938 George Sterling - THE RADIO MANUAL 3rd ed.  It does not get into feedback much though.  See RDH for that, though much more intensive.
Title: RCA Ba2A Pre amp qestions & answers
Post by: craptical on October 15, 2008, 06:01:34 PM
For anyones interest (in cap types etc), a BA-2C is on evilbay now, including picture of guts:

http://ainaz.pair.com/radioman/rcabc2ab.jpg

By the way: I´m thinking of giving this beast a try, using a Kenyon T-101 as output. Any comments?

/Dave
Title: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on October 16, 2008, 12:19:49 AM
The Kenyon iron is very good.  I don't know offhand if it takes DC or not.
Title: RCA Ba2A Pre amp qestions & answers
Post by: craptical on October 16, 2008, 03:37:32 AM
Thanks Doug! :thumb:

 The (very limited) specsheet says it´s suitable output for 56/76/6C5, 500 ohms sec. Doesn´t say anything about DC. However it´s listed in the 1941 catalog, was parafeed standard back then? If I understood you right in an earlier post 6C5 is almost a triode version of 6J7/6SJ7 (or at least interchangable in this circuit)?

Best regards

/Dave
Title: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on October 16, 2008, 12:47:23 PM
DC or parafeed both possible all the way back to the 1920's.  Safer assumption of course is if no DC listed, then not meant to have it.  Probably have to try to know.  6C5 and triode 6J7 always on the same page of values in the tube manuals.  I've heard highly opinionated claims that 6C5 sounds superior.  I've never done direct A/B.    Certainly easier to work with.  Hum rejection and capacitance likely a bit different due to grid input point.  Haven't looked at data sheets in awhile.
Title: RCA Ba2A Pre amp qestions & answers
Post by: CJ on October 16, 2008, 04:16:48 PM
Kenyon made some really good stuff, and some not so good stuff.
Not so good, meaning lossy.

One model I tested drug down the sig gen so bad, I gave up.

UTC was sleeping with RCA for many years, thus, all the UTC iron on the BA stuff.
Some of it is private labeled, but it is still UTC.
'
Title: RCA Ba2A Pre amp qestions & answers
Post by: craptical on October 16, 2008, 06:56:17 PM
Hmm... looks like I have to get my hands on a couple of 6C5s! :green:

No DC listed on any class A/AB/B output transformer in the catalog, only on modulation types (but they are rated at 150mA or more!). A schematic describing a transmitter with a mic input (in one of the old Kenyon papers) puts DC through an unrated (interstage) transformer, though. Well I´ll build a prototype and check it out. If the Kenyon behaves badly on DC, are there any direct problems with trying parafeed (except the obvious; a large cap in the signal path...)? Which UTC is the "correct" iron for this circuit?

Thanks!

/Dave
Title: RCA Ba2A Pre amp qestions & answers
Post by: CJ on October 16, 2008, 07:28:18 PM
Here is an old RCA, does ity look similar to yours?
If so, maybe the iron would sound great.
LS 27 is the big boy.
LS 10 is kind of pricey.
Same exact xfmr as the A-10, only wound on a bigger core.
And more shielding.

(http://i33.tinypic.com/24pc6ix.jpg)
Title: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on October 16, 2008, 07:33:44 PM
That's the BC-2B console preamp.  LS-10 and LS-27 is incorrect, but nice substitutes.
Title: RCA Ba2A Pre amp qestions & answers
Post by: CJ on October 16, 2008, 07:52:40 PM
OT, check out the octagon transformer on this BA 1:

http://i33.tinypic.com/6e2asl.jpg

http://i34.tinypic.com/2ykxh7m.jpg

Look at this weird thing:

http://i38.tinypic.com/34ni89f.jpg
Title: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on October 16, 2008, 09:31:28 PM
Pretty sure the octagonal on the BA-6 is a Thordarson.  Probably says on it.  All the interchangeable power outputs have the same drawing #, regardless of manufacturer.   Usually there's a specific manufacturer # also.  

2nd one is the 41-B preamp output.  Good example of the pre-war mystery manufacturer iron.  Maybe in-house, maybe not.  Maybe Ferranti or Amertran.
Title: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on October 16, 2008, 09:51:48 PM
This a copy of a post I put in the Brewery, since it will probably disappear over there.  

Comparison of RCA tube type input transformers used in broadcast and theater equipment from 1933-1967.  Cases and sizes compared with UTC LS, HA, and A types in the background for comparison.  There are of course a few others; for instance 1940's Hollywood Photophone production used Hollytran square can iron, in many cases with the same part #s as the broadcast stuff.  There's also some square cased transformers pre-1933.  This is effectively representative of everything they used in broadcast audio during these years.  The PA/industrial line type is thrown in as another example, and seen on the lower end non-broadcast stuff, as are some novar socketed plug-in types.  

Case types, left to right: 1933-41, 1937-41 (DeLuxe line), 1938-57 (2 pieces of differing sizes, program amp and preamp, representative of BA-3A and BA-1A among others), 1945-50 (OP-6), 1945-approx. 1956 (PA/industrial lines), 1954-1967 (BA-21A and BC-3/5/6 consoles).   UTC LS, HA, and A in background of two pics.

The first one weighs 10 lbs, and is encased in 1/2" solid copper shielding.  You can get 1/2" scale by comparing the thickness of the top on it.  It goes with the output CJ showed above.

Only the last two shown are UTC, with the PA type also having been previously available in RCA winding.

(http://farm4.static.flickr.com/3245/2741373863_4d05c32b2d_b.jpg)

(http://farm4.static.flickr.com/3060/2742206372_5b84af3985_b.jpg)

(http://farm4.static.flickr.com/3009/2742208924_985e19434c_b.jpg)
Title: RCA Ba2A Pre amp qestions & answers
Post by: CJ on October 17, 2008, 01:56:07 AM
Ya know, I don't think I could hack that chrome one.
Even me.
Title: RCA Ba2A Pre amp qestions & answers
Post by: craptical on October 19, 2008, 03:19:31 PM
Wow, nice collection! Should be put to work ASAP.  :wink:

CJ: I was planning to build from the BA-2C schematic in the begining of this thread, but with 6SJ7´s (or 6C5´s) instead of the 1620´s. Any suggestions iron-wise?

Found this in one of my docs... Quite a drop (-2dB´s) at both ends. (Diagram shows 60Hz to 15kHz. My apologies for the poor quality!)

(http://www.twin-x.com/groupdiy/albums/userpics/normal_Bild_1.png) :sad:

Best,

/Dave
Title: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on October 19, 2008, 06:09:15 PM
I doubt you can hear 2 dB at those extremes.  I use a lot of gear that's much worse.   I just bought a new CD that's low passed (for some reason) and -120 dB down at 16 kHz.  You can't really tell.
Title: RCA Ba2A Pre amp qestions & answers
Post by: craptical on October 20, 2008, 04:42:47 AM
Well, I do like gear that has a sound, so I´ll go ahead and build the little beast! (I´ll post some pics later if you like!) Thanks for your help and opinions so far. :thumb:

By the way: What are the function(s) of the meter in this circuit?

/Dave
Title: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on October 20, 2008, 11:04:00 AM
Quote from: "craptical"


By the way: What are the function(s) of the meter in this circuit?

/Dave


cathode current
Title: RCA Ba2A Pre amp qestions & answers
Post by: craptical on October 23, 2008, 03:26:32 PM
It´s alive! Running silently, despite the lack of hum balance control and point-to-point mess etc. (powered by a tube variable psu my father and grandfather built). Bass roll-off is ok/moderate, very defined and classy mid-highs! Sadly not much shiny treble, so I might have to change that output tx anyway... Or could it be something else? Anyway, here it is (my first on the forum!):

(http://c2.ac-images.myspacecdn.com/images02/33/l_03dd2b8657c74ccc8fc5998e7e6ca509.jpg)

Insides (before 600 ohm term. switch was added):

(http://c3.ac-images.myspacecdn.com/images02/58/l_75347b216cc64b228ee557e05fbc264a.jpg)

Thanks everybody!

:sam: :sam: :sam:

/Dave
Title: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on October 23, 2008, 10:10:56 PM
Looks nicer than the inside of a real one.   Tailor the interstage filter until you get the highs back.  You may have to give up some gain to get more high boost.   Make some real plots to confirm.  Then compare against a real one here:

http://www.groupdiy.com/index.php?topic=29509&start=0&postdays=0&postorder=asc&highlight=mccurdy
Title: RCA Ba2A Pre amp qestions & answers
Post by: craptical on October 24, 2008, 06:00:02 AM
Thank you, sir! Tweaking, as in increasing resistor/decreasing cap value? The real one doesn´t seem to have too much highs either... Is there any point in trying to imply some of the BA-11A hi-frq compensation network? The Collins looks great, though! Tried to find a schemo, but no success. Well, back to tweaking then! :green:
Title: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on October 24, 2008, 10:50:25 AM
The roll-off is certainly in the RCA iron.   Increase the resistor to vary blend of hi-passed signal passing through cap and full range passing through voltage divider; decreases gain.    Vary cap value to select boost point.  RCA generally selected to extend LPF point or give very slight boost.      BA-11 tailoring is in feedback loop, so not an option.    

Collins has 25ish dB of feedback loop through a tertiary output winding.  

I use the Collins and the RCA side by side frequently.  The charts imply more difference than the casual user would observe, and don't necessarily suggest what sounds best either.   The RCA has more character and richness.  The Collins sounds like clean tube.
Title: RCA Ba2A Pre amp qestions & answers
Post by: craptical on October 24, 2008, 07:05:26 PM
Thanks again, emrr! :thumb: Tried switching the 600 ohm termination out, voila!, extended (expanded?) treble (but bass suffers). No other tweaking as of yet. Total gain: 43/47.7dB @ 1kHz term./unterm.. Here comes an old school hand-made plot:

(http://c4.ac-images.myspacecdn.com/images02/8/l_6e1cb6495a324e408cef1e04482be727.jpg)

Looks as if I have slightly more treble and less bass compared to your original (when terminated). I also tested my proto "6SN7 one-bottle" with A24 output iron, it´s within tenths of a dB 20-20k :shock: But I really LOVE the sound of the RCA!

Quote from: "emrr"
The roll-off is certainly in the RCA iron

Might go for another "hifi-ish" build to try out the 6C5 option, is there any suitable iron coming to mind?

Best,

/Dave
Title: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on October 24, 2008, 08:07:56 PM
Sounds kind of normal on term/unterm.   Try terminating with 1K and see if bass stays close to best and highs improve.  

Suitable iron is always a matter of budget; hard to say.  Lots of choices.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: craptical on November 12, 2008, 03:11:57 PM
Work got in the way. Had a band in and tried the preamp on guitar, very nice! Anyway: I installed a 5K pot to have some options, but treble suffers long before I get something back in the lows, so I´ll use it where bass isn´t critical. And I´ll leave the term switch in, in case you want the "older" sound... Maybe I´ll look into a parafeed (con)version of the circuit to make choice of OP TX easier.

Thanks

/Dave
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: CJ on November 18, 2008, 03:57:49 AM
OK, I found the output iron that emrr sent, full props, no wonder this thing rolls off.

Anybody know the turns ratio of the output?

10:1 WTF, over?

The windings were cooked, so no DCR or Inductance specs were taken.

looks like

Pri: 6700 Turns, split inside - outside
Sec: 690 Turns, split - side by side

Coil Structure: 1/2 Pri - Sec - 1/2 Pri

The Pri is wound back and forth, layered paper, thin wire. 3/16" margins
The Sec sits between the two secondaries and is wound as side by side pies.

36 Layers Pri 185 turns per
14 Layers Sec 50 turns per


One of the Sec pies is reverse wound for CMMR.

Core looks like 50-50 Ni

625 EI

Not gapped, but stacked in a staggered form.
7/8 inch stack

Core staggered into 4 by 4 chunks.

No E shields

Sorry it took forever, eventually everything gets done around here.
Pics if you want.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on November 18, 2008, 07:48:43 AM
Awesome.  Glad to see this.  Now that you've seen the inside, any feeling on who wound it?  Does it bear any clues?  Pics would be cool if you have them. 

This was a preamp output from an RCA 76 series console, which would date it 1941-50 roughly.   250 ohm secondary; triode connected 1620 (6J7) driving it.  I have seen a significant number of these that were cooked, and have wondered why.   One thought was that over-voltage was easily possible in a 76 console if the monitor amp (well more than half the total current draw) was disconnected.   Tiny wire windings sounds like a good reason too. 

10:1; crazy.  I measure a lot of voltage loss through almost all of the octal era RCA preamp output transformers, and have always guessed 30K or more on the primaries.  This one is about the same as that found on the BA-1 / BA-2 except for the sec Z, and would be substitutable. 

Thanks for having a look.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: CJ on November 19, 2008, 10:57:10 PM
OK, RCA winding print on the way...
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: CJ on November 20, 2008, 01:01:05 AM
Just a few bad shots from the ailing Cannon, too many xfmr fumes I guess.

(http://vacuumbr.ipower.com//The_Lab/RCA/BA2a/rcaba2a_outa.jpg)

(http://vacuumbr.ipower.com//The_Lab/RCA/BA2a/rcaba2a_outb.jpg)

(http://vacuumbr.ipower.com//The_Lab/RCA/BA2a/rcaba2a_outc.jpg)
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on November 21, 2008, 09:42:13 AM
excellent.  remind you of anyone else's winds in particular?
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: CJ on November 21, 2008, 11:08:39 AM
i can not say for sure, it has the thin light brown inter-layer inulation like UTC, but everybody uded that.
I took apart a Western Electric that had a similar core lap job.
the potting compound is usually a giveaway, everybody has their own stink, this stuff is a little different from utc, triad, peerless, langevin.

i will look in the catalog and see if the numbers match anything.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: craptical on November 21, 2008, 03:51:59 PM
How far a throw do you think an A-25 would be?

/Dave
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on November 21, 2008, 04:42:35 PM
How far a throw do you think an A-25 would be?

Would work fine.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: craptical on November 21, 2008, 07:25:17 PM
How far a throw do you think an A-25 would be?

Would work fine.

Thanks!

Got some 6C5´s in the mail as well. ;D

/Dave
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: CJ on November 22, 2008, 12:17:52 PM
Hey EMRR are you sure that was not instead of an output transformer you sent me?

It looks a lot like a UTC Commercial Grade - CG - 235 50-200-500 to 80,000 Grids.
The can has the  RC-75 or whatever dimensions.
There is an alloy liner in the can, but the lam stacking is more like an output, weird.

 
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on November 22, 2008, 03:08:21 PM
It's officially the output.  Yes.  I have a whole stack of them, and many original preamps using them. 

The crimps in the case differ from any UTC I've ever seen.   Craft was one manufacturer of transformer potting cases, and many people used their stuff.  You see CG type cans and the can generally considered to be Stancor output type in the Craft ads of the late 40's-early 50's.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: CJ on November 22, 2008, 09:44:11 PM
Cool, if you are ever under the hood, get the DCR.
I can add it to the chart.

BTW, Wire is #40 for pri and #37 for sec.

Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on November 23, 2008, 11:37:47 AM
I measured 4 of them.  Average is 2.6K:120 DCR.

Part # is XT-2747.


A similar RCA output is part #900542-501 (same as XT-2875).   I have one of same undetermined manufacture and another made by Hollytran in LA.  Both measure 2550:210 DCR.  Sec on these is 500. 

Note the K-901740-501   (XT-2875-2) is the BA-1A and BA-2A output. 
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: CJ on November 23, 2008, 11:47:27 AM
Awesome.
See here:

(http://i36.tinypic.com/2ah8hvl.jpg)
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on November 23, 2008, 01:25:54 PM
Correction:  250 ohm secondary on the XT-2747.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: abby normal on November 24, 2008, 03:34:01 PM
OT, check out the octagon transformer on this BA 1:

CJ you know way more about transformers than me, could that be a UTC "S" series?
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on October 22, 2009, 10:58:20 AM
Did some experimenting with another members BA-2 clone yesterday.  It uses original input iron, and UTC A-25 outputs. 

We switched in an RCA output for direct measurement comparison, and the A-25 is microscopically better on the top end, and worse on the bottom end.    I'm not sure enough of a difference to hear, in regards to freq. response.  The RCA is built on a larger core than the UTC. 

The most interesting thing was to see that the original input primary connection, as shown on the schematic (one side grounded), gives a treble peak about 16K.   When connected as in the BA-1A, CT grounded or floating, it gives no treble peak.   The original BA-2 schematic shows input connection as would be found in this amps normal occupation; turntable output amp, fed by an unbalanced turntable EQ network. 
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: lassoharp on October 22, 2009, 12:12:23 PM
Aha you beat me to it.

But yes, take heed of emrr's observation - very important to proper freq response of the BA-2.

FLOAT (the cold side of the input pri) and ye shall be FLAT again!

Thanks again Doug for pinpointing that problem.

BTW - amp has noticeably different character now - seems richer in the mids, plus no sibilance band dist.  Interesting also that A/Bing each channel - hi boost IN/OUT gives no audible difference like it did before.  Much better overall this way though.  ;D
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on August 29, 2011, 04:02:44 PM
*bump*

I'm gearing up to build a BA-2C pre amp, here's the schematic:
(http://www.preservationsound.com/wp-content/uploads/2010/10/ba2c.jpg)

I've drawn out my turret board layout, but I have a couple of questions:

a) I might be able to get hold of a BA-2 output transformer but I don't have an input transformer.  I was thinking of going Sowter, what sort of specs should I be looking for?

b) I can't quite make out what the values of R1 and R2 are, does anyone know?

c) Was the original coupling cap between the stages paper-in-oil or film?

d) What voltage rating should C7 be?  I know it's a power filter and I know I should be able to work this out with some maths, but I don't know the maths!  I know Ohms Law is my friend here, but I don't know what the power or current would be to work out the voltage.

Thanks.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: gary o on August 29, 2011, 08:21:20 PM
I think R1 & 2 are 820R & 680R or there abouts in my ghetto DIY version I popped a 1K5 in there as didnt need the meter...
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: lassoharp on August 29, 2011, 10:17:22 PM
Quote
b) I can't quite make out what the values of R1 and R2 are, does anyone know?


Unless you are wanting to add metering just use a 1.5K  (820 + 680)



Quote
d) What voltage rating should C7 be?  I know it's a power filter and I know I should be able to work this out with some maths, but I don't know the maths!  I know Ohms Law is my friend here, but I don't know what the power or current would be to work out the voltage.

You have a 230 volt winding times peak value.  350V is the lowest standard value you want to use.  Anything higher is just fine.   Same goes for all the coupling caps.


Quote
a) I might be able to get hold of a BA-2 output transformer but I don't have an input transformer.  I was thinking of going Sowter, what sort of specs should I be looking for?

Anyone of their dedicated mic to grid transformers should work.  The REDD 47 model or the generic mic to grid (4935).   They will not have the same ratio as the original but this is only a small difference.  I would email Sowter and ask about recommended resistive loading on the secondary - ask what the expected input impedance would be if left unterminated into an open grid as the original is.
 
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on August 30, 2011, 05:12:33 AM
Great, thanks guys!

Attached is my turret board layout for the amp (excuse the dodgy drawing).  I'm going to be using screened wire for the grid connections to the 1620s.  Is it ok to solder the screen to the ground on the input XLR or should I try and get a connection to one of the two star grounds?
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on August 30, 2011, 10:24:57 AM
This might be a stupid question, but could I add a DI by connecting a jack to the grid of the 1620 with a couple of resistors like this:

(http://www.gearslutz.com/board/attachments/high-end/179661d1277235737-rca-op-6-powerhouse-op-6-di.jpg)
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: lassoharp on August 30, 2011, 11:38:17 AM
Is this really for an OP-6?  Or for your BA-2 clone?

If for your BA-2 you can do a standard type DI with a 1M resistor from grid to ground - omit the series resistor.  Should work just fine.

Theory wise doing that for an OP-6 probably won't be as straightforward as a BA-2.  Someone else will have to answer.

I probably won't be the only person to advise against hacking a real OP-6 for a DI - even if it works without issue.  I would say use a DI box for instrument use.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on August 30, 2011, 11:43:02 AM
Great, thanks for the answer.  This is for my BA-2 clone, I just came across that schematic whilst browsing through Gearslutz.

Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on August 30, 2011, 02:35:33 PM
Me again!

What's the best type of capacitor to use as C8?  The treble boost bit of the circuit (so signal flows through it).  It's 330pf and I can find silver mica and polystyrene.  What was used in the original?

Thanks!
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on August 30, 2011, 02:46:10 PM
It'd be mica in the original.  I wouldn't install R11/C8 unless needed, since it's really transformer specific.  The values may need to be modified. 
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on August 30, 2011, 02:55:13 PM
Great thanks Doug.  Hopefully I'll be using the original output transformer so I guess I will have to install it.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on August 31, 2011, 01:01:21 PM
It compensates for the combo of input and output, it's not exclusive to one or the other.  So, you'll really need to measure it with and without to see.   It's not a lot of boost in the first place, and elimination will give you a little more total gain since R11 is part of the volume voltage divider with a cap bypass. 
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on August 31, 2011, 01:07:54 PM
Ah I see, looking forward to experimenting with it.

As for a PT, I had my eyes on the Hammond 370x which has a secondary winding with 240v-0v-240v, is that going to be ok?  The original had a PT with 230v-0v-230v on the secondary, but I can't find one like that.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on August 31, 2011, 02:52:38 PM
That sort of PT voltage will be fine.   Could add up to a bit more current through T2, can always increase size of R8-9 to compensate.   

Another trivia bit I am remembering.  I have seen two variations in the transformer for T2, depending on the era the part is from.   The schematic shows 2570 ohm DCR on the pri, there are also later transformers where it's 10K.  I thought that was a bad transformer, the first one I came across, but it worked correctly and I then found others that read the same.   
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on August 31, 2011, 03:20:37 PM
A better schematic, camera phone style.  Someday I'll have time to scan properly.  Amazing there's not a single better one on the web already.

http://www.groupdiy.com/index.php?topic=44740.msg571378#msg571378
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: gary o on September 02, 2011, 03:27:34 PM
Aha I had R11 & C8 instaled in my DIY BA2A.......I just bypassed them & it sounds better to me so I think I will leave them out then , could put a switch in their maybe & try some other values for fun.....Thanks Doug.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on September 07, 2011, 04:13:12 PM
That sort of PT voltage will be fine.   Could add up to a bit more current through T2, can always increase size of R8-9 to compensate.

Thanks Doug.  If there was too much current through T2, what would happen?  I'm trying to apply Ohm's Law to the schematic so I can work out how much current is flowing through with the correct transformer, but I can't find out anywhere what the voltage drop is across the 6X5 rectifier.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on September 07, 2011, 04:28:06 PM
You might cook the transformer with a higher voltage, or a funky tube drawing too much current through it.   It's an unknown spec, so hard to say. 
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on September 07, 2011, 04:37:54 PM
V3 / 215AC-0-215AC according to voltage chart, differs from schematic 230-0-230 / 288VDC
V2 plate 232VDC cathode 8.8VDC
V1 plate 73VDC cathode 2.4VDC
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on September 07, 2011, 05:05:15 PM
Hmmm, interesting.  If the voltage chart says 215ac-0-215ac (despite what the schematic says) then is my 240-0-240 PT going to be way too much?  Bit scared of cooking my output transformer!!

Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: lassoharp on September 07, 2011, 05:26:47 PM
The schematic for the 230-0-230 version calls for dropping resistor values of 3.3K & 3.9K.  Use 5ma total draw as the ceiling.  You will be ~10V high with the 240-0-240.  Off the cuff I'd say a pair of 4.7Ks for R8 & R9 should get you in the ball park.  Tweak with standard value combos from there if needed.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on September 07, 2011, 06:14:23 PM
Thanks lassoharp.

Quote
V3 / 215AC-0-215AC according to voltage chart, differs from schematic 230-0-230 / 288VDC
V2 plate 232VDC cathode 8.8VDC
V1 plate 73VDC cathode 2.4VDC

Sorry if I'm being a bit slow, but do I need to tweak R8 & R9 to get 288VDC at the output transformer?

One more PT question (apologies, but this is my first undocumented project!).  Would the 370x in the following table have enough power?  It has a secondary of 240-0-240 and a total power of 41VA.  So that's 41/ 480 = 0.08 total supply amps.  Not much, but is it enough? I can't work it out from the schematic.  What would you recommend from the following table:

(http://www.tube-town.net/info/hammond/hammond_pt_classic.jpg)
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: lassoharp on September 07, 2011, 06:41:22 PM
Yes, more or less.  It's really just a straightforward application of Ohm's law but for best results I would suggest using a breadboard set up to tweak the resistor values.  This is to take into account  the power transformer's relative sag or stiffness & the fact that different tubes (1620s) even if same brand will give slightly different voltages under the same circuit conditions.  Different 6X5s will also vary slightly in their output voltage.

If you have an RCA tube book handy, look up 6X5 or 6X4.  They will give you a chart that allows you to predict what the output voltage will be at the rectifier's output for a given total current draw.  You must pick a current draw - 4.5-5ma for one channel or double that for two - then work from there to determine the total resistance value needed to get to your target ~288VDC. 
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on September 07, 2011, 06:47:33 PM
Right, I get it.  Is the 4.5-5ma current draw an educated guess?  If the maximum current draw of a channel is 5ma, then the 8ma ceiling of the 370x should be fine, I think.

Thanks very much - looks like I've got some maths to do!
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: lassoharp on September 07, 2011, 07:32:58 PM
It was a rough guess.  Using the factory numbers that emrr posted - 5.6ma would be the exact number to aim for.  Should be easy to get with modern 1-2% resistors.

I'm not sure what you mean by the 8ma figure for the 370X.  What you typically want to do is aim for running the PT at 50% of it's max current handling capability.  You'll be well within that.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on September 08, 2011, 03:24:15 AM
The 370X will put out 240-0-240 with a 40 mA load.   It will give a higher voltage with 5-6 mA draw.  All of which can be mitigated by trimming R8/9. 
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on September 08, 2011, 11:04:40 AM
Ok, so I may have got this totally wrong, but here goes...!

I build the circuit, and tweak R8 & R9 to bring the voltage at pin 1 of the output transformer down to 288VDC.  So to protect the transformer, can I replace it with a 2.2k 2W resistor so there is a dummy load across the power supply?

lassoharp - how did you get the 5.6ma figure?  I'm still getting to grips with Ohms Law, and no matter what figures I try out from the ones emrr gave earlier, I can never get 5.6ma.  I'm keen to learn so your workings out would be of interest.

Thanks!
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on September 08, 2011, 11:25:00 AM
Not grasping the usefulness of a 2k2 dummy load.   Just start with higher value R8/9 on first power.    Use 10K to be safe, and work down.    Or use stock values and bring up with a variac while measuring, don't go past 290VDC.   
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: lassoharp on September 08, 2011, 11:52:03 AM
Quote
lassoharp - how did you get the 5.6ma figure?  I'm still getting to grips with Ohms Law, and no matter what figures I try out from the ones emrr gave earlier, I can never get 5.6ma.  I'm keen to learn so your workings out would be of interest.


From the factory chart you're given a cathode voltage (V).  You then have to refer to the schematic to get the cathode resistor value (R).  Since RCA has meter insert points in the cathode circuit the total cathode resistance is broken up into two portions.  So for this calculation just ignore any small effects a meter may have and just add the resistances to get the total.  I used a standard value (2.2K) for V2 even though  it's actually 2270ohms.  So now it's a matter of just solving the equation for current (I) through each tube and adding them together for the total.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on September 08, 2011, 01:25:25 PM
Thanks both of you - you both make perfect sense!
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on September 14, 2011, 07:57:54 AM
So I'm just sitting around waiting for all the parts to turn up now -slightly frustrating!

In the meantime, I'm looking at adding an output attenuator after the transformer so I can drive the amp a bit if I feel the urge!  I was thinking of building NYD's design from this thread: http://www.groupdiy.com/index.php?topic=4825.0 (http://www.groupdiy.com/index.php?topic=4825.0) and adding another pole to make it balanced. 

(http://electronicdave.myhosting.net/miscimages/600bridgedt.gif)

The OP transformer on the BA-2 has an output impedance of 500ohms, so if I change the 600ohm resistors on the attenuator schematic to 500ohm, then is should work, right?
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: lassoharp on September 14, 2011, 09:45:36 AM
Forum member nielsk had some nice balanced T attenuators for sale.  I've used one for the output of an Altec 436 clone I built and they work great.  Price is great too.

http://www.groupdiy.com/index.php?topic=44746.msg560548#msg560548


 
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on September 14, 2011, 10:12:20 AM
Great, thanks for that.  Looks like they're 600ohm, will that work ok with the BA-2C?

Cheers!
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: lassoharp on September 14, 2011, 01:29:26 PM
It should work fine.  I think mine actually measured about 570ohms with a DMM.

If you have a software setup for measuring freq response you can always compare a 600r termination to a 500r termination and see if there are any differences worth worrying about.

Check emrr's thread here for some relevant info on differing terminations:

http://www.groupdiy.com/index.php?topic=41032.0

Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on September 14, 2011, 04:49:55 PM
500-600; effectively the same thing. 

Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on September 23, 2011, 10:08:22 AM
Still twiddling my thumbs until some essential bits turn up.  I've soldered the turret boards though, so that's some progress.  Starting to think I should have done it just P2P as the turret boards take up a fair bit of space!

C-2 - the coupling cap between the two tube stages, what would be a safe voltage rating for it?  I've got a .47uf Vitamin Q that I was thinking of using, but it's only rated at 100v.  R-10 and R-3 should be dropping the voltage a fair bit though, shouldn't they?
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: lassoharp on September 23, 2011, 10:27:23 AM
C2 must be able to handle the full peak voltage of the PT that appears during power up before the tubes start conducting.  (way way >> 100V)

Now, if you were to install a stand-by switch, this would prevent C2 from seeing such a high spike in the voltage at power up, but it will still be seeing the full plate voltage of V1 plus a smaller momentary rise after the stand-by switch is hit.

100V is within limits for the ~73V plate voltage of V1 but the momentary surge would probably be over 100V.   It may take it for hundreds of turn ons or it may start leaking oil after a few hits.  Again, this is for a stand-by switch case - normal turn on would greatly over voltage a 100V cap.  I'd save that Vit-Q for an EQ project
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on September 25, 2011, 07:02:24 PM
Thanks lassoharp, I've managed to get hold of one rated at 630v now.

I've been reading up on grounding and I've come up with the attached simplified diagram which shows how I'm going to attempt to wire the grounds in the pre amp.  It's extremely simplified and only shows the ground wires.

I've been using this page as a reference:  http://www.aikenamps.com/StarGround.html

I have two turret boards in the enclosure, one containing the pre amp parts and the other the power supply parts.  The 40uf cap shown on the power supply board is the first filter cap in the power supply circuit.

The dark green line represents the bus ground, while the lighter greens represents the ground wires going from the off board components to the turret boards.

Have I done anything terribly wrong here?  I'm not entirely sure I've got the shields on the input/ output cables wired correctly.  I'd really appreciate it if someone could look over this diagram and flag up anything that doesn't look right.

Thanks a lot!
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: lassoharp on September 25, 2011, 07:30:51 PM
The sec of your OT should be floating, so the "lo" side to pin 3, not earth - if that's the way you had it shown.  You would usually want to either connect pin 1 (shield) of the output jack directly to the chassis (ground tab or screw on jack) or just leave it disconnected.  I've done it both ways. 

Same thing with pin 1 of the input jack.

The other grounds look reasonable.  You may want to try grouping the two 1620 stage grounds and the input transformer sec ground to a single short local point and then running a single longer wire from that back to the main star - as opposed to running the individual long wires from each stage back to the star.  It may not create any problems as is, but the reasoning behind the shorter wires makes sense for heading off any loops. 
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on September 26, 2011, 04:51:53 AM
Thanks for the feedback.

So for the 1620 shields and the input secondary I should make a sort of mini star point which should then connect to the main star?  I just realised I missed out the shields on the wires leading to the grid caps, I'll connect that to that mini star too I think.

One thing I don't fully understand is why, in the article I linked to, it says that the star chassis ground point shouldn't be connected directly to the earth ground point.  If they're not directly connected, they're surely still connected via the chassis, so why does it make a difference?
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: DaveP on September 26, 2011, 07:09:23 AM
Letterbeacon,

Remember that all connections whether bolted or soldered have a resistance even if milli-ohms.

It is good practice, not to use the chassis as a universal ground but to use it as an extension of the XLR in/out shielding, so that a screened cable comes in, extends around the circuit in the chassis then goes on out via the XLR out.  So the XLR's pin 1 needs to be soldered with a thick wire to the lug on the socket that connects to the chassis.  Any screened wire within the circuit can also have the screen connected to the pin 1.

Star grounding works, as does a large dia busbar, both should be connected to the chassis at the same point with the Mains earth/ground wire.  The logic is that due to the star, every circuit's earth travels directly to earth, it does not have to carry the earth currents of another part of the circuit, or indeed any hash or RF picked up by the in/out cables or chassis.  A busbar works slightly differently in that the sensitive parts of a circuit are always "upstream" from the heavy current parts.  Electron earth currents are like water, they don't run upstream, but always towards the nearest earth.

When you take these principles onboard you can confidently design your own layouts for any circuit.

best
DaveP
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on September 26, 2011, 06:25:26 PM
Thanks Dave.

I've attached another (scruffy!) Paint drawing taking on board yours and lassoharp's notes.  I don't quite understand why the noise picked up by the shields which is supposed to flow out of pin 1 of the output XLR won't flow out of the power inlet if they're both connected to the chassis.

I've attached all the shields to the shield of the input 1620 which then goes to pin 1 on the input XLR.

I've placed the components on the pre amp turret board so the sensitive input components are over on the left and is far away from the power supply grounds.

As before, I'd appreciate any notes - thank you!
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: DaveP on September 26, 2011, 07:26:28 PM
Letterbeacon,

I can't make out what all those green wires are doing, there does not appear to be a star point.

This Jensen schematic is the business:-

(http://i54.tinypic.com/21lkjn4.jpg)

Maybe the picture will explain it better

best
DaveP
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: lassoharp on September 26, 2011, 08:06:56 PM
I think what letterbeacon is asking - and it's a logical and natural question - has to do with why the "common Z coupling" is prevented when everything ultimately references the chassis. 

They speak of separation - keeping shield noise out of signal commons - and it's natural to ask how this is achieved when electrically they are at one level all equivalent.   When you put your DMM probe on the chassis at one end and measure either to the other end of the chassis or to any of the ground points the meter says:  "Same Place".

I like to think in terms of loop potentials.  If you ground pin 1 XLR shield direct to chassis at POE and also run a wire to the 1st stage ground bundle you have a potential to get that noise going around in circles though some of it still going to ground.  With a single path to chassis only it's more likely to go straight to ground and stay out of everything else.  If pin 1 is ran straight to circuit grounds a similar thing could happen - part of it goes to ground and part of it loops around the amp circuit - and it only takes a very small amount in a gain stage to be heard.

Sometimes anomalies crop up.  I've had to break away from otherwise good star grounding in guitar amps before - separating the V1 cathode ground from the star and placing it on the input jack ground - because nothing else would work to stop the humming.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on September 27, 2011, 03:29:58 AM
Between your two replies, I think I get it now.

lassoharp, that is exactly one of the questions I was asking, thank you.

DaveP, thanks for the diagram -I think that is pretty much what I have.  All the green lines that go to one of the 1620 stages are the shields from the signal carrying cables which are going to a mini star point (isolated from the chassis) and then go to pin 1 of the input XLR.

There is no star point because, for some reason, in the article I linked to earlier it said don't link the signal ground to the safety ground, whereas in the Jensen diagram the two are joined.  On my diagram, if I join the points labeled 'bus bar ground' and 'earth' then I think it will match the Jensen diagram.

Thanks a lot!
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: DaveP on September 27, 2011, 03:56:01 AM
Ok I see what you are getting at,

The meter will register lots of points as at ground potential when in fact there are subtle differences as I indicated in my earlier post.  Most DMM's don't register milli ohms and the subsequent voltage drops, but the amps amplify them up to nuisance levels.  As Lassoharp says, the last thing you want is a loop, its a recipe for hum.

Here's an analogy, if a piece of guttering is dead level, water may collect in some parts because there is no incentive to move in any direction, when its full it will flow but when the currents are low it doesn't flow.  By putting a slight fall from one end to the other there is always the incentive to move to the lower level, that is what a busbar or a star ground does because it is the lowest level possible.

One other point, if there is a decoupling cap for each stage, return the negative as close as possible to where the cathode resistor is grounded, this reduces the length of current carrying wire to a minimum and there is less chance of inducing a current into sensitive areas.  Same goes for large dia. wiring, there is less of a voltage drop in heavy wires to be picked up and amplified (milli ohms again)

best
DaveP
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on September 27, 2011, 09:03:17 AM
I'd be very interested in seeing that!
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: gary o on September 27, 2011, 10:16:55 AM
Interesting info.....I enjoy DIY ing & usually learn the hard way trial and error....Im starting to understand earthing & things like that....The pres i have DIYed have a separate PSU in a box 6 feet away...

My question is will carefully laying out the components on turrets make much difference if there is no nearby power supply part like the power transformer...

does the layout keep noise lower in general ?

In the past in my ignorance & lack of info or pictures I have built fairly complex tube gear just bird nesting everything the shortest route I could see & then tried different earth paths to hear which is best ...it usually ended up looking like the star ground diagram it was nice too see that my ears led me to the star grounding before I came across the star grounding info on the web

For instance my ghetto version of the BA2A is just 2 tube sockets close together with components soldered between the 2.... the whole powers supply is elswhere in a metal case

So Im wondering if a better layout would make lower noise in my circuits, not that they sound noisy at the moment but if my messy layout may be noisier it could be time for a tidy up....maybe I need to measure what noise i do have....

Looking forward to seeing letterbeacons BA2A & hearing his reports on the sound......

Thanks for all info here am learning lots.....

Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on September 27, 2011, 10:54:16 AM
I don't think turret board layouts are necessarily quieter at all.  My choice was purely because I had never made a turret board before and wanted to have a go.  In fact, I wish I had gone point to point with this one because the turret boards are taking up a fair bit of space inside my fairly small enclosure.

I can't wait to hear my BA-2 either!  Just waiting on the transformers now!
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on September 27, 2011, 10:57:37 AM
Back on page 4:

http://www.groupdiy.com/index.php?topic=26675.msg363779#msg363779
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: DaveP on September 27, 2011, 10:58:16 AM
Gary,

Sounds like shortest route between components is a good way to go as long as the earthing is correct, remote power supply is never a bad idea.

Laying components on turrets does look neat and is handy for removal for servicing, but it can often mean longer wiring runs as a result, a kind of trade-off of neatness for noise if its not done carefully.

Re, measuring noise, I'd welcome some input on that myself.  Just today I've been measuring some triode wired RF pentodes to find some low noise types, but my scope only goes down to 1mV/div and you can just about see the difference between 100uv and 200uV on that.  Steve Bench reported figures of 0.67 uV in his tests.

best
DaveP
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: gary o on September 27, 2011, 12:38:15 PM
Thanks dave yeah the turret board designs usually make the signal path longer so more chance to pick up something un wanted l...like and ariel i guess & therefore more important to layout sensibly....I nearly put that in my previous post...

I read somewhere about tubes can be too close to each other too.....

sadly I have no noise measuring gear & wouldnt know how to use it really.....for now I been using some RME software, basically a huge sensitive meter, I can see the noise on the meter & adjust the circuit to watch the level go down hopefully its been handy to tune my gear....

thanks once again....
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on October 01, 2011, 12:55:15 PM
BA-2 layout (http://farm7.static.flickr.com/6141/6200470189_65458d448e_o.jpg)

BA-2 guts (http://farm7.static.flickr.com/6159/6200986994_959256aee9_o.jpg)
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on October 02, 2011, 01:48:31 PM
Great, thanks for that Doug, very interesting!
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on November 20, 2011, 11:12:24 AM
So I've finally got this thing powered up and I have a few questions regarding the power supply.

Here are some of voltages I should be expecting:

Quote from: emrr
V3 / 215AC-0-215AC according to voltage chart, differs from schematic 230-0-230 / 288VDC
V2 plate 232VDC cathode 8.8VDC
V1 plate 73VDC cathode 2.4VDC

I'm using a Hammond 370x for the PT which is 240-0-240 rather than the 230-0-230 specified by the original schematic, so as discussed earlier in the thread, I'm going to have to change R8 & R9 to get the voltages in spec.

The voltage at the plate of the 6X5 is 288VDC so, sing Ohm's law, the voltage after the R9 (ie. the voltage that goes to pin 1 of the OT) is 248VDC.  The current draw of the amp is 5.6ma x 7.2k (which is the combined resistance of R8 & R9) = 40v voltage drop.  288 - 40 = 248VDC.

As I'm just getting my head around all this, could someone confirm that is correct please?

Thank you.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: lassoharp on November 20, 2011, 11:45:24 AM
Quote
The voltage at the plate of the 6X5 is 288VDC so, sing Ohm's law, the voltage after the R9 (ie. the voltage that goes to pin 1 of the OT) is 248VDC.  The current draw of the amp is 5.6ma x 7.2k (which is the combined resistance of R8 & R9) = 40v voltage drop.  288 - 40 = 248VDC.


Yes, this is correct.


ps - (probably a typo but technically you would mean the cathode of the 6X5)
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on November 20, 2011, 12:13:28 PM
Great, thanks lassoharp.

So I've powered up the unit and have the following voltages with the schematic's voltages in brackets:

v3 plate = 295 (288)

after r8(4.7k) & r9(3.9k) = 247 (248)

v2 plate = 211 (232)
v2 cathode = 7.83 (8.8)

v1 plate = 80 (73)
v1 cathode = 2.3 (2.4)

So none of them are exactly right, but are in the right ball park.  I'm slightly worried about V2's voltages, are the too far out do you think? Apart from R7 R8 & r9 all the resistors are carbon comps which might go some way to explain the discrepancies.

I plugged a mic in and it works!  Kind of.  I can hear myself but there is a very loud buzz, which sounds like a grounding problem.  I laid out the ground schemes as per that Jensen diagram that was posted in the other thread but I'll double check them.

I haven't done anything with the 6.3VAC CT at the moment - shall I ground this?

The Hammond 370x PT is also very hot.  I can touch it for a couple of seconds before it gets too hot to touch.  Is this normal?  It's the first time I've used one so I have no idea.

emrr (who sold me the input transformer) recommended that:
Quote
On the secondary [of the input trasnformer], try connection both ways when you have test gear, and see which has the better top end response.  Usually one way does, due to capacitive distribution within the coil.   Once you arrive at the better connection, then flip-flop input terminals as needed for correct polarity through unit. 

I don't have much in the way to test frequency response, is there any software that you can recommend?  How about RMAA?
Thanks for all your help so far!
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on November 20, 2011, 12:26:09 PM
The voltages you show are fine; your output is the higher primary resistance type, and that will give a lower voltage like you are seeing.  The 6.3CT is part of your loud buzz.  The Hammond probably shouldn't get that hot. 
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on November 20, 2011, 12:42:44 PM
Thanks Doug, I'll ground the CT and see what happens.  Glad to hear the voltages look about right.

I'm slightly worried about the PT.  I'll do some more research and re check my wiring, but if anyone has any ideas why it is so hot please let me know!
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: lassoharp on November 20, 2011, 12:51:48 PM
Quote
I haven't done anything with the 6.3VAC CT at the moment - shall I ground this?

Yes.  I had a PS with filament CT disconnected for a while - sometimes it would hum sometimes it wouldn't.  It needs a permanent reference.



Quote
The Hammond 370x PT is also very hot.  I can touch it for a couple of seconds before it gets too hot to touch.  Is this normal?  It's the first time I've used one so I have no idea.


That sounds like trouble.  At that current draw that PT should be cool for a good bit of time.  Carefully recheck and confirm all your connections with the schematic.  With the voltages you gave it seems strange that the PT would be getting that hot - no evidence of excessive current draw.  Can you confirm to us how you have the 6X5 rectifier wired using the PT wire color codes and tube socket numbers?
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on November 21, 2011, 04:06:51 PM
That sounds like trouble.  At that current draw that PT should be cool for a good bit of time.  Carefully recheck and confirm all your connections with the schematic.  With the voltages you gave it seems strange that the PT would be getting that hot - no evidence of excessive current draw.  Can you confirm to us how you have the 6X5 rectifier wired using the PT wire color codes and tube socket numbers?

Here's the Hammond wiring diagram for reference:
(http://www.hammondmfg.com/jpeg/300sch.jpg)

Ok I've just rechecked my wiring of the 6X5 and the PT and here's what I have:

Power transformer primary:
White + Blue wires connected to IEC inlet via a 1A fuse and switch
Black + Brown tied together
All other wires isolated

Power transformer secondary:
Green + Green 6.3VAC connected to V3 pins 2 & 7
Green/Yellow 6.3CT  not connected at the moment (but will be connected to ground when I get a chance)

Red + Red 240VAC connected to V3 3 & 5
Violet + Red/Yellow 240CT tied together and connected to ground

Yellow +  Yellow 5VAC isolated (not needed)
Yellow/ Black 5CT isolated (not needed)

Does that look about right to you?
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: lassoharp on November 21, 2011, 05:13:47 PM
Quote
Violet + Red/Yellow 240CT tied together and connected to ground


That may be the issue.  Only tie the CT (Red/Yel) to ground)  leave the 50V tap (vio) isolated.

  The 50V tap is probably a bias tap.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on November 21, 2011, 05:31:29 PM
Quote
Violet + Red/Yellow 240CT tied together and connected to ground


That may be the issue.  Only tie the CT (Red/Yel) to ground)  leave the 50V tap (vio) isolated.

  The 50V tap is probably a bias tap.

Ah yes, the thought crossed my mind as I was typing it out. Thank you, I'll try isolating it tomorrow when I next have the iron out.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: lassoharp on November 21, 2011, 08:24:54 PM
Another way to look at it - you have 50V going directly to ground with only some of the PTs internal DCR in between - maybe several hundred ohms . . .
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on November 22, 2011, 06:21:09 PM
So I've grounded the 6.3V CT and isolated the 50V bias tap on the secondary and... no more hot PT and no more buzz!

Kind of...

There is a buzz but it's so faint that I can only hear it when I turn the gain up full record a clip in my DAW and then increase the clip by 24dB, and turn the volume of my speakers way up, and only then I can hear it faintly in the silence between audio.  I'm not too worried by it, but I'm hoping to reduce it further by using shielded cable from the XLRs to the audio transformers (at the moment I'm using twisted cables).

Also now the 50V bias tap isn't going to ground, the PT is outputting a higher voltage so instead of getting 248VDC after R8 & R9 I'm getting 270VDC.  I've run out of resistors to drop the voltage to 248VDC for the time being so I'll have to look at that later.

The audio sounds like it hasn't got much top end at the moment which I put down to three things: 1) I'm using a Shinybox ribbon mic which are quite dark anyway 2) I may need to swap the polarity of the secondary on the input transformer as Doug recommended in an email and 3) I haven't put in the treble boost part of the circuit yet.

When I have a spare minute later this week I'm going to install RMAA and have a play with the polarity and treble boost circuit, but first I need to build some attenuators so the pre amp input sees the correct impedance from my Fireface.

Here are a couple of pictures of it so far - apologies for the bad camera phone, I'll take proper pictures when I've got a proper front panel to show off (this one's just temporary).

(http://img.photobucket.com/albums/v428/Senorpants1/photofront.jpg)

(http://img.photobucket.com/albums/v428/Senorpants1/photoback.jpg)
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on November 22, 2011, 07:22:26 PM
Nice build!
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on November 27, 2011, 07:53:33 PM
Thank you, I'll get better ones up once I get the front panel done and have finished installing the JLM stuff.  In the meantime I've been messing around with measuring the frequency response of the pre amp.

Attached is a pdf with the graphs and findings of the pre amp after running some tests via RMAA.  As you can see the frequency response drops off pretty steadily after about 1k and before 100Hz.  Does this look normal?  I mean, as it is a tube pre amp I expected it to be a little 'dark'.

I tried adding the treble boost circuit to extend the top end a bit but didn't notice any difference.  How subtle is the treble boost supposed to be?

Another thing you'll notice is the 50Hz boost in the noise levels, which I assume is the mains hum.  I can try rotating the PT, but are there any other tips for reducing this hum?  I'm going to replace the twisted wires that connect the input XLR to the input transformer with shielded cable, but how much of a difference do you think this will make?

Thank you.

Hmmm, it won't let me upload the pdf -it says the upload folder is full.  I'll attach a picture of the frequency response and the noise levels.  The left channel has the BA-2C across it (via 75 ohm resistors across the + and - to bring the impedance up to 150 ohm) and the right channel is my Fireface looped into itself.

(http://img.photobucket.com/albums/v428/Senorpants1/ba2cfrequencyresponse.jpg)
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on November 27, 2011, 09:11:59 PM
The bottom end looks better than some, the top can be highly variable depending on test setup.  Look at my 87-A plots in my rack up thread for examples of a real one with two different input loads.  Consider that a mic will deliver a totally different load than a resistive or amp driven setup, regardless of the nominal expected Z. 

Noise looks respectable.  I'd try power rotation just to see.  Is the shield tap on the input trans grounded?  That measurement should be taken with a 150-200r across the input terminals.  Real RCA quotes -69dB (0.001 watt reference) with input terminated. 

High boost is in the 2-3 dB range, and set high enough that it usually only influences the shape of the roll off. The earliest versions used 56k/270pfd rather than 33k/330pfd, which would be a slight difference.  Consider that if r11 was 100k then c8 would be a 6 dB boost bypassing half of the voltage divider, and overall gain would fall by the same amount.  So 33-56k is a reasonable eq loss to balance against top end response leveling.  Tune shape by changing cap values, may take a handful and some test leads to see what looks best.

Q?s:

Output load?  Look at it with and without a 600-1k across the output.  Sometimes it makes no diff, sometimes a lot.

Did you try reversing the secondary of the input trans?

Did you have a look with a mic pad between the source and the input?
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on November 28, 2011, 05:19:09 AM
Yes comparing my BA-2 frequency response with the plots for your 87A, the bottom end is about the same, maybe even a little better.

Yes the shield tap on the input transformer is grounded.  I'm pretty happy with the noise level, I did use carbon comp resistors throughout so I've only myself to blame!  It's just the 50Hz hum I  might try and lower somehow.

As:

Yes the amp was driving into a 10k load.  What do you suggest I do to change this?  Use a U pad with a 600r shunt resistor?

Yes I did try reversing on the secondary of the input transformer.  Weirdly, the level was much louder the other way, and the frequency response dropped off very rapidly after 1k.

No, I haven't tried it with a mic pad between the source and input.  Again, what would you suggest?  A U pad with 10k in series and a 75ohm as a shunt?

Don't get me wrong, I'm really pleased with it so far - I recorded some vocals using it with a ribbon mic last night, and it did sound a little dark, but when I dialled in some top end with an EQ it sounded great!
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on November 28, 2011, 10:29:15 AM
Can we see a clear shot of the connections you made on the input transformer?  Or tell me what pins are connected to what?  Louder one way versus the other makes me suspicious, but not sure of what.   Do you have gain measurements noted? 

output load, strap a 680r across the output terminals.   1K is also fine.   May or may not see anything other than gain change. 

standard mic U pad 620/169/620, or similar, a la Jensen.   10K would be a line pad, which usually shows even greater roll-offs in the highs. 

Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on November 28, 2011, 10:37:51 AM
I found this section of the BA-2 manual online (http://www.bigdmc.com/ba-2c.html):

(http://img.photobucket.com/albums/v428/Senorpants1/ba-2C-4.jpg)

I suppose I should be aiming for a frequency response similar to the one in the manual.  My bass response is better, the manual's plot shows it dropping off at about 200Hz, but the high end is much better.

Aha, I was just typing this and can see that you've replied.  I'll try and post a close up of the input transformer tonight, and will also try the mic U pad.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on November 28, 2011, 10:54:40 AM
Note the stock input connection has one side of the primary grounded, and in a real one this causes a large treble boost.  Having a different input transformer, you should expect differently.  You don't want one side grounded, you want the entire primary floating.   I've never measured a BA-2 that looked like that drawing, in regards to the treble boost. 
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on November 28, 2011, 11:14:12 AM
Ah I see.  On mine I have both sides floating, with one side of the secondary tied to ground.  I'll post a detailed picture of the input tx soonish, which should clear some things up.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on November 30, 2011, 06:27:54 PM
Here's a picture of the input transformer wiring which produces the frequency response I posted earlier in the thread:

(http://img.photobucket.com/albums/v428/Senorpants1/BA-2-TX.jpg)

The blue and brown wire comes from the + and - of the input XLR.
The grey shielded cable goes to the grid of V1.
The shield of the shielded cable goes to the shield tap on the input tx which, via the green wire, goes straight to pin 1 of the output XLR, which is connected to the chassis.
The green wire in the middle is connected to signal ground.

Unfortunately I had to send my Fireface interface to Germany yesterday as the rear input jacks have corroded (a known issue apparently), so I won't be able to do any more tests for a bit.  I'll probably borrow an interface from work this weekend to test out the mic pad you suggested earlier.

Can we see a clear shot of the connections you made on the input transformer?  Or tell me what pins are connected to what?  Louder one way versus the other makes me suspicious, but not sure of what.   Do you have gain measurements noted?

Here's a plot that I did before I sent the Fireface back.  It's with the secondary of the input transformer reversed (i.e. opposite to the way it's pictured in the photo above). 

(http://img.photobucket.com/albums/v428/Senorpants1/ba-2txrev.jpg)
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on November 30, 2011, 06:45:05 PM
Looks right from I can see on my phone. 

My only immediate thought is the shielded grid wire.  Shield capacitance and long wire run can kill treble.   I might replace it with unshielded for comparison.  If it's a contributor, weigh interference versus roll-off.  They used to sell low capacitance shielded for crystal mics, which is what you find on old grid caps.  Single solid conductor with a large rubber fill between it and shield. 
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on November 30, 2011, 07:53:25 PM
Thanks Doug. Hmmm, I'm not using very much shielded cable in the amp, maybe about a foot in total, but I'll give swapping it for regular cable a shot.

Do you have an idea as to why the frequency response graph of the amp is so different when the secondary is reveresed?
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on November 30, 2011, 08:39:55 PM
Relationship of capacitance to ground.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on December 01, 2011, 06:40:06 AM
Another question that I've been meaning to ask is that on the schematic pin 1 of the 1620 tubes are connected to ground and are presumably connected to the metal case of the tubes.  My 1620s don't have a pin 1, is this normal?
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on December 01, 2011, 11:32:04 AM
I haven't noticed one that lacked pin 1.   
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on December 01, 2011, 12:45:47 PM
Hmmm, very strange as both my 1620 tubes are missing them -I just assumed that they weren't supposed to have them.  Perhaps it would bring down the noise a bit more if they were screened?  I'll have a chat with the guy who sold them to me.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on December 01, 2011, 12:59:18 PM
might or might not.  Your noise seems typical and normal from what I can see, and from your description.   Ground the outer cans yourself and see. 
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: lassoharp on December 01, 2011, 01:03:33 PM
Pin 6/ NC is the only pin that's missing on all the ones I have.  Seems strange for that particular tube.  I suppose you could run a test wire from the shell to ground and see if it helps but I doubt you'll hear any audible difference.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on December 05, 2011, 03:27:49 PM
I managed to grab some time to perform some more frequency analysis, and this is what I've come up with:

Firstly this is the frequency response with no pad whatsoever, it's just being driven by the 600r line outputs:
(http://img.photobucket.com/albums/v428/Senorpants1/BA-2MR816nopad.jpg)

Now this is looking more like what I was expecting, and more like emrr's plots on his RCA pre amps.

This is the frequency response with the mic pad emrr suggested earlier in the thread - 620r series resistors and a 160r shunt resistor:
(http://img.photobucket.com/albums/v428/Senorpants1/BA-2MR816micpad.jpg)
This one has much better top end than the responses I measured previously.  Curiously though it doesn't have the treble boost in it that the other plot has.  Both tests were taken with the treble boost circuitry in. It looks like the pad is taking out the treble boost for some reason.

So what does this tell me?  I guess that driving the BA-2 with the 600r line outputs is optimal, and with the mic pad in the response is still respectable.  I'm assuming that using a mic pad is the closest to simulating a mic, is that correct?
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: lassoharp on December 05, 2011, 04:17:39 PM
Quote
So what does this tell me?  I guess that driving the BA-2 with the 600r line outputs is optimal, and with the mic pad in the response is still respectable.  I'm assuming that using a mic pad is the closest to simulating a mic, is that correct?


I will second CJ's remark of "If it sounds good - go with it".   I haven't had a chance to use my BA2s on a wide selection of mics - just M-500 ribbon, SM-7 dynamic and a pair of tube mics, all of which use output transformers.  I expect there will be L to L interactions that behave differently than a pure resistive source.  I've been pleased with how all those mics respond.

I also have another tube pre that clearly starts rolling of around 2K - plot looks like your original BA-2, but doesn't sound like it's missing an excessive amount of high end.

I tend to think that with most mics the tube pre with transformer input sound like they're behaving closer to the 1st of the 2 plots you just posted - driven by 600r inputs, and with the rising response around 5K.


Quote
and it did sound a little dark, but when I dialled in some top end with an EQ it sounded great!
 

Has been pretty much standard protocol for me and that's where a Pultec style EQ comes in handy.



During the BA2 build, I sat for a while doing some A/Bing of w/w/out treble boosting circuit.  The source was line level audio going through mic to line pad.  My ear was telling me that the midrange was being altered more than just a 'straight' treble boost.  I can't recall the RDH4 page but I believe this is to be expected with this type of boosting circuit - a slight alteration of the mid response.  If I had to say which way, I would have chosen slightly dipped.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on December 05, 2011, 05:20:08 PM
I will second CJ's remark of "If it sounds good - go with it".
I've just recorded myself playing guitar and singing into it and I have to say I love it now.  It's got a really nice vintage vibe.  It does sound good, and I shall go with it!

The only thing now is the 50Hz hum.  I said it wasn't an issue before, but now I've done a bit of multitracking and also kicking up the volume in my headphones whilst tracking, the hum is definitely there and pretty distracting.  I'm going to try using shielded cable to and from the input tx to see if that will help.   I know that rotating the PT will probably help a hell of a lot, but I'd have to cut new holes in the back, and even then it's not guaranteed to help.

Quote
Quote
and it did sound a little dark, but when I dialled in some top end with an EQ it sounded great!
 

Has been pretty much standard protocol for me and that's where a Pultec style EQ comes in handy.
A p2p Pultec is definitely on my diy list now!
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on December 05, 2011, 05:33:25 PM
How effective is elevating the 6.3VAC to reduce the hum?  On this page: http://www.freewebs.com/valvewizard/heater.html they mention adding a DC reference by using a voltage divider on the B+ to get between 50VDC and 90VDC and then add that to the 6.3VAC CT.

(http://www.freewebs.com/valvewizard/heater5.jpg)
My HT is about 326VDC.  If I make R1 180k and R2 50k that should give me 70VDC.  I'll try connecting that to the CT to see where that gets me.

Does anyone have any experience of this?
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: lassoharp on December 05, 2011, 05:54:25 PM
Even though my 6.3VAC tap on the PT has a centre tap, do you think elevating the heater's voltages would reduce hum?

It's supposed to in theory.  But bigger Q:  Do you have CT connected straight to ground?  If so, try installing two 100r 2W resistors and leave the CT tied off, or better yet, install a 100-200r pot.

Before attempting I would go over the layout and make sure the filament wires aren't too near any sensitive audio wires.  I've personally never had filament hum due to layout issues and have even tried to purposefully induce it by pushing fil wires into close proximity of audio wires.

I'm going in reverse order here - rule out tubes by swapping, etc.  Also make sure that tubes are well seated into their sockets.  I just had a case of this on the recent preamp I built.  Brand new sockets that felt ***** tight, but tubes weren't all the way pushed in.  It was creating hum in output and a simple reseat cured it.

Have you done step wise troubleshooting to isolate what stage hum is coming in?  I'm assuming maybe 1st since you say you have to crank it to hear it.  You can try to check output stage by shorting V2 grid to ground (or just turning vol pot all the way down), then run another small amp of some type on the output and bring vol up and see if it's still there in the same place.

Nice build! - BTW
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on December 05, 2011, 06:04:59 PM
Looks like I edited my post at the same time as you were posting yours!

Thanks for the compliments on the build - I'll get some proper pictures up once I've finished the front panel.

Quote
It's supposed to in theory.  But bigger Q:  Do you have CT connected straight to ground?  If so, try installing two 100r 2W resistors and leave the CT tied off, or better yet, install a 100-200r pot.
Do you mean connect the two 100r resistors between the 6.3VAC and ground?

Quote
Before attempting I would go over the layout and make sure the filament wires aren't too near any sensitive audio wires.  I've personally never had filament hum due to layout issues and have even tried to purposefully induce it by pushing fil wires into close proximity of audio wires.
There is one instance (from V2 to the output TX) where the signal wire is fairly close to the heaters, but I used shielded cable at that point, to be on the safe side.

Quote
I'm going in reverse order here - rule out tubes by swapping, etc.  Also make sure that tubes are well seated into their sockets.  I just had a case of this on the recent preamp I built.  Brand new sockets that felt ***** tight, but tubes weren't all the way pushed in.  It was creating hum in output and a simple reseat cured it.
I'll try reseating the tubes, but in the back of my mind I'm thinking about the missing shield pin on both my 1620s!

Quote
Have you done step wise troubleshooting to isolate what stage hum is coming in?  I'm assuming maybe 1st since you say you have to crank it to hear it.  You can try to check output stage by shorting V2 grid to ground (or just turning vol pot all the way down), then run another small amp of some type on the output and bring vol up and see if it's still there in the same place.
So my plan will be to have the volume pot right down and then connect the output to one of the pre amps in my interface to and crank it up.  If there is hum this means it's coming from the 2nd stage, right?
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: lassoharp on December 05, 2011, 06:38:20 PM
Quote
Do you mean connect the two 100r resistors between the 6.3VAC and ground?

yes


Quote
There is one instance (from V2 to the output TX) where the signal wire is fairly close to the heaters, but I used shielded cable at that point, to be on the safe side.

The quick test there is to use a chopstick and push the wires away from each other and  see if it changes hum level


Quote
I'll try reseating the tubes, but in the back of my mind I'm thinking about the missing shield pin on both my 1620s!

I forgot about that.  I suppose it's a possibility.


Quote
So my plan will be to have the volume pot right down and then connect the output to one of the pre amps in my interface to and crank it up.  If there is hum this means it's coming from the 2nd stage, right?

With this method it will be showing any residual hum in 2nd stage - there could still be hum originating upstream too.  You'd be looking for any significant amounts here. If you have virtually none down to a low low level -80db?  Then likely it's all originating from somewhere in first stage.  If equally distributed in 2nd stage then maybe pointing to heater or PT related hum.

Do you have a  rough db figure for the overall hum level when you have it cranked? 
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on December 06, 2011, 07:21:32 AM
Quote
With this method it will be showing any residual hum in 2nd stage - there could still be hum originating upstream too.  You'd be looking for any significant amounts here. If you have virtually none down to a low low level -80db?  Then likely it's all originating from somewhere in first stage.  If equally distributed in 2nd stage then maybe pointing to heater or PT related hum.
With the volume pot completely off, there is no noise whatsoever when I crank the inputs of my MR816 right up until the gain on the interface is up fully.  Then there is a hiss but to me that sounds like the noise floor and nothing like the hum I'm hearing when the volume pot on the BA-2 is up.

Quote
Do you have a  rough db figure for the overall hum level when you have it cranked?
With the volume on the BA-2 fully cranked, looking at the meters in my DAW, I'm getting around -3dB.

When I took the frequency response measurements I didn't have the amp cranked so as not to overload the convertors on the interface.  Thinking about it now, the noise figures aren't an accurate figure on the frequency response if I don't have it cranked.  D'oh!

I'm going to try and head to the shop today to get some resistors to try the artificial CT and maybe elevating the heaters.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: DaveP on December 06, 2011, 10:47:05 AM
Hi Letterbeacon,

Just noticed your post about no Pin 1 on the 1620.  I have almost 100 RCA metal tubes of all types and every single one is earthed through pin 1.  Look at the original RCA spec on Frank's site, it clearly shows pin 1 earthed.

To have an unearthed chunk of steel covering the most sensitive part of your circuit is very bad news.  I think you have been sold some tubes that someone has messed about with.

A 1620 is just a low microphonic version of a 6J7.

I buy all my tubes from http://www.vacuumtubesinc.com/ in florida and if you keep the total below $18 you dont pay any customs duty or VAT.

best
DaveP
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: lassoharp on December 06, 2011, 12:37:26 PM
Quote
With the volume on the BA-2 fully cranked, looking at the meters in my DAW, I'm getting around -3dB

If I'm interpreting that figure correctly that sounds crazy loud for hum?


Q: Is the input terminated when hum is being checked?   If not, clip on a 150 or 250r resistor to the pri or plug a microphone into it (cover capsule with cloth to keep out room noise) and see if that makes a difference.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on December 07, 2011, 06:10:13 AM
To have an unearthed chunk of steel covering the most sensitive part of your circuit is very bad news.  I think you have been sold some tubes that someone has messed about with.

I think you're probably right.  I ordered some more in preparation for my BA-6A build, so they should be arriving soonish.

Quote
With the volume on the BA-2 fully cranked, looking at the meters in my DAW, I'm getting around -3dB

If I'm interpreting that figure correctly that sounds crazy loud for hum?


Q: Is the input terminated when hum is being checked?   If not, clip on a 150 or 250r resistor to the pri or plug a microphone into it (cover capsule with cloth to keep out room noise) and see if that makes a difference.

Yes, that's -3dBFS when fully cranked.  The reason I didn't notice it being so high before is because I never fully cranked it before.

No that wasn't with the input terminated, I'll have a go at plugging a mic in when I get some time tonight.

As we have established the hum is coming from the first stage, does this mean that the hum is unlikely to be coming from the PT?  The input stage is on the far side of the case to the PT, with the second stage 1620 and output transformer in the middle.  If the hum was coming from the PT then surely hum would be present in the second stage and output tx as that is physically closer to the PT.   Is that correct?

Another thing I want to try is to use shielded cable from the input XLR to the input TX.  At the moment I'm using tightly twisted wires.

I'll also try and get a diagram of my grounding system up tonight, to see if you guys can spot any school boy errors.  Basically the signal ground and power ground are starred at the same point on the chassis.  Both XLR pin 1s are connected to the chassis and all the shields (the shielded cables, the shield tap on the input tx etc.) are connected to pin 1 of the output XLR.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on December 08, 2011, 06:05:12 PM
I had some time to work on the BA-2 tonight - I've got the new tubes in, I've put shielded cable between the input XLR and the input transformer and I've put 100r resistors in to create an artificial centre tap for the heaters.

I've now got the hum down to about -27dBFS when fully cranked with a microphone plugged in (with many cloths covering the mic).  So I'm pleased with that, however I'm sure I can get it lower.  What sort of figure should I be aiming for?

I was thinking of trying to elevate the heaters by about 50VDC by using a potential divider across a feed from the B+.  It's the first time I've ever done this, is there anything to watch out for when doing this?
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on December 08, 2011, 06:10:30 PM
I am missing something about your hum measurement.   It appears very good compared to system noise in your plots, unless I'm seeing them incorrectly.  There it appears nowhere near the level you are mentioning, which would be unusable.   What am I missing? 

You shouldn't use a mic at all when doing this test, just a 150-250 ohm resistor. 

I didn't think elevated heaters addressed hum, only other noises.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on December 08, 2011, 06:23:43 PM
Apologies, I'm not being very clear.

When I made the plots I didn't have the gain of the amp fully cranked.  I only had it turned up enough so it wouldn't overload the inputs to my interface.  Therefore the noise plots are unreliable because they only show the level of the noise when the gain knob is about a quarter of the way up.

When I later did some real world tests recording an acoustic guitar with a ribbon mic, I found the hum to be very loud especially when multitracking.

lassoharp asked me:
"Do you have a  rough db figure for the overall hum level when you have it cranked?"

So I tried it the other day and got -3dB.  Tonight after using new tubes, shielded cable between the input XLR and input TX and with the artificial centre tap I got -27dB.

I'm slightly loath to rotate the PT as I've (stupidly) fastened it so it's a pain to move.  I will try that as a last resort though.  The thing that makes me think that it isn't the PT is that the second stage, which is closest to the PT, has no hum in it when the gain pot is turned right down (off).

You shouldn't use a mic at all when doing this test, just a 150-250 ohm resistor. 
Gotcha, I'll give that a go tomorrow -work has a habit of getting in the way of my DIYing!
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on December 08, 2011, 06:46:08 PM
Gotcha. 

The hum is still coming from the power transformer, one way or the other.   If the volume is down or off, the input transformer (most likely to pick up hum of anything) is not part of the picture.   Nor is the gain of the first tube there to amplify anything of low level.  The 2nd tube and output transformer are themselves alone likely not accountable for any gain at all, more like unity gain.  Orientation of power transformer and input transformer relative to one another are key.  Gates used that transformer on a 75 dB preamp with built in PSU, and hum was acceptable in it (I heard several of them), so it can be done.   
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: lassoharp on December 08, 2011, 07:03:28 PM
Quote
So I tried it the other day and got -3dB.  Tonight after using new tubes, shielded cable between the input XLR and input TX and with the artificial centre tap I got -27dB.

That's quite a change.  So, that's 3 new things.  First thing, differentiate some of what's doing what.  Put in old tubes with new CT & shielding and note change. 

  To further sort out possible filament balance issues a balance pot will be most helpful.  I've only really had to use a hum pot once for a final build and I'm inclined to feel that the 2 100r resistors does adequate job in most cases.  If it's not too much trouble I would install a balance pot and see if it helps get the figure lower.  I've found that on PTs where a simple CT or 100r to ground does the job, that if a hum pot is used you will notice nearly no noticeable appearance of hum until pot is rotated to the extremes.  On the one circuit that was troublesome the hum level changed perceptibly and constant through the range and the null was within a smaller range.

If rotating iron I would try to rotate input instead of PT if possible though it looks to be fairly tight for both.  Maybe a perfect 90 deg rot will do the trick.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: DaveP on December 09, 2011, 05:31:20 AM
Hi letterbeacon,

This is a straightforward way to measure hum.

Put a standard 200 ohm metal film resistor across the input terminals (inside an XLR plug if possible).

Put a DMM across the output load (620 ohms?) then turn it up fully and measure the level.

A V77 amp is about 4mv so that is the kind of amount to expect.

Next check the gain of the amp with a 1kHz signal.  Now divide the hum level by the gain to get the E.I.N. (equivalent input noise).  Say the gain is 60dB and your hum is 7.7mv, then your EIN will be 7.7uV, this is 100,000 times less than 0dB which is -100dB.  The V series amps are -110 to -120dB for comparison.
If its a cheap DMM then it won't measure high frequencies very well but it will sure measure 50~120Hz as mains frequencies were what they were designed for.

best
DaveP
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on December 09, 2011, 07:20:33 PM
Thanks all three of you for your replies -it's fascinating to me this whole troubleshooting process.

Orientation of power transformer and input transformer relative to one another are key.

If rotating iron I would try to rotate input instead of PT if possible though it looks to be fairly tight for both.  Maybe a perfect 90 deg rot will do the trick.
I think I'm going to have to bite the bullet and try rotating the PT at some stage.  Unfortunately the input TX can't be rotated as it's too wide to stand vertical.

That's quite a change.  So, that's 3 new things.  First thing, differentiate some of what's doing what.  Put in old tubes with new CT & shielding and note change.
Good point, I should have done it in stages and noted the stages really.

Next check the gain of the amp with a 1kHz signal.  Now divide the hum level by the gain to get the E.I.N. (equivalent input noise).  Say the gain is 60dB and your hum is 7.7mv, then your EIN will be 7.7uV, this is 100,000 times less than 0dB which is -100dB.  The V series amps are -110 to -120dB for comparison.
If its a cheap DMM then it won't measure high frequencies very well but it will sure measure 50~120Hz as mains frequencies were what they were designed for.
I don't quite understand the maths behind this I'm afraid.  You say divide the gain by the level of the hum.  So 60 / 7.7.  You get 7.7uv, but I get 0.128mv.  I'm obviously doing something wrong here, but I can't figure out!  Do you mean divide by 7.7mv expressed in dB?

Unfortunately I've got family commitments this weekend so I'm going to struggle to find time to do what you guys recommend, but I'll report back as soon as I do!

Thank you!
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: DaveP on December 11, 2011, 06:57:30 AM
Letterbeacon,

You simply mis-read my post,  for arguements sake say the gain is 60dB or 1000 times.

Next measure the hum/noise on full gain, say its 7.7mv.  You simply divide this by the gain 1000 to get 7.7uV,which was what it was before it was amplified.

I used 7.7 because the reference point 0dB is 0.77 V (actually 0.77459V) so it just makes the maths easier.

Obviously your figures will be different from this and at some stage you will have to convert a number to a log and multiply by 20 to get dB.

best
DaveP
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on December 11, 2011, 04:39:16 PM
Thanks Dave, I understand that now.

Unfortunately I don't think my DMM goes low enough to measure mV accurately.  I tried the following but with not much success, I'll write out what I did in case I'm doing anything obviously wrong.

First I measured the gain of the pre amp by outputting a 1kHz sine from my DAW @ -80dBFS.  I ran it through the BA-2, with the gain up full, and back into my DAW.  The incoming sine measured -3dBFS meaning my BA-2 has a gain of 77dB.

Then I soldered a 200 ohm resistor across the + and - terminals of the input XLR and a 620 ohm resistor across the secondary of the output transformer (I couldn't easily reach the output XLR, but I'm pretty sure this does the same thing).  I then switched my DMM to it's 2VAC setting (the lowest it goes) and measured across the output tx secondary.  My DMM has three decimal places so it can in theory show mV, but I was getting 0.000VAC.

It's funny because when I had my leads not connected to anything, the DMM would read random numbers as i it was reading something in thin air (maybe the AC fields) but as soon as I connected the leads to the output tx secondary it would start to read 0.000VAC.

Have I done something glaringly obviously wrong here?  If not, it might be that I need a better DMM with a higher resolution.  I'm working at the BBC for a couple of days this week, I might see if I can use some of their equipment (if there's any left these days!).

Quote
So I tried it the other day and got -3dB.  Tonight after using new tubes, shielded cable between the input XLR and input TX and with the artificial centre tap I got -27dB.

That's quite a change.  So, that's 3 new things.  First thing, differentiate some of what's doing what.  Put in old tubes with new CT & shielding and note change. 
Now I'm using a 200 ohm resistor across the input I'm getting a reading of -33dBFS, I guess the 6dB difference must have been the room noise the mic was picking up.

I tried swapping out the tubes, the new CT and the shielding between the input XLR and the input TX and the tubes and the artificial CT don't do anything noticeable.  It is the shielding between the XLR and the input TX that is reducing the hum considerably.

I noticed that the wire coming from the first 1620 plate runs fairly close to the AC heaters wire.  I used a pencil to move the wire away from the heaters while the unit was on but I didn't notice anything change hum wise.  I'm thinking I might shield the wire anyway though, just in case.  It's only a about 3" long so hopefully capacitance won't be an issue.

Thanks again for sticking with me on this - it's been hugely educational so far.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: DaveP on December 11, 2011, 05:11:04 PM
Hi Letterbeacon,

It wasn't a good day to go visiting was it!  What with practically blind drivers leading vast convoys of cars at 29mph and wind and rain to boot eurgh!

Anyway, its good news that you can't see any AC on your DMM, you would be in trouble if you could!

best
DaveP
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on December 11, 2011, 05:26:43 PM
Hi Letterbeacon,

It wasn't a good day to go visiting was it!  What with practically blind drivers leading vast convoys of cars at 29mph and wind and rain to boot eurgh!
Ha!  No, it wasn't too pleasant!  More bad weather on the way down from Scotland apparently too!

Quote
Anyway, its good news that you can't see any AC on your DMM, you would be in trouble if you could!

best
DaveP

I think I may have misunderstood - wouldn't the hum be an AC (audio) signal? I thought I needed to measure the AC (in mV) present and then convert it to dB to figure out how loud the hum is in my pre amp.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: DaveP on December 11, 2011, 06:04:55 PM
Letterbeacon,

No I mean that its good news that the hum is so low it doesn't show on your DMM!

best
DaveP
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on December 11, 2011, 06:14:00 PM
Aha!  Sorry a bit slow here tonight!
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on December 11, 2011, 07:01:39 PM
so.....
.....why does your build have 27 dB more gain than it should?   Am I reading that correctly?
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on December 11, 2011, 07:23:13 PM
I did think that was slightly odd. I have a higher B+ than the one specified, if that makes a difference.

Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on December 11, 2011, 07:27:42 PM
It shouldn't.  You have measured as much more gain as an additional tube might give.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on December 12, 2011, 10:30:05 AM
I must have measured wrong or I'm making a mistake somewhere along the line in the set up.  I'll double check tonight whether the interface added any gain at some stage.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on December 27, 2011, 02:11:10 PM
so.....
.....why does your build have 27 dB more gain than it should?   Am I reading that correctly?

I tried the sine wave test again without the BA-2 in the chain - plugging straight from the out of the MR816 to the in of the MR816 and it looks like the unit adds 19.5dB of gain somewhere along the way.  As it's a loan unit until my RME gets fixed I'm not very familiar with it and can't figure out where this extra gain is coming from.

So it looks like my amp has a gain of 57dB.  Does the extra 7dB sound reasonable?

Seeing as my MR816 has a gain of 19.5dB, then my hum figure of -33dbFS is more like -52.5dBFS, right?  That's with a 200r resistor across the input.

I'm going to attempt to rotate the PT later to see if I can lower the hum anymore.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on December 27, 2011, 02:54:04 PM
I bet the MR816 has programmable input gain that's set higher.   7 dB sounds like reasonable variance with transformer differences and tube gain differences. 
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on December 27, 2011, 04:55:02 PM
I think I may have discovered where the hum is coming from and it's NOT the power transformer.

I took the amp out of the rack and laid it on the floor so I could reach behind it to rotate the PT at the back.  Even before I had turned the PT I noticed that the hum level was reading -45dBFS (down from -33dBFS).  When I turned the transformer, this figure stayed the same, nothing changed.  When I put it back in the rack, the hum level went back up to -33dbFS.

If the hum was coming from the PT then turning it might have made some difference, right?

This is my home studio and at the bottom of the 10U rack is my PC.  I think the hum is coming from the PC.  I set the BA-2 up at the other end of the room to the PC (about six feet) and found the hum had gone down to -52dBFS.

With the BA-2 still at the other end of the room, I took the 200r resistor out and plugged in my mic turned the gain up to about 2 o clock, and recorded myself playing a bit of acoustic guitar.  The hum was still there, but much less noticeable, you could only really hear it when I wasn't playing.  I A/B'd that guitar recording with one I had recorded when the amp was in the rack, and difference in hum is amazing.

Does my PC causing the hum sound plausible to you?  I suppose the switch mode power supply must be having an effect on my input transformer.

So the hum reading is -52dBFS, take way 19dB which the MR816 adds so that means a hum of about -71dBFS. That sounds about right, right?
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: lassoharp on December 27, 2011, 06:26:46 PM
Quote
that means a hum of about -71dBFS. That sounds about right, right?

Yes.


I know that both of my DIY Pultecs picked up hum from adjacent other tube gear's PTs.  I felt it that in those cases it was more the passive circuitry via rails and case that was transmitting it to the makeup amps than the input transformer alone which was a vintage shielded type.  Either way, moving out of the rack was the solution.  Glad to hear the problem is solved.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on December 27, 2011, 06:35:00 PM
Thank you very much to both of you with all your help on this project.  It's been incredibly educational and hugely enjoyable.  I'm going to put a JLM go between in and a nice engraved front panel and then it's all done.  I'll get some proper pics up when I'm completely finished.

lassoharp - when you're recording with your BA-2-esque pre amps, can you hear a hum faintly in the pauses between vocals etc.?
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: lassoharp on December 27, 2011, 07:10:32 PM
I can't hear hum on either the BA2 clones or the 3 stage RCA-ish pres.  I don't have an official figure yet on where exactly the hum resides db wise but a rough figure would be somewhere below -80db, or below the hiss level.  My crude method for checking hum involves running the preamp wide open into a power amp that has ~ 60db of gain and pushing my ear to the loudspeaker.  If I can't detect it in that set up I know it won't be audible on recordings.  I hear only faint hiss on BA2 when the preamps are cranked wide open while monitoring on the DAW.

The DIY tube pres (2 at current) I have that do have small amounts of low level hum seem to be caused by PT field pickup to some degree.  On one I had to rotate the PT to reduce the hum and it seemed to be going directly into the output circuit immediately at power on (It was a 3 stage amp and I had the 2nd stage grids shorted to ground).  That being said, if you still find the hum level objectionable, if possible,  you could try rotating the PT and see if it changes anything.  On mine the PT was perfectly square mounted with bell towards the other iron as yours is but it wound up being just slightly off this at an odd angle that gave the best hum reduction for some reason. 
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on January 01, 2012, 12:13:51 PM
That being said, if you still find the hum level objectionable, if possible,  you could try rotating the PT and see if it changes anything.

Unfortunately I've tried this to no noticeable affect.  It doesn't bother me though as I have to have the gain cranked up pretty high to notice the hum.

I installed the JLM go-between the other day and now have phantom power.  Using a condenser mic, I don't need to crank the gain up much and you definitely can't hear any hum.  Very pleased with that!

I did some measuring the other day and I noticed my B+ is about 20v lower than what it should be, due to me not having exactly the right dropping resistors on hand.  Is it worth me trying to get it up to what it should be?  It works as it is, but will it make a difference if I get it up to what it should be according to the voltage chart?

There's one last thing I want to do and then this project is complete and that's installing an output attenuator so I can crank the tubes into distortion.  I have a 600r t attenuator that I bought from Nielsk and I'm wondering how to hook it up.  I looked at the 1176 schematic to see how that attenuator is hooked up on that unit and I've come up with the attached diagram.  How does that look?

Thank you!
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: lassoharp on January 01, 2012, 12:35:53 PM
It's still bothersome that you're having residual hum.  This may create issues if you decide to chain in the BA6 downstream as it will amplify the hum level. 


Quote
Unfortunately I've tried this to no noticeable affect

So you're saying nothing happens - no increase or decrease?  Apparent immunity?  That would seem to suggest the problem lies elsewhere.

One other relatively easy thing to try, just in case.  Clip in (Parallel) equivalent size filter caps in the PS.  This is kind of covering two things:  1) the odd possibility that one of the new caps isn't up to snuff, and 2) the possibility that the circuit wants to see more filtering.  I realize the theoretical numbers for the stock values are plenty for the amount of current this amp draws but it's easy enough to clip those extra caps in to see.  I did have one SE preamp that seemed to want above and beyond what typical filtering should have provided and I wound up having to use a choke input filter in the PS to get things right. 
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on January 01, 2012, 01:53:31 PM
Quote
One other relatively easy thing to try, just in case.  Clip in (Parallel) equivalent size filter caps in the PS.  This is kind of covering two things:  1) the odd possibility that one of the new caps isn't up to snuff, and 2) the possibility that the circuit wants to see more filtering.  I realize the theoretical numbers for the stock values are plenty for the amount of current this amp draws but it's easy enough to clip those extra caps in to see.  I did have one SE preamp that seemed to want above and beyond what typical filtering should have provided and I wound up having to use a choke input filter in the PS to get things right.

That's a good idea.  I'll have to order some more in so won't be able to try it right away, but I'm willing to give it a go.

I might try and draw my grounding scheme out and post that in case I've done something wrong there.  I'm pretty confident (it's basically the Jensen diagram posted earlier in the thread), but it'll be good to see what you guys make if it.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: Bren-hur on February 06, 2012, 04:18:07 PM
Hi, this seems to be the go-to Ba2 thread so I thought I'd post a couple of Q's... I have this devilsh gremlin crackle that makes the unit unusable, it used to work fine but now it crackles... kind of like a bad phone line when it rains... it's totally random, sometimes there sometimes not, which is a total pain in the ass to troubleshoot... The gain pot has no effect on the crackle, it's there no matter what, so to try and lock down the location of the crackle, i ran a signal through the input and traced backwards from the output. The crackle disappears when i get past the output trans... (clipped a coupling cap to the output of V2 to test the tube). This pretty much leaves the only culprit to be the output transformer... or the power transformer maybe...

I have a UTC A25 as the output, an ebay purchase, it tested out ok, but i dunno... Is there any off the shelf brand new output transformers that will work in the BA2 circuit? I want to try a 100% known good in there... I was looking at the cinemag outputs...

http://cinemag.biz/output/output.html

It looks like the CMO-10/600-150 or the CM9589 could be possibilities...

Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on February 06, 2012, 04:35:18 PM
Could be current draw in output causing crackle.  Put a larger resistance between v2 plate and transformer to see if reduction of current flow decreases crackle.  Might reveal something.  If it goes away with an extra 10k-20k then it's probable the output is going. Yes, B+ to plate will fall also, but not enough to invalidate test.   Output might still be fine in a parallel feed circuit, and no longer a dc coupled one.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: Bren-hur on February 06, 2012, 07:36:33 PM
Hi, Doug! ... i went to try what you said and of course the crackle decided to disappear, before i tried to add the resistor... guess I'll wait till it starts doing it again...

any comments on those cinemags? or is there any other new transformers you know would work?
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on February 06, 2012, 07:47:30 PM
 CMO-30/600 with allowance for DC is the closest Cinemag. 
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: Bren-hur on February 07, 2012, 11:30:01 PM
Well... the problem is solved... but not in a good way... :(

Turned it on this morning and it was buggered.. total distorted crackle... checked the plate voltage and V2 had 0V... voltage is getting to the A-25 and not passing through. I Pulled the tranny and there is no DCR on the primaries.. waaaahH!!! pretty over it... paid 100 bucks for the tranny on rapEbay and barely saw a few hours use... ggrrrrr. It's funny though, coz the other night i was recording and it totally died, like the same thing i heard today, but the next day it was back online, with the crackle i mentioned earlier, anyway looks like i need a new tranny...
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: lassoharp on February 07, 2012, 11:46:54 PM
Sorry to hear the bad news.

I suppose at this point it wouldn't hurt to try and open the bottom plate and see if the open has occurred right at the pin connections.  Is there any potting compound showing around the base of the pri pins?

It may have had an excessive amount of current ran through it over a long period of time by previous owner.    Mistakenly running the plate lead to ground could burn one up in a hurry too.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: Bren-hur on February 07, 2012, 11:51:46 PM
I was thinking bout taking it apart... do you know how to get the bottom plate off? I don't want to wreck it trying to dismantle it if it's actually fixable.... don't see any potting compound anywhere on the bottom...

thanks!
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: lassoharp on February 08, 2012, 12:06:43 AM
Doug and CJ have taken UTCs apart.  Hopefully they will chime in shortly with some good advice for you.  Good luck.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: Bren-hur on February 08, 2012, 01:51:37 AM
Well I got the A25 dismantled, partly anyway... i very carefully and slowly drilled out the holes where the screws go in the bottom, on pressing the base plate with a screwdriver they were the only parts on the bottom that didn't move. Once i drilled about 1/8th of an inch in on each hole, the bottom popped right off! I desoldered the primary wires and checked them... unfortunately, it seems the transformer is well and truly dead....

dang it...
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: CJ on February 08, 2012, 02:12:30 AM
possibly a busted wire at the splice of the cloth leads/pri enamel wire.

i think the sec. is wound first,

so the pri will be on the outside of the coil,

this means you can get to the splices,

if the sec splice was bad, then you have to unwrap the whole pri to get to it and then rewind the pri, not fun..

so before you put it in the scrap heap or salvage it for lams for a diy transformer,
heat it up and gently unwrap it like an egyptian mummy,

pretend you are doing cosmetic surgery on madonna,

you will see a piece of cloth tape with a rivet in it for splicing small wires to big wires, sometimes too much tension snaps the small wire at this junction.



Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: Bren-hur on February 08, 2012, 03:43:39 AM
wooaahhh!!! holy fckn sh*t... I feel like a super ninja!!  8)

Did as you said, cooked it for a bit, pulled out the coils (forgot to disconnect the ground so had a wee bit of a hearattack moment). I cleaned off some goo, carefully peeled back the tape till I got to the wire joins, and sure enough, one of them had come off. I thort I might've torn it opening it, but the other one was real secure... had another heart attack moment when i pulled a bit too hard and snapped off the wire that was still attached... peeled back some more tape to find it again, gently scraped both of the teeny wires to get rid of the coating and soldered on two new big wires... connected the multimeter... 1k! wot's upppppppp! it's back. A bit lower than spec DCR... i got 1K on primarys and 80ohm on secondaries, but sh*t damn! I can't believe it...

Only problem now is... i can't remember what wire went where on the bottom tabs, i didn't pay attention coz i was so sure the tranny was dead... help!!

There was a red wire on the topside and a black wire on the bottom, any clue which goes to P and which goes to B?

Thanks for the help...
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: lassoharp on February 08, 2012, 11:17:11 AM
Red is often a standard color code for the B+ wire but no idea if that translates 100% to UTC innards.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: Bren-hur on February 08, 2012, 08:22:32 PM
Well, I put the a-25 back in my BA2 and it's cranking! no more crackles ( so far ! ) and it sounds a darn sight louder than it did before... I have a bit of a bad hum going on, from the power tranny, it's a temp one I had in the cupboard and is unshielded... the hum changes volume as i move the mic around like a hum sensor. Got a nice new transformer on the way and I'm going to rework my component positioning as was talked about earlier in this thread...

On a side note, what would be considered excess current for an A25? Like I said... the power tranny I used wasn't a pretty one, an old unsheilded RCA 270 0 270... I wanted to see if it was ME running excess current through it... :-[

Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: lassoharp on February 08, 2012, 09:49:52 PM
Quote
On a side note, what would be considered excess current for an A25? Like I said... the power tranny I used wasn't a pretty one, an old unsheilded RCA 270 0 270... I wanted to see if it was ME running excess current through it...


It's rated for the quoted response at 8ma unbal DC, which is fairly high.  I don't know that the question of how is too much and for how long has been reliably answered other than to say I doubt, say 10ma of normal use time probably wouldn't hurt it a bit.  I've ran dead short - plate to ground with full B+, long enough (15 to 20 sec?) to heat the transformer to blistering hot and it's still kicking.  I = about 235V/whatever the DCR on the primary is = oh sh*t!

Your PT is higher HV than the stock so will draw slightly more.  Easiest to just measure current from cathode voltage.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: Bren-hur on February 15, 2012, 03:43:21 PM
Well I've been waiting to ask some questions till I worked some stuff out myself... I got my BA2 Clone up and running and VERY quiet as per suggestions in this thread, rotating Power tranny and position of input to get zero hum... that truly was an awakening experience... I then got RMAA and started doing sweeps and noise tests... Now I kept getting really bad results, a bass rolloff starting at 300hz, I kept trying stuff to remedy, increase coupling cap, got the B+ voltage SPOT ON, got minor improvements but still not good enough... anyway, i read through this whole thread again and twigged onto what doug was saying about running tests with load resistors across the input pins when running these tests... so I tried that, 600 ohm wired into the ouput XLR and 150ohm wired into the input XLR.. and BAM... perfect results... ultra flat response beyond 10K and the noise floor dropped down to -100... with some niggly 60hz peaks at about -80, though I think these might be in my soundcard/cable setup, because I could still see the peaks, albeit smaller, when testing the transformers outside the case with teh unit unplugged... on listening tests in cubase, gain all the way up, headphones all the way up.. .it's really quiet, and at normal recording levels... silent.

ANYWAY... to get to the point... these load resistors got me thinking.... when recording, the input is loaded as in the tests, but if I'm going into my cheesy MAUDIO, the output is not loaded like in the test... now when I do proper recording, I use my friends RME UFX... which I imagine is better... but I start to wonder about the output loading and response...

I'm going to run some more tests now... take off the output loading resistor and see what I get... just thought I'd post my musings... and thank the fellas on this thread for the hum-killing advice... total gold.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: lassoharp on February 15, 2012, 04:19:44 PM
Quote
when recording, the input is loaded as in the tests, 

Not exactly, as the mic is now the load or termination, so it's not purely resistive.  This assuming you are going directly in to the primary and not using a pad

 
Quote
the output is not loaded like in the test.

It should essentially be if you terminate the OT with a resistor.  I built switches for this in mine.  Sometimes you may like the differing response without the termination.  Sometimes it can sound horrible.  My BA2s don't seem to stray too far without termination going into Hi Z loads.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: Bren-hur on February 15, 2012, 04:39:52 PM
What kind of value do you have on the switch? I get quite a bit of gain loss with 600ohm across the terminals...

Also, I'm trying to wrap my head around RMMA and the frequency graphs... I'm not sure if I'm reading things wrong, but looking at my graph in log view (no output termination)  I have a slope starting at 300hz, sloping from +0db to -10db @ 40hZ.... pretty extreme... but then it says on the numbers readout, the detail summary, that from 40Hz - 15Khz is +1db to -4db deviation... which is totally fine...
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: lassoharp on February 15, 2012, 05:29:32 PM
I think I used 620r on mine.  600-1K or so is a decent range.  As your tests seem to have shown, removing the termination has an effect the response, as it should in theory as loading on the output tube is changed.

It seems like a lot of people react unfavorably to the loss of gain from termination.  On most circuits I've listened to the response has been more balanced and less distorted with the resistor in place.  I've heard some say they like the sound of an unterminated output.  It's hard to say if they were responding to the difference in apparent gain/vol or other.

The difference may not show up until numerous tracks have been cut and the more subtle differences begin to stand out, whatever they may be. 

Your numbers sound about right for an A-25.  I think Doug may still have sweeps of mine on file to compare.  What inputs are on yours?
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: Bren-hur on February 15, 2012, 05:51:11 PM
I have a Cinemag 3440 as the input... I think that too has a bit to do with with the low end. I tested all the components out of the unit, the A25 was pretty much dead flat across the board, the cinemag on the other hand does have a drop in the bottom end, can't remember where, but I twigged that's where the loss may be occuring...

On the termination, I think I'll do as you say.. put it on a switch... terminated it did sound a lot smoother, unterminated had a much more open sound. On some instruments/mics I can see both working...

I actually have the tilt EQ setup in this unit too, on a hardbypass switch, I've been doing all my testing with it off... then I switched it in... and had a laugh... it has such an extreme affect on the sound... but sounds REALLY good... if you haven't tried it yet, I highly reccomend it, such a cool little EQ! 
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: lassoharp on February 15, 2012, 06:02:56 PM
Quote
I actually have the tilt EQ setup in this unit too

Do you mean the stock // resistor-cap network? or something else? 


Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: Bren-hur on February 15, 2012, 07:08:14 PM
No... its from the MI-12241 preamp, pretty much identical to the BA2, except the tilt EQ section replaces the BA2 treble boost...

Doug kindly drew it out...

http://www.groupdiy.com/index.php?topic=16210.msg215423#msg215423
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on February 15, 2012, 08:15:04 PM
That Cinemag needs correct loading to sound right.  I recall they recommend something like 360K on the sec?  Whatever is in all the Quad Eight schematics.    It all sounds hosed if you run it wired 600 and feed it a 150 mic.  Really weird.  A real BA-2 doesn't care too much whether loaded or unloaded on output, and gets a treble boost with a typical mic pad on the front end. 
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: lassoharp on February 15, 2012, 08:15:47 PM
Oh, that one.  That is a fun one.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: Bren-hur on February 15, 2012, 08:47:49 PM
It all sounds hosed if you run it wired 600 and feed it a 150 mic.  Really weird. 

That's exactly what I'm doing ... and why I keep on damn tinkering with it not quite satisified with the sound! I looked at the Cinemag schem's, and they say to put a 47K across the secondaries to match a phono preamp... I'll try that a bit later and see what I get....
 
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on February 15, 2012, 09:39:47 PM
set primaries for 150.   Load secondary at 360K.   If you load 47K you are making the primary reflect true 150, which you don't want. 
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: Bren-hur on February 16, 2012, 01:08:43 AM
Hey Doug, thanks for the info! I tried what you said... at least what I think you said (check my attached drawing). The frequency response remain unchanged, but I got a lot more gain, the larger step up from 150 instead of 600 i take it... problem is, I think it's too much, I'm getting a glassy, feedbacky sound in the top end, testing vocals... do I have it connected up right?

Interesting note.. it's a bit noiser, because of the gain i spose, but its far less susceptibale to power transformer hum pickup... anyway, pretty underwhelmed by the cinemag input at this point...  :-\
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on February 16, 2012, 10:00:52 AM
Gain should only be about 4 dB hotter wired for 150.  The Cinemag is a fine sounding transformer when set properly. 
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: Bren-hur on February 17, 2012, 11:46:41 PM
So on some more fiddling around, I got the noise down to a normal level, just input transformer positioning and making sure the case was touching the chassis and grounding. I was curious...  on the reflecting back of the 360K resistor... I'm curious on the theory behind it, what is the impedance seen by the mic? and how does the math work?

On another note... I was theorizing that back in the day, the BA2 was likely going out to another unit with an input transformer, and it was getting loaded up that way... I say this because my edcor's arrived today and just for the hell of it, I went out of the BA2 into the 600 side of the edcor, then out the 15k and ran it through RMAA... the response was exactly where it should be, pretty much dead flat from 40hz... like when I load up with a 600ohm resistor, but without as much gain loss... and on listening tests in cubase... FINALLY the Ba2 sounded solid, i A/B'd with and without the transformer load up... and without the transformer/load resistor going straight into the soundcard, it sounds real thin and honky... ie: not right....

Is there any value in having a 600 ohm : 600ohm transformer to load up the BA2? or is a resistor just as good... i like the *idea* of having a transformer at the back end... because i like transformers..
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: lassoharp on February 18, 2012, 01:20:37 AM
Quote
I went out of the BA2 into the 600 side of the edcor, then out the 15k and ran it through RMAA... 


Quote
but without as much gain loss.

If I'm reading you right then you should be having no gain loss - just the opposite - 600:15K is about + 14db of gain . . .


Quote
how does the math work?


turn ratio = sq root of impedance ratio


Quote
and without the transformer/load resistor going straight into the soundcard, it sounds real thin and honky... ie: not right....


Soundcard input impedance is something high, say 10K.  With a 600r resistor on the sec, the primary is presenting a 15K load on the tube.  With a 10K load on the sec . . .


The 15K:600 I have in front of me now says that with the sec loaded with  600r, I get a primary impedance of 18K at 1KHz and 13K at 120Hz.  Not perfect, but close enough to give near constant impedance on the primary.


Now, when I load that same transformer with 10K I get a 95K reflected load at 1KHz on the primary, But at 120Hz the load has dropped to only 4.6K.  That's a 20X difference.


I have another 15K:600 transformer that will measure a 17K pri load with a 600r sec load at 1KHz and will measure 17K at the pri with a 600r sec load at 120Hz.  Pretty much spot on.

When I load the 2nd transformer sec with a 10K load I get a pri impedance of 214K at 1KHz,  and 78K at 120Hz.  That's only 2.7 times difference.

If I had to pick which transformer might sound more toppy when misloaded, I'd guess the first one.

Transformer No 2 has a primary inductance that is nearly six times higher than No 1.

Doug has mentioned that his stock BA2s aren't fussy about what resistance the sec is loaded with.  You can see from his plots that the stock BA2 extends to a lower corner frequency than A-25/BA2.

My A-25 BA2s change some without termination but I haven't tried them on a lot of different Hi Z inputs.  But your description sounds typical of what can happen in some situations without termination.  Try something higher, like 1K, 1.5K, etc and see if you can reach a happy medium of overall gain loss and tonal balance/distortion. 



Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: Bren-hur on February 18, 2012, 08:02:47 PM
*DING!* The penny just dropped... i think...  I read and read that last post and couldn't quite understand it. I googled and read some docs about transformers from Jensen... and I think I got it... but please correct me if i'm wrong...

The impedance designations given on a transformer AREN'T the actual impedance... ie: the transformer isn't generating impedance, it's merely reflecting the impedance of the mic/amp. I have been hung up on the numbers, thinking they were the actual impedance and therefore have been very confused...

Going back to Dougs post on the cinemag...

Having the cinemag wired for "150ohm : 50K", which is an impedance ratio of 1 : 333, a 150ohm mic plugged in would reflect a 50K load on the first tube.

Having the cinemag wired for "600ohm : 50k" which is an impedance ratio of 1 : 83, a 150Ohm mic would reflect a 12K load on the first tube.

In either scenario, a load resistor on the secondary could be adjusted to reflect the same impedance to the mic:

Wired 150:50K, a 360K resistor would reflect around a 1k load to the mic.

Wired  600:50K, an 80K resistor would reflect around a1K load to the mic.

SO logically, the next thought is, what is the optimal impedance that the first tube in the circuit wants to see? And I'm going to guess it's 50K as per Dougs reccomendation... my tube theory isn't deep enough to figure it out for myself yet... out of curiosty, what impedance does the first tube reflect to the mic without any resistor in place?

Thanks for the tutelage... I sure hope I'm right on this transformer stuff... coz it was a bit of an awakening....
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on February 18, 2012, 08:09:34 PM
It's not about the tube, it's about the mic, and the optimal transformer conditions considering all factors.  Original ba2 input designed for unloaded sec, Cinemag not. 
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: lassoharp on February 18, 2012, 09:11:27 PM
Quote
out of curiosty, what impedance does the first tube reflect to the mic without any resistor in place?

This is an excellent question and one that does not have a lot of printed data of exact numbers for reference.  For amps like a BA2 and others with an input transformer working into an open grid we usually assume that it's at least 1.5K or higher through most of the audio band.  The grid circuit acts like a very, very high impedance.

Remember that since you're dealing with an inductance, you have to ask - "what impedance at what frequency" because the impedance changes with frequency, which is what those measurements in the previous post were showing.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: Bren-hur on March 02, 2012, 12:09:23 AM
So I want to hook up a VU to my BA2... on the schem it's got taps off both V1 and V2 cathode resistors going to a switch i assume, then off to a meter... is this where I'd hook up the VU meter too? and if so, whats the deal with the switch for V1 cathode and V2 cathode?

Thanks!
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on March 02, 2012, 12:23:35 AM
that's for measuring cathode current.  VU would connect to audio output, through the correct resistor network. 
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on April 23, 2012, 12:40:54 PM
I FINALLY got the front panel powder coated and engraved.  Below is the obligatory porn:

(http://img.photobucket.com/albums/v428/Senorpants1/BA-2-front.jpg)

(http://img.photobucket.com/albums/v428/Senorpants1/BA-2-front2.jpg)

(http://img.photobucket.com/albums/v428/Senorpants1/BA-2-front-top-of.jpg)

(http://img.photobucket.com/albums/v428/Senorpants1/BA-2-above.jpg)

(http://img.photobucket.com/albums/v428/Senorpants1/BA-2-above-2.jpg)

If you look closely at the back, you can see the green heaters wires twisted together.  Do they look twisted tight enough?
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on April 23, 2012, 12:47:11 PM
Looks fantastic.  Wires twisted enough if you're happy with hum level; can't answer that. 
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on April 23, 2012, 01:09:52 PM
Thanks Doug!

I've just finished spraying the BA-6A transformers I bought from you -they're looking good as new now!  Can't wait to get cracking on that one.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: lassoharp on April 23, 2012, 04:50:12 PM
Nice job!
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: buildafriend on July 10, 2012, 08:40:16 PM
C1 and C3 are cathode bypass, and 25V rating is more than plenty. 

C7 is power filter.

C2 is already overkill large.  Nothing to change there.  Nornal to see 0.1 or 0.25 in that impedance scenario.

R1-R2 affect bias point, and will make your distortion point move if you are into running up into distortion with test equipment and fiddling with values for most headroom.  They picked that spot for a reason; you may find you have a different reason to pick a different spot.  At normal operating levels (with plenty of headroom) you may not hear any difference between any reasonable chosen point.   Not uncommon to see 2.2K or 2.7K biasing that tube in triode in low level preamp stages.

As a general statement, it's not uncommon to find that two stage preamps of this vintage have essentially the same overload point in each stage, meaning there's great headroom in the 1st and nowhere left to go in the 2nd if the 1st is going full blast already.

Now I see why you said to go here. Thanks.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: buildafriend on August 22, 2012, 11:15:40 PM
(http://img.photobucket.com/albums/v207/whpnewjersey/9A05AE21-17CD-478A-9944-947E3F7CD0F7-1152-00000100A27A6217.jpg)

Thanks for the inspiration guys! I hope that I can pull this off.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on August 23, 2012, 03:48:00 AM
Have fun!  What input and output transformers did you go with in the end?
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: buildafriend on August 23, 2012, 12:57:08 PM
Thanks letter-B.

Input will be CMMI-10C, output I am still deciding but I think I will end up just waiting for the right NOS A-25 to pop up on eBay.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: lassoharp on August 23, 2012, 01:21:32 PM
If we're lucky Cinemag may have a good SE OT for this project at some point.  They already have a 7:1 that was kind of made for one of the UA console pres that should work ok for BA2.  Apparently David is working on a new design.

A-25 has pretty much already made it into "AY(F)KM" market price territory.   Lundahl may even be less $ now.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: buildafriend on August 23, 2012, 04:28:02 PM
I think that C2 should be carbon film, right? I think something like a nice WIMA cap would do. I have a polarized JJ brand 47uF electrolytic cap rated at 500v, but if I think electro might be wrong for this application.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: dmp on August 23, 2012, 04:58:56 PM
47uf would be very big also. The schematic says 0.5 uf. I used a 0.47uf Mallory 150, which is a film cap. Wima would also be a film cap.
Film caps are not 'carbon' film, they are different kinds of plastics. Check out the different film for MKS vs MKP Wima via google.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: buildafriend on August 23, 2012, 05:06:28 PM
whoops! I meant 0.47uF! sorry about that.

i'll do the research that you have suggested I do.

Thanks!!!
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on August 23, 2012, 05:59:45 PM
I used a Russian PIO for C2.  I can't remember where I read they used PIO caps in that position, it might have been this thread (or I could have imagined it).

EDIT: It was this thread:

Quote from: emrr
The metal cased caps are paper/oil types, and the 0.047 and 0.1 types probably are also.  I've never been entirely sure about that type, but cap info in RDH4 suggests it as a primary manufacturing method.

On exact values, older types are usually +100%/-10% versus new types which seem to be +/-10 or 20%.  Older types are seldom low, and frequently much higher than specified, 100% high not being abnormal in an original cap.  New types seem to always be slightly low to dead on, seldom higher than quoted.  Sometimes you have to take this into account in restorations, and buy higher than quoted to match what you are actually replacing.  This of course takes some intelligent 'reading between the lines' on the original design.   It's not at all uncommon to recap an old piece and have less low end that you had before the cap job, simply because you went by the book.

EDIT NO 2: There's some good cap info on page 1 of this thread from emrr.  You can lower the capacitance of C2 apparently.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: buildafriend on August 23, 2012, 08:22:34 PM
A Russian PIO eh?

I just ordered a pair of wima 400v 0.47uF film caps, and a 400v ceramic disc cap for C8.

If I make R11 variable, I wonder how it would effect things. I have to read up on the tilt option.

My first dual layer high voltage PCB attempt. I'm going to look it over tomorrow and hopefully etch it. Most yellow traces are symbolizing ground connections, if they are not supposed to be there they will be removed.

(http://img.photobucket.com/albums/v207/whpnewjersey/CaptureBA-2A.png)
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: buildafriend on August 25, 2012, 12:23:26 AM
(http://img.photobucket.com/albums/v207/whpnewjersey/E14F0783-64C7-4408-A7EC-F142DBE2998C-876-0000015D77CD385C.jpg)

Alright, I fixed a few funky traces from my layout, etched it, built it etc.. I seem to be getting all the right voltages. Everything looks good except for my output. My output was a acting a little strange and I think it was my output transformer selection that caused it.

I used a 0.1uF 250v WIMA film cap in place of C2 as a temporary part while I wait for the 0.47uF.

Since I still don't have my UTC A-25, would an edcor 2-1/2W 10K Ohms to 600 Ohms (4.1:1) line level matching transformer work?

I only ask because I have one available that I thought I might try to get this thing running.

URL is below.

http://www.edcorusa.com/p/160/xsm10k-600
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: lassoharp on August 25, 2012, 12:38:44 AM
Quote
Since I still don't have my UTC A-25, would an edcor 2-1/2W 10K Ohms to 600 Ohms (4.1:1) line level matching transformer work?

Electrically it should work and pass signal,  but it will not give bass response using it in this circuit.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: buildafriend on November 07, 2012, 07:00:39 PM
This project is begging for my first ever stepped gain pot. 12 step rotary it is.

I'm assuming this circuit uses a log style pot? .. gotta learn the math behind this. .. what is shorting, what is non shorting?

everything is good to go. I just ordered a custom chassis. I'll post another pic when all is together.



Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: dustbro on November 13, 2012, 07:54:25 PM
I'm assuming this circuit uses a log style pot? .. gotta learn the math behind this. ..

NewYorkDave had a great informational post about making a stepped attenuator. I'm too lazy to find it for ya, but do a search. Has all the info you need.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: buildafriend on December 31, 2012, 12:25:14 AM
Is there any way I can get this circuit to produce a little more gain? Are all you other BA-2A cloners happy with the amount of gain it produces for you?

Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on December 31, 2012, 01:01:30 AM
Not much left to do, beyond using an input transformer closer to the original ratio. Cinemag cmqee-3440a is most appropriate in their line.  50k instead of 15K, buys a few more dB.   

 Bypass the eq (r11) should buy 3dB more. 

 
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: lassoharp on December 31, 2012, 01:02:29 AM
Quote
Are all you other BA-2A cloners happy with the amount of gain it produces for you?

Yup.  I often use SM-7 for vocals and get plenty.   Everything beyond that mic and source wise keeps vol less than 12 oclock or padded for heavy stuff.

What input iron did you use?  If modern lower ratio step up you'd lose a little relative to the original but probably not a deal breaker.

Should be getting around 50db or so.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on December 31, 2012, 01:07:31 AM
Never any issue with gain here on an original RCA.   It might fail for spoken word with a ribbon, but probably not.  The real thing is stupidly quiet and tolerant with good tubes.  I have to pad mine 20 db on the front frequently.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: buildafriend on December 31, 2012, 02:24:55 AM
Input Cinemeg CMMI-10C
Output Cinemag  CM-2092 (7:1, 30K-600)

I can get what I want out of it. It's very clean. I wish it had a little more gain. I think it would fail for spoken word with a ribbon, but it just sounded great through my peluso U47 copy. A phantom power circuit is all this guy really needs.

Now I REALLY WANT to start a BA-6 to pair this thing up with.

Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: buildafriend on December 31, 2012, 02:31:59 AM
Not much left to do, beyond using an input transformer closer to the original ratio. Cinemag cmqee-3440a is most appropriate in their line.  50k instead of 15K, buys a few more dB.   

 Bypass the eq (r11).

 Connect the input transformer secondary to v1 cathode through 0.5 mfd or more, rather than ground.

Maybe I should switch to using that transformer?
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on December 31, 2012, 09:07:03 AM
It's a lot of expense for a few more dB
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: buildafriend on January 15, 2013, 01:06:12 AM
quick bass sample after adding DI
cheapie gibson bass, increased gain digitally but no compression.

https://soundcloud.com/user464938406/bass-ba-2a-completely
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on January 15, 2013, 07:35:19 PM
Sounds great! Full!
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on January 18, 2013, 02:35:16 PM
So I finally got round to replacing the old ADC input transformer I had in my BA-2 build with an original RCA one.  While I did this, I also re-did the heater wiring in an effort to reduce the slight hum I can hear in the amp.

I've managed to shave another 6dB off the hum, which I'm pleased about.  However, when I touch the case of the unit when it's out of the rack, the hum drops in level quite a bit which makes me suspect I've got a grounding issue somewhere in there.  I'm going to investigate further this weekend.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on January 20, 2013, 04:55:33 PM
When I have the BA-2 plugged into my RME FF400 interface fully cranked, according to my DAW I measure hum at -38dBfs.  When I touch my case the hum lowers in volume.

I have phantom power in my unit via a JLM Go Between that is fed by a ruffrecords phantom power PCB.  When I disconnect the 48v I'm feeding to the JLM PCB I measure hum at -68dBfs.  A whopping 30dB reduction in hum!

So there's obviously something wrong with the way I'm connecting the 48v, and I wondered if anyone has any insights.  Here's how I've got it set up so far:

I have a 15VA 25v-0v 25v-0v transformer connected in series to give me 50VAC.  i.e.  I have tied the 0v of one winding to the 25v of the other, giving me 50v across the other two windings on the secondaries (if that makes sense!).
The 50VAC is then connected to the phantom PCB which outputs 48VDC.
The 48VDC is connected to the 48v input on the JLM PCB
The ground output of the phantom PCB is connected to the input XLR.

Regarding the 25V and 0V winding I have tied together on the transformers secondaries -do I need to ground this?

Thank you.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: MagnetoSound on January 20, 2013, 05:44:47 PM
Regarding the 25V and 0V winding I have tied together on the transformers secondaries -do I need to ground this?

Thank you.

No, I'm not familiar with Ian's phantom board but if you do this you will probably create a short across the bottom winding, assuming there is a full-wave bridge on the board.

Do you have a good ground at the smoothing caps on the 48v board? You may need to connect this to your star ground (if you have one) so that the ground on (mic XLR) pin 1 is not left floating.

Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on January 20, 2013, 05:55:09 PM
Do you have a good ground at the smoothing caps on the 48v board? You may need to connect this to your star ground (if you have one) so that the ground on (mic XLR) pin 1 is not left floating.

Mic XLR 1 is connected to the chassis as well as the ground output of the Phantom PCB so it's not floating.  Do you think this could be causing a ground loop?
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: MagnetoSound on January 20, 2013, 06:04:30 PM
Phantom ground could probably go via the ground pad on the Go-Between and up to the XLR on the screen.

Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on January 20, 2013, 06:13:01 PM
Thanks, I'll give it a go.  However I just found this in the Phantom PCB thread where I asked a year ago (wow, time flies) how to hook up the 0VDC:

Quote from: ruffrecords
The 0VDC should go to pin 1 of the XLR input. This same pin 1 should also be connected directly to the chassis as close to the XLR as possible. The system 0V should be connected to the chassis at the power supply.

Maybe I should take this to the Phantom PCB thread...
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on January 21, 2013, 12:41:46 PM
It turns out I shouldn't have connected the shield on the pre amp side pin 1 of the XLR output.  I've disconnected it and now it's hum free.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: My3gger on January 21, 2013, 03:08:24 PM
Quote
Since I still don't have my UTC A-25, would an edcor 2-1/2W 10K Ohms to 600 Ohms (4.1:1) line level matching transformer work?

Electrically it should work and pass signal,  but it will not give bass response using it in this circuit.

Hi,

i'm watching this thread very closely and have similar problem; all of my output transformers are ungapped, most of them are 50Ni/50Fe Cinemags (one with huge core), others are Sowter 8650s:
http://www.sowter.co.uk/specs/8650.htm
At the moment i can't afford another transformer and i would really like to try BA2 if it makes sense. What do you guys think about using one of transformers i mentioned, anyone tried something similar, what are your thoughts about it? I red some good posts about this matter in RCA BC-2B thread, as i understand it i could loose some bottom, but ungapped model should otherwise act similar to gapped with little current as here and big core.
IIRC UA originally used ungapped tx in 610 and there is 8mA in primary, here we have 4mA which doesn't seem much. But i can't really measure what i get with ungapped, this is why i would like to hear opinions. If any of my transformers could work ok for test and i like the sound i'm sure i will somehow buy proper transformer.
Thanks for any help!
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: My3gger on January 23, 2013, 02:49:11 PM
I hope i'm not asking too many questions and too much. I really searched well to find at least some answer which don't tell me much. The problem is that i can't measure what i get in lows; i have a scope and am collecting money for good generator.
I would be very happy to know about experiences with dc in ungapped tx, or maybe what you think about it.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: buildafriend on January 23, 2013, 03:03:57 PM
In the kindest possible way, what is holding you back from getting a proper OP transformer? mine is listed here a 1 or 2 pages back. I believe the UTC A25 is an option as it is for the BA-2C.

I like people who ask questions.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: lassoharp on January 24, 2013, 02:27:02 AM
Quote
Since I still don't have my UTC A-25, would an edcor 2-1/2W 10K Ohms to 600 Ohms (4.1:1) line level matching transformer work?

Electrically it should work and pass signal,  but it will not give bass response using it in this circuit.

Hi,

i'm watching this thread very closely and have similar problem; all of my output transformers are ungapped, most of them are 50Ni/50Fe Cinemags (one with huge core), others are Sowter 8650s:
http://www.sowter.co.uk/specs/8650.htm
At the moment i can't afford another transformer and i would really like to try BA2 if it makes sense. What do you guys think about using one of transformers i mentioned, anyone tried something similar, what are your thoughts about it? I red some good posts about this matter in RCA BC-2B thread, as i understand it i could loose some bottom, but ungapped model should otherwise act similar to gapped with little current as here and big core.
IIRC UA originally used ungapped tx in 610 and there is 8mA in primary, here we have 4mA which doesn't seem much. But i can't really measure what i get with ungapped, this is why i would like to hear opinions. If any of my transformers could work ok for test and i like the sound i'm sure i will somehow buy proper transformer.
Thanks for any help!



For the ungapped Sowter you'd want to go to AC coupled (cap feed) output stage for best results.  That would require a redesign.  There are some potentially simple ways to do that.

If you want to stay DC coupled then you're narrowed down a bit on choices.  UTC A-25 and LS-27 . . . they're out there but already in the nutball price zone for the most part.  Cinemag has the one 7:1 that's similar to recreation of the one for the old UA preamp.  I'd email David at Cinemag and tell him what you're looking for - he may already have something in the files.  You can also try Lundahl.  They have a couple different ones in the right ratio range.

http://www.kandkaudio.com/transformers.html   
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: My3gger on January 24, 2013, 03:19:13 AM
In the kindest possible way, what is holding you back from getting a proper OP transformer? mine is listed here a 1 or 2 pages back. I believe the UTC A25 is an option as it is for the BA-2C.

I like people who ask questions.

Money is holding me back, i wrote i can't afford another transformer at the moment. Someone just asked me if i would sell one or two of my ungapped OTs i don't really need now, so it seems i have a solution.
Thanks for replies.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: dmp on January 24, 2013, 10:15:50 AM
Quote
i'm watching this thread very closely and have similar problem; all of my output transformers are ungapped, most of them are 50Ni/50Fe Cinemags (one with huge core), others are Sowter 8650s:
http://www.sowter.co.uk/specs/8650.htm
At the moment i can't afford another transformer and i would really like to try BA2 if it makes sense.

I don't think you'll be building a BA-2 unless you have the right kind of transformers. But you may still build a great preamp, though.
Look at Gates designs for alternatives that are resistor coupled (SA-70, for instance). It's a very similar preamp to the ba-2
Lassoharp describes the situation well with crazy prices for the relatively few options out there for transformer coupled..
Of course, you can try if you want and I would be interested to hear how successful you are.  You sometimes learn the most from trying things that don't work out.
This forum needs people to start winding there own transformers for this application!
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: peters on January 24, 2013, 02:54:21 PM
Lundahl LL1680 might be an option. Its advertised as "made to match or exceed the specs of the UTC transformer LS-27". Its not cheap though.

I'd be curious if anyone has used it...

Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: My3gger on January 25, 2013, 06:14:01 AM
Quote
i'm watching this thread very closely and have similar problem; all of my output transformers are ungapped, most of them are 50Ni/50Fe Cinemags (one with huge core), others are Sowter 8650s:
http://www.sowter.co.uk/specs/8650.htm
At the moment i can't afford another transformer and i would really like to try BA2 if it makes sense.

I don't think you'll be building a BA-2 unless you have the right kind of transformers. But you may still build a great preamp, though.
Look at Gates designs for alternatives that are resistor coupled (SA-70, for instance). It's a very similar preamp to the ba-2
Lassoharp describes the situation well with crazy prices for the relatively few options out there for transformer coupled..
Of course, you can try if you want and I would be interested to hear how successful you are.  You sometimes learn the most from trying things that don't work out.
This forum needs people to start winding there own transformers for this application!

Yes, SA-70 is another cap coupled option. I built one with EF86/half 6CG7 and later changed it to EMRR's WE 141-A mash-up (also EF86 and 6CG7). Since then it stayed this way, i like it because it sounds good and different from anything i tried (no OT). A friend who has it now uses it all the time.
I thought about just trying ungapped OT in BA-2, but it would only make me think obsessive about how it would sound with right OT :) It will take some time until i get it together with gapped tx and compare to ungapped. I will ask guys at Cinemag about BA-2 OT this weekend and report back about model and price.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: lassoharp on January 25, 2013, 06:40:16 AM
There's also NYDave's 6SN7 One Bottle, and while on the subject of mash-ups - I don't see any major reason why subbing NYD's OB output stage onto BA-2 wouldn't work just fine.  That would keep output stage open to subbing any non gapped OT of choice.  May have to make some minor tweaks in the power supply.



Also - Ian's(ruffrecords)  mu-follower mic pre is an excellent sounding design that uses no OT, or can use a 4:1 if desired.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: My3gger on January 26, 2013, 04:29:59 PM
6SN7 One Bottle mash-up is a good idea, i've just done some work about it. It is incredible  how solutions are found in this place :) For mash-up i should probably leave 10K R (from reg. dc) for BA-2 input anode, do you maybe know which R is needed for One Bottle output part?

Do you find Ian's mu-follower very clean in use even with transformer, could you explain that a bit? I built a few European classics and many of them are pretty clean. This, interstage pot and trioded pentodes made me explore American no-nfb designs. I red Bob Olhsson's post where he says RCA and WE controlled most of audio patents worldwide until late 40's, which is interesting because not long ago people in Europe didn't know much about American electronics.

Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: lassoharp on January 26, 2013, 11:35:08 PM
Quote
do you maybe know which R is needed for One Bottle output part

Plate load resistor?    Think Dave's SN7 one used 10K. 


I built up Ian's circuit on breadboard and played with for a few weeks.  Yes, it's a very clean low distortion design but the distortion profile leans toward predominant 2nd order.   I'd suggest going to Ian's website for in depth reading.  He explains that particular circuit. 

Are you looking for something to give more distortion?  The Collins 6Q is good one for that and use of non DC coupled output.  Interstage vol is easily added.   
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: ruffrecords on January 27, 2013, 07:33:22 AM
Do you find Ian's mu-follower very clean in use even with transformer, could you explain that a bit? I built a few European classics and many of them are pretty clean. This, interstage pot and trioded pentodes made me explore American no-nfb designs. I red Bob Olhsson's post where he says RCA and WE controlled most of audio patents worldwide until late 40's, which is interesting because not long ago people in Europe didn't know much about American electronics.

It is pretty clean and for two reasons:

1. The 6CG7 tube is electrically identical to the 6SN7 which well known to have the lowest intrinsic distortion of all the popular audio triodes.

2. It is no good using an inherently low distortion tube if you put it in a circuit that prevents it achieving its potential - like putting an Olympic swimmer in a pool of treacle. The main cause of distortion in triodes is loading the tube's plate resistance ra. The mu follower uses a clever bootstrapping technigue to make the plate load look like a much higher value resistance which reduces distortion to close to the tube's intrinsic level.

The distortion produced by a mu follower not surprisingly depends on the load. That's why I recommend only feeding it into 10K bridging load. If you want to make it distort more then load it more but be aware this also reduces headroom.

A circuit closely related to the mu follower is the SRPP. This has much better drive capability than the mu so it is ideal if you want a 'dirtier, sound. If you look at the REDDI thread you will will see where I gave details of converting the mu follower to the very similarvSRPP.

Cheers

Ian
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: lassoharp on January 27, 2013, 11:19:15 AM
Since Ian's here to explain the details I'll just say that it's a very good sounding circuit and I have it on my to build list of preamps.

Good versatility on output options - transformerless or use of non exotic low ratio stepdown.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: My3gger on January 27, 2013, 11:38:37 AM
Thanks Ian for explaining it. I followed your writing when you told us how to change mu-follower into srpp, it was in another thread some time before REDDI. Your pdf's about 6SN7/6CG7 differences and mu-follower distortion are reference reading for me, very revealing. I learned a lot and it also made redesigns easier because i didn't have to change sockets to octals.
Lassoharp, i meant R8 (4k7) where HT comes into NYD's Simple6SN7 preamp (sorry i can't add schematic for you, i'm having some problems). I think it should be different if i put BA-2 input in front.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: lassoharp on January 27, 2013, 12:59:11 PM
Yes, it will likely take some redesign on the PS decoupling resistor.  Dave's design calls for a higher supply voltage.  Should be no be problem as there are a wide range of PTs readily available.  Might be good to start that as separate thread with both schematics on hand.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: peters on April 12, 2013, 06:09:57 PM
This is my first attempt at a turret style build. The plan is for 2 amp channels with shared power supply in a 2RU case. I'll be using 5693 (6SJ7) tubes, Lundahl LL1680 and LL1578XL transformers. This will be kind of a modern version. I did splurge on some Jupiter Beeswax caps for C2 for some old time flavor.


Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on April 12, 2013, 06:16:09 PM
Cool.

I wouldn't waste a 5693 on V2, only on V1.  It will make no appreciable difference at V2.  In fact, I'd be sure to check and see if it even really does at V1.  A good 6SJ7 may equal it at this gain. 
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: lassoharp on April 12, 2013, 06:50:35 PM
When I get time I'll try and post my turret layout.   Just finished a single channel of these.  Took about 15 slots I think.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: buildafriend on April 13, 2013, 11:36:58 AM
When I get time I'll try and post my turret layout.   Just finished a single channel of these.  Took about 15 slots I think.

That would be great! I would definitely like to see have a print out of it in my files.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: CJ on April 13, 2013, 01:26:17 PM
you might want to mount the 6SJ7 tube sockets on rubber grommets, or mount the sockets all on one plate and then mount the plate on rubber grommets,

this is because the 6SJ7 can make a lot of noise when you tap on them.

they make a glass version that sounds real smooth,

Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: My3gger on April 14, 2013, 12:58:30 PM
I forgot to let you guys know what trafos Cinemag has for this amp: inputs are CMMI-10PCA and CMMI-10B, gapped outputs are CM-2092G and  CM-27101G. There are datasheets for inputs on their site.
I bought CMMI-10B because it has leads which means a less work. Next time i will try pins because shielded secondary wires really makes difference when runs are a little longer. Manufacturer told me that shield capacitance can influence frequency response, so i removed mesh shield from coaxial wire and i will put it over secondary wires to see what happens.
OT's don't have datasheets posted, here are some information: CM-2092G is gapped in the standard configuration, i didn't ask for max I, ratio is 7:1. CM-27101G can be gapped (special laminating structure with 4 "E"s in on direction and 4 in the other), it's higher quality transformer, takes up to 10mA, iirc max signal is +18dBu, also with 7:1 ratio. It was recommended to me, so i ordered one and didn't pay anything more for gapping. Maybe because i ordered more than 10 trafos all together for some other guys beside me.
So this is not that expensive after all and i really like their transformers. With all costs for shipping from US to EU they are still cheaper than Sowters and at least inputs sound better to me.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: MatthisD on April 14, 2013, 04:04:04 PM
Thanks for posting these Cinemag details, would you happen to have the DC resistance of the windings? particularly the output primary of CM-27101G and if you know the dimensions of it.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: My3gger on April 15, 2013, 03:07:27 PM
Thanks for posting these Cinemag details, would you happen to have the DC resistance of the windings? particularly the output primary of CM-27101G and if you know the dimensions of it.

Transformers still didn't go out of factory, we are waiting around month and a half. I will ask them if everything is ok, a friend already wrote one mail about three weeks ago and they said they will ship in a few days. This is very unusual for them, maybe it's because of this custom trafo... I think ungapped version of CM-27101 have the same core dimension as their CM-2810, i have both of them at home. When gapped version of CM-27101 arrives i will measure it, take some pics of lamination stack and let you know DCR reading.
There is also CM-9589 with 4:1 ratio which costs a little more than CM-27101. I think it takes up to 20mA with steel laminations, have to check email if anyone is interested.
Btw, compare CMMI-10PCA datasheet to Jensen JE-115KE, they look almost identical. I heard 115KE is based on old Reichenbach input transformer which is now CMMI-10PCA.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: MatthisD on April 15, 2013, 04:17:55 PM
How long did your previous order from Cinemag take to arrive, 3 or 4 weeks?
I couldn't find it listed on the site but the datasheet is here;
http://cinemag.biz/output/PDF/CM-27101.pdf
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: My3gger on April 16, 2013, 10:05:16 AM
More like 2-3 weeks to Europe, trafos went out of factory a few days after payment for about 5-6 orders until now. I never ordered custom product or something that wasn't in stock, maybe it's this or because it was two of us talking to them this time. Anyway, they used to ship fast, communication was good and they also proposed correct ratios/models when i asked.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: buildafriend on April 16, 2013, 01:47:56 PM
Just call dave at cinemag and ask him. He's a great communicator and he is on point.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: My3gger on April 16, 2013, 02:18:37 PM
You are right, no problem there. I wrote an email a few hours ago and got reply from David right away. They have some lead time and also had Frankfurt. I should ask them myself before posting here...
So we have easily available trafos for BA-2, i think pretty cheap for what they are.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: My3gger on May 06, 2013, 02:36:21 PM
Ok guys, i got CM-27101G (gapped version), here are results compared to ungapped i also have:

Ungapped CM-27101
Primary DCR: 1371R
Secondary DCR: 30R

Gapped CM-27101#G
Primary DCR: 1335R
Secondary DCR: 29,5R

My DMM is a cheap one, so this numbers are little off but consistent.
Both transformers have wire for Faraday shield, same color code and core dimensions. They sent photo of the same stacking method used for CMOQ-2x, i will try to make it smaller and attach it. Mine has 12 stacks of what looks like Iron and Nickel, so 4Fe/4Ni lams 6 times. I'm pretty sure shiny lams are Ni and other Fe. Hope you understand what i want to say, if you have any question let me know. Unfortunately i can't say how it sounds...
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: MatthisD on May 06, 2013, 04:54:59 PM
Ok guys, i got CM-27101G (gapped version), here are results compared to ungapped i also have:

Ungapped CM-27101
Primary DCR: 1371R
Secondary DCR: 30R

Gapped CM-27101#G
Primary DCR: 1335R
Secondary DCR: 29,5R


Thanks very much!
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: My3gger on May 07, 2013, 02:34:45 PM
MatthisD, why did you ask for dcr? I'm sure it reveals something, i just don't know what and why. Can you please explain me this if you can?
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: MatthisD on May 08, 2013, 07:22:45 PM
MatthisD, why did you ask for dcr? I'm sure it reveals something, i just don't know what and why. Can you please explain me this if you can?

To get an idea of the number of turns and the wire gauge that make up the primary, and to compare the various transformers available with this lamination size. I'm using a Lundahl for the moment which is a c-core dual bobbin type. The cost is equally high for this Cinemag once postage and additional taxes are included so I like to have as many details as possible before buying a couple of these.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: buildafriend on July 03, 2013, 04:31:38 PM
I was wondering if I could beg for a little help here...

I have a friend with a UTC CR 725

I got some possibly correct specs on it through and ebay auction for one


"This sale is UTC hermaeticaly sealed audio output transformer model CR-725. This transformer was manufactured for RCA under part number B-8480798-1. The response specs are: 1/2 DB 30 to 15 KC at +20 DBM. The primary impedance is 100 ohms and the secondary can be configured for either 150 or 600 ohms. Size is 2 1/4 inches high and 1 5/8 by 1 3/16 mounting base. There are threaded studs on the bottom for mounting. This is a commercial grade transformer."

My BA-2A has a low output. I've deemed it too low for actual use. It cant even drive a 58 for a vocal. Not okay. It sounds great and its clean but using make up gain in post from a mic pre is not okay with me. I need to get it working. I'm guessing its the output transformer.

Are there any specs that I can give you guys that might help me?
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: dmp on July 03, 2013, 04:43:11 PM

Mine has about 45-50 dB I think (I omitted any tone adj)?
I wouldn't say this is "too low for actual use" (for vocals from a dynamic mic).
I would start by measuring the gain (in dB) of the whole preamp, then the gain of the input and output transformers and see if things make sense. Then go from there on making changes.

Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: dmp on July 03, 2013, 04:44:46 PM
Also, it's not clear to me what the UTC CR 725 has to do with your BA-2A. Are you saying you are thinking of using the UTC 725 for the output transformer? It doesn't sound remotely like the correct output transformer for a ba-2a.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: lassoharp on July 03, 2013, 04:50:43 PM
Quote
My BA-2A has a low output. I've deemed it too low for actual use. It cant even drive a 58 for a vocal. Not okay. It sounds great and its clean but using make up gain in post from a mic pre is not okay with me. I need to get it working. I'm guessing its the output transformer.

Then i'd say there is problem with your BA-2.  Mine is chugging away like a champ in the studio.  Really no situation I've found close to not having enough output.

A low Z 1:1 for the OT . . . . I suppose it will pass signal with unloaded secondary.  If you terminate the sec with 600r it will give the output tube a hernia.  It's the wrong tool for the job and it will likely sound not so good.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: My3gger on July 21, 2013, 04:13:44 AM
Hi guys,

i'm changing my psu from RC filters as on schematic to fully regulated. I'm not sure what to do with R10/10k. Looks like it makes part of psu decoupling with C7, so it should probably stay in the circuit with reg. psu. Is this correct?
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: lassoharp on July 21, 2013, 08:52:53 AM
Hi guys,

i'm changing my psu from RC filters as on schematic to fully regulated. I'm not sure what to do with R10/10k. Looks like it makes part of psu decoupling with C7, so it should probably stay in the circuit with reg. psu. Is this correct?


Yes, R10 decouples and sets supply voltage for V1.   Probably wouldn't hurt to use a 22uF there.

Am curious why you are choosing a regulated supply for this circuit.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: My3gger on July 22, 2013, 03:53:48 PM
I'm using 22u C7 (might go even higher, as on BA-11A) for a little more filtering and because of emrr's remark about old lytic's values. Both cathode caps are 68u for this second reason.
I'm changing psu because i want to have all three voltages on one board. HT is actually capacitance multiplier with zeners which comes very handy as i don't have to experiment with resistors to get right voltage. The last time it had only 1mV of ripple and almost no heat at ~270V/15mA... I think this are good reasons to use it, beside somehow liking them.
Have you ever noticed any difference in sound from simple circuits like this when using reg. vs unreg.? I'm asking because never got a chance to compare on various sources, gains, etc.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: lassoharp on July 22, 2013, 08:44:38 PM
Quote
Have you ever noticed any difference in sound from simple circuits like this when using reg. vs unreg.? I'm asking because never got a chance to compare on various sources, gains, etc.

I've never tried a regulated supply on any of the preamps I've built so can't really say.  Certainly on vari-mu limiters you can hear the increased distortion and overall poorer performance with an unregulated PS design.

If your line voltage ever dropped enough to bring the V1 plate voltage down towards 50V or so you'd likely hear the typical results for that area of operation.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: My3gger on July 26, 2013, 06:55:17 AM
I'm making layout for BA-2 and chose RCA knob, like on Letterbeacon's build here:
http://www.groupdiy.com/index.php?topic=26675.240
There is eBay offer with similar NOS knob which has 1,5 inch diameter. It doesn't say if this includes pointer. Does anyone know if it is 1,5 inch with pointer or not? Here is eBay add, can't ask because i'm not a member:
http://www.ebay.com/itm/NOS-Original-Vintage-1-5-RCA-Viz-Bakelite-Knob-/320984836948?pt=US_Vintage_Pro_Audio_Equipment&hash=item4abc2feb54
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: lassoharp on July 26, 2013, 08:23:43 AM
Yes, that is a 1.5 inch knob.  I'm not sure what you mean by pointer.  What's pictured is all there is to the knob.  Did you mean a metal pointer as found on some vintage knobs?   I've never seen a metal pointer on an RCA knob of that era.  The earlier ones had them but that is cosmetically a totally different knob type.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: My3gger on July 27, 2013, 06:56:51 AM
I mean little "nose" or "pointer" at the bottom of the knob, it has white infill. I'm not sure if diameter of knob with this pointer is 1,5 inch or a little more. Letterbeacon told me from memory that diameter with this pointer is a little more than 1,5 inch, probably 40mm. Those 2mm make a difference because i would like to make proper scale for knob. Thanks!
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: letterbeacon on July 27, 2013, 07:02:10 AM
Thanks for reminding me to measure!

The diameter of the round bit (i.e. not including the pointer) measures 38.4mm (1.5"). Measuring from the tip of the pointer to the other side is 41.2mm.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: My3gger on July 27, 2013, 09:39:38 AM
Hey, thanks for helping me out. I've done 40mm scale and left 1,5mm more. This is just right, i don't have to redo it.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: buildafriend on October 06, 2013, 07:07:08 PM
Never any issue with gain here on an original RCA.   It might fail for spoken word with a ribbon, but probably not.  The real thing is stupidly quiet and tolerant with good tubes.  I have to pad mine 20 db on the front frequently.

Wait, my brain just did something logical.. (thats rare).

I have low gain from my DI signal and from my mic signal. Maybe my output transformer choice sucks!? maybe thats why I have low gain.. RRRG JUST WORK.

Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: My3gger on May 21, 2014, 08:52:58 AM
Hi guys,

i got back to BA-2 and measured some important voltages. First grid shorted and pot wide open, output trafo is Cinemag CM-9661 that takes up to 10mA unbal. current (50:50 version), sounds fine. Voltages seem quite a bit off compared to others, except for HT:
HT=263V
Anode 1 108V, cathode 1,79V
Anode 2 262V, cathode 5,67V

Also measured AC at reg. HT where it is 1mV, heaters have 50mV which seems quite high compared to previous builds.
 
This is compared to voltages i found here, they might be taken by old voltmeter which causes this difference:
HT 248VDC/ 5,6mA
V1 plate 73VDC cathode 2.4VDC 
V2 plate 232VDC cathode 8.8VDC 

I'm using EH EF86, looking at anode voltages it seems like they take a little less current than they should. Should i do something about it, or go on and close the lid? I built quite a few tube preamps where voltages were 10% off at most, often closer to reference.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: lassoharp on May 21, 2014, 10:56:01 AM
Quote
I'm using EH EF86, looking at anode voltages it seems like they take a little less current than they should. Should i do something about it, or go on and close the lid? I built quite a few tube preamps where voltages were 10% off at most, often closer to reference.

The curves for EF-86 triode mode are probably a little different than 6J7.  If that extra bit of biasing difference is something worth pursuing then maybe.  If you're driving 10K or > inputs only you probably have enough current there.  You can always compare headroom with 4ma biasing and see if it's worth changing.


I'd be interested in seeing freq response in general for the Cinemag OT if you have a set up for it.  It sounded pretty good on paper as I recall but no idea what the actual inductance figures were.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: My3gger on May 21, 2014, 04:03:32 PM
Yeah, EF86 and 6J7 are a bit different. I will probably leave first stage as it is and on second gradually make cathode resistor higher by 500R. Output stage needs good drive because it will often go into UA176 which has 600ohm input. Direct coupling is new to me, so i hope this plan is ok for the start. If anyone thinks i could do differently without changing tubes please let me know.
Those 50mVAC is still a mystery to me, it could be one of diodes from bridge, or maybe to low voltage into reg. (there is another 1,5v higher tap to try). I will scope it tomorrow unloaded because i know previous regs had 1mV this way...
Don't have a setup to measure inductance of 9661, if you tell me what you would like to know, my toroid winder could probably measure it. It was quite a revelation when i made A/B tests with good big vs small core trafos, thanks for that!



Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: My3gger on July 13, 2014, 06:07:50 PM
Cinemag CM-9661A-L can't take those few mA of DC from output stage, no way! I wrote it can do 10mA because it was mentioned by a guy here who used to give very good information... With only a few mA it distorts pretty bad and also limits the output signal. Correct trafo is on the way, can't wait to play with it a little further. Calculating and understanding direct coupling gave me a little headache, but it feels so good to understand it properly.
I was pretty surprised at how warm EF86's get at this bias. Tried Tesla EF806's, RFT EF86s, Svetlana EF86, Philips EF86 and new production EH EF86. Beside Svetlana which had really low gain and barely glow, all of them get similarly warm. It was surprising to hear new production EH EF86's sounding as good as nos i mentioned, only RFT was a little more "alive", although it had quite higher gain than the others. I also tried to run EH as pentode with NFB, can't say it is different than nos. Two of them run for a few months wired as pentodes or triodes, they seem to be fine. Hope we finally have good new production EF86.
Datasheet for EF86 shows max Ik at 6mA, operating characteristics when wired as triode give 3,6mA at 400V, BA-2 biasing seems well below limiting values. Output stage has ~4mA which should be fine, i'm probably just used to very low current and heat at pentode wired input stages.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: lassoharp on July 13, 2014, 06:17:18 PM
I think that unit is designed for PP stages.  I imagine it would distort.  From David Geren:


"The CM-9661A-x is suitable for push-pull applications.  The Altec 436 used the 16402, which used "L" laminations instead of E-I. Those laminations are "unobtanium" at this time because the tooling ended up at Tempel Steel who sold it for scrap a number of years ago.  The CM-9661A-x has far more level capability with a turns ratio of 7:1.  The turns ratio for the 16402 was 6.2:1.  The 16402 laminations were 49% nickel.  The CM-9661A-L uses a mix of 50% high-nickel and 50% steel laminations.  They are $105.87 in quantities of 1-10 pieces.
Best regards,
David"


The CM-2092CF Steel Lams version sounded like their best bet for a suitable sub.  I've never tried it but was sent some test data. From David Geren:

"Here is what it looks like with a 20K source impedance.  Because you will be driving it from a significantly lower impedance, it will have better low frequency characteristics.  It will have no problem taking 9mA on the primary.  Nickel laminations are not used for butt-stacked transformers."

Source Z for V2 is probably around 12-13K.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: My3gger on July 14, 2014, 01:36:54 PM
I used CM-9661A-L only for R-C coupled preamps, it was very popular OT for Redd47 and similar. David Geren proposed it to me for 47 years ago, though about PP because of CT and core but never got any data until now.
Hope it will survive at this current until it will be replaced by CM-27101. I like this one a lot for R-C coupling too when ungapped, iirc David wrote it is a bit higher quality than CM-2092 when used in BA-2. Stock gapped 27101 has a stack of 4x4 Ni and Fe lams (hope i wrote this correctly, it is 4 Ni lams in one direction, 4 Fe in other, each end has 3x3) and is pretty cheap too. I really like how it sounds in this preamp with CMMI-10B...
Thanks for sharing data for CM-2092CF, hope i will have a chance to properly test it side by side with gapped CM-27101.
I will report my results with EH EF86 because random samples work fine for quite some time.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: lassoharp on July 14, 2014, 01:44:18 PM
I used CM-9661A-L only for R-C coupled preamps, it was very popular OT for Redd47 and similar. David Geren proposed it to me for 47 years ago, though about PP because of CT and core but never got any data until now.
Hope it will survive at this current until it will be replaced by CM-27101. I like this one a lot for R-C coupling too when ungapped, iirc David wrote it is a bit higher quality than CM-2092 when used in BA-2. Stock gapped 27101 has a stack of 4x4 Ni and Fe lams (hope i wrote this correctly, it is 4 Ni lams in one direction, 4 Fe in other, each end has 3x3) and is pretty cheap too. I really like how it sounds in this preamp with CMMI-10B...
Thanks for sharing data for CM-2092CF, hope i will have a chance to properly test it side by side with gapped CM-27101.
I will report my results with EH EF86 because random samples work fine for quite some time.


Ahh . . . . cool. So do you happen to know if the 27101 is a new design?   I recall David mentioning working on a gapped high ratio OT design using Ni.  This was during discussions several years ago on what might work best for BA1/2 outputs.  I never asked again to see if he'd come up with a new transformer but it sounded promising.  How big does the core size look?
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: My3gger on July 14, 2014, 03:32:23 PM
First time i heard about and bought ungapped 27101's in 2009. In 2012 we exchanged some mails with David, this is what he wrote:

"I have been considering re-working the CM-27101 which has a turns ratio of 7:1.  It is not the highest performance design, but it is very good overall. Because you want a gapped transformer to take the dc current, it will have to be made with steel laminations.  If you are interested, let me know and I will look into it further."

After my reply he wrote more, remarks about "butt stacked", larger transformer and more complex winding structure might be related to CMO-30/600 i asked about too:

"The new part number is CM-2092G.  The "G" suffix means "gapped".  We did not want to adopt something for "butt stacked"... that one will be a good choice, too.  It is a larger transformer with a more complex winding structure for better bandwidth at the high end.  The CM-27101 is great at high frequencies, but the CM-2092 is better.

In mid 2013 i bought first gapped 27101, it came with Ni and Fe lams stacked as i mentioned in last post. Core is EI48, lams look like a little less than 0,3mm thick.
Judging only by price and Ni/Fe lams stacking (at first it was supposed to contain only Fe), 27101 might be better transformer now. I think David confirmed my observation, can't say for sure because gmail search can't find it in so many mails we wrote. He is a very, very patient guy :-) I rather call now to make things easier.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: lassoharp on July 14, 2014, 03:49:00 PM
Ok, it must have been something else or maybe he was just referring to the same one.  What's your opinion of the overall sound?  Does it seem balanced in the bottom and top?
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: My3gger on July 14, 2014, 04:53:11 PM
I just found an email from 2013 where David wrote that gapped 27101 is better transformer than 2092.
Compared to CM-9661, Sowter 9980 and some others, 27101 seems more balanced in the bottom. 9980 is quite similar at the top. I didn't pay much attention to Lundahl because they cost much more and i don't need all those taps.
Can't say about gapped because i didn't have a chance to really compare side by side. With CMMI-10B input in BA-2 it sounded just beautiful. Not very very clean like some cap coupled with nfb, and not distorted like others. It has a very nice color, i wouldn't change anything at the bottom or top, mids are good too. Ok, now i'm talking about the circuit with one OT transformer, but it really sounds very good and is not too colored or distorted.
Well, when i listened to BA-2 in this configurtion for the first time, i felt like a very happy guy with a white coat in some 50's US lab, with this preamp and Protools. Thanks to you guys :-)
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: mitsos on July 15, 2014, 12:25:59 PM
does anyone have L and DCR measurements for the 2092 or 27101?  Would like to wind a gapped tube output, looks like a nice starting place.  I think the core is 1/2" (same as the smaller API 2623 output).
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: My3gger on July 17, 2014, 03:55:20 AM
Don't have a setup to meassure L, i wrote DCR readings for 27101, not sure if it is in this thread. I will check later for this info.
Core has ~ 16,5mm. If right thing to do (27101 is in production), i can write exactly how it is stacked and how winding looks from outside. Can't comment about 2092 because i haven't seen it.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: mitsos on July 18, 2014, 12:01:02 PM
Of course, one page back... sorry, should have looked better.
Quote
Ungapped CM-27101
Primary DCR: 1371R
Secondary DCR: 30R

Gapped CM-27101#G
Primary DCR: 1335R
Secondary DCR: 29,5R

So it looks like a 5/8" core (same as API 2503 outputs).  I can see how you'd get 30K using nickel/steel lams, but when you go just straight steel you lose a ton of inductance, and if you gap it, you lose even more.  But if it sounds good, then it must be right...
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: My3gger on July 22, 2014, 03:38:28 PM
Compared to CM-9661, Sowter 9980 and some others, 27101 seems more balanced in the bottom. 9980 is quite similar at the top. I didn't pay much attention to Lundahl because they cost much more and i don't need all those taps.

Thought about this some more and should correct myself. Popular Redd47 has this nice edge at the top which stands out a little, so CM-9661A-x balances things nicely. This OT also has nice mid bass to it, so is very good choice for this preamp. I always used it with CMMI-7C input which takes a lot of signal and seems it is easier to work with it (avoiding clipping it, setting pad, etc). They are very nice pair and i want to check better their HiNi version too.
If i take in account suitability for Altec 436, primary tap and high level capability at more than 24dBu at 20Hz, it makes sense to have one.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: buildafriend on October 24, 2014, 09:57:47 PM
Would CMMI-10C work on input? At a quick glimpse at it seems similar to the suggested CMMI-10PCA..I think? I'm not so amazing at reading transformer datasheets. I'm using the 10C.. 

CMMI-10C Datasheet:
http://cinemag.biz/mic_input/PDF/CMMI-10C.pdf

CMMI-10-PCA
http://cinemag.biz/mic_input/PDF/CMMI-10PCA.pdf

BA2A Schematic
http://www.preservationsound.com/wp-content/uploads/2010/10/ba2c.jpg

It's been over two years since I built this and ended with low output. I would like to think that I can go back over it and fix it after putting it away for so long and successfully building so many other projects.

My output transformer is the Cinemag 2092G which was also suggested earlier on ( I think page 16 )

Maybe I should update this post with voltages soon.. that and some transformer DCR to see how it compares to the others that seemed to have worked for most of you.

Power transformer is this guy:
269JX
http://www.hammondmfg.com/pdf/5c0026-27.pdf

AC heaters running at about about 7.4V (weird right?.. go easy on em jeeze)

Top of my power supply caps measure 366VDC, 325VDC, and 292VDC.

520Vac are hitting the 6x6 rectifier tube

plate voltage on V2 is 240VDC

plate voltage on V1 is 293VDC
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: buildafriend on October 24, 2014, 10:10:25 PM
does anyone have L and DCR measurements for the 2092 or 27101?  Would like to wind a gapped tube output, looks like a nice starting place.  I think the core is 1/2" (same as the smaller API 2623 output).

I can measure and confirm the DCR for you about the 2092. If you care for it still.. since I have one.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: lassoharp on October 24, 2014, 10:50:26 PM
Quote
Would CMMI-10C work on input? At a quick glimpse at it seems similar to the suggested CMMI-10PCA..I think? I'm not so amazing at reading transformer datasheets. I'm using the 10C.. 

Yes.  I think that's their best one and I would recommend it over the B version.

Quote
It's been over two years since I built this and ended with low output.

Real BC2 is about 50db  of gain.  Both Cinemag inputs have less gain than original. 10C will have slightly more gain than PCA.  Original OT is 7:1 so compare that to Cinemag to see where gain loss stands.  You'll likely end up around 40db of gain.

Quote
Maybe I should update this post with voltages soon.. that and some transformer DCR to see how it compares to the others that seemed to have worked for most of you.

Assuming you have right spec power transformer I would look at what happens to bottom end with the stock cathode resistor of V2.  You should be drawing around 3-4ma on V2.  You can adjust cathode resistor for less current and slightly better bottom but it may not net you huge improvement.  I'd forget what is said about the Cinemag OT "being good up to 8ma" or whatever that value is.  3-4ma is plenty for driving high-Z loads and the bottom response gets slightly worse as current goes up.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: buildafriend on October 25, 2014, 06:54:03 PM
Thanks lassoharp. I added some voltages to my post near the bottom of page 17. Maybe you guys will spot something weird about them. Should I add any other test measurements?
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: lassoharp on October 25, 2014, 08:57:17 PM
Quote
plate voltage on V1 is 293VDC

Typo?  V1 on yours should probably be around 75-100V.    V2 voltage looks right.

Quote
AC heaters running at about about 7.4V (weird right?.. go easy on em jeeze)


no doubt.  That does seem really high for that filament rating.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: mitsos on October 25, 2014, 10:51:01 PM

I can measure and confirm the DCR for you about the 2092. If you care for it still.. since I have one.
[/quote]

hey man, that'd be cool. Still haven't gotten around to winding an output for my "BA2" so this info would be nice to have.  thanks!
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: buildafriend on December 12, 2014, 08:13:27 PM
I know this is really simple project, but is there any interest in a PCB here?  I might print a few for myself... still deciding on a few design ideas though.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: buildafriend on December 16, 2014, 01:28:40 PM
I just re ordered all my parts for round two. I can't understand why my last build had low gain so it's time to start over and shelf the old parts.

here's a quick BOM. Maybe you guys will spot something weird.



Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: mitsos on December 16, 2014, 02:10:28 PM
hey, I just ordered a couple of gapped 2092s today! 
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: Joechris on March 22, 2015, 10:04:25 AM
Finaly I got around to make a version of this preamp. And it works!! And it sounds very good...big open, lots of detail.
I used a EF37a tube with a 600/100k JS input transformer. Output is a 6SJ7 to a LL 1680/5ma transformer.
Old Siemens 0.5mf PIO cap inbetween. Used a guitar amp chassiss from Tube Town...a bit large but with the power supply on the other end this is dead quiet.  This will be my goto pre for vocals from now on.... Thanks to everybody on this thread for inspiration and help.

J

(http://i354.photobucket.com/albums/r419/joechris3/BA-2c%20EF37a_zpsm0ngb9oi.jpg) (http://s354.photobucket.com/user/joechris3/media/BA-2c%20EF37a_zpsm0ngb9oi.jpg.html)

(http://i354.photobucket.com/albums/r419/joechris3/Ba-2c%20diy_zpssrnqtzqp.jpg) (http://s354.photobucket.com/user/joechris3/media/Ba-2c%20diy_zpssrnqtzqp.jpg.html)
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: My3gger on May 13, 2015, 05:14:44 PM
Hi,
got my BA-2 back home to try different input transformers, i did  many listening tests between them and also run frequency response tests.
Inputs are CMMI-10C:
http://cinemag.biz/mic_input/PDF/CMMI-10C.pdf
and CMMI-10B:
http://cinemag.biz/mic_input/PDF/CMMI-10B.pdf
Output is kind of CM-27101 with gapped iron core and several other differences like max 10mA DC.
Both inputs with this OT have -3dB points at ~35Hz and 28kHz. CMMI 10C has a bit better response between this two points, but this differences are very small and might be caused by my error, although i'm pretty sure i got it right this time.
Both inputs were also measured out of circuit, properly loaded and with 20dB pad (this part is the same as when they were in circuit).
Results are pretty much the same as in datasheets, but i found out 10B's response falls much faster than 10C's under 20Hz. One of more interesting things i don't really understand is that 10C has higher -3dB point at 82kHz, 10B has it much higher at 115kHz.
Higher frequency response can clearly be heard on 10B, highs are more pronounced compared to 10C. Whole picture is kind of shifted to highs, although it still has bass and all, just different. I like the way 10B sounds with this particular combination of trafos, circuit and applications, but would like to understand better why this is so. Looking at older inputs i don't often see response to so high frequency, they are a bit similar to 10C, sound is also less toppy, except for those tiny Beyer.
I also exchange some PMs about this matter and was told modern transformer designers seem to design for much more highs than needed i guess. Measurements and listening tests confirm this to a point, of course i should meassure and listen to many more models to really confirm this. Anyone has any thoughts about it, do you want me to measure some more as i have a day or two more before preamp goes back to the studio? To me it looks a bit like OTs where bigger cores have bigger bass.
Like i said, 10B sound very nice for my needs and i also tried another OT which was a little too clean for what we are doing. I love BA-2, open and lots of detail is very good description and i just started understanding what it can do with different iron, bias, RL, output loading, etc. :)
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: My3gger on May 13, 2015, 05:15:55 PM
Seems i can't add two pics, here is another
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on May 13, 2015, 06:28:56 PM
Your measurements on the bottom don't sound out of line with the RCA catalog spec. 

 For mic pad in and 620 ohm load (best bottom), my original RCA is -3 about 12Hz, -2/16, -1/27.  Worst loading perspective is -3/18, -2/25, -1/43.   That's better than specified by RCA.  I don't have measurements of others to know if mine is better than average or not. 

Top extension:  modern designers are also chasing linear phase which requires linear frequency.  The original has a treble resonance boost around 15K with certain load conditions. 
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: My3gger on May 15, 2015, 02:49:32 PM
Your measurements on the bottom don't sound out of line with the RCA catalog spec. 

 For mic pad in and 620 ohm load (best bottom), my original RCA is -3 about 12Hz, -2/16, -1/27.  Worst loading perspective is -3/18, -2/25, -1/43.   That's better than specified by RCA.  I don't have measurements of others to know if mine is better than average or not. 

Top extension:  modern designers are also chasing linear phase which requires linear frequency.  The original has a treble resonance boost around 15K with certain load conditions.

Thanks, very helpful as are your measurements i have in toolbar for a long time:
http://groupdiy.com/index.php?topic=41032.msg513738#msg513738
I didn't focus so much on OT loading this time, when i did at least two good models with proper driving stages didn't complain about loading (600r-1k). One of better UA converters had ~2k input impedance which looks strange in days of bridging, but this is another thing, it made me think at a time they maybe wanted to get "more sound" this way...
Also checked some Sowter, Jensen and all Cinemag inputs i could find. They seem to confirm your remark about top end extension, many "better" models have phase vs frequency plots and not all of them are on sites. I think we have enough choice this days, limit seem to be experiences with all this new and old iron.

I promised report of new nickel pin EH EF86 after some time in circuit. Now they have year and a half of quite frequent use and a lot of testing (turning on and off, heaters are reg., HT cap. multipliers, micalex sockets). A few samples work well with the same current, no microphonics or changes of any kind, only slight expected change in current in first days or so. One runs as pentode, i think this tells good news anyway.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: gato on October 07, 2015, 03:43:09 PM
Why triode strap a pentode as opposed to using triodes?
Was it for supply reasons? I see that the Ba1a uses 1620s as pentodes (and I assume it was in production first), so maybe this is why? Is there any sonic advantage?
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on October 07, 2015, 04:41:49 PM
Radio Designers Handbook addresses this I believe.  The 1930's RCA use the 77 predecessor in triode 90% of the time, they could have easily gone with a triode. 

It was as well for supply reasons, easier to stock one type. 

BA-1/2/3/4/6 all came out the same time.  BA-1/2/4 all exist in previous incarnations with different model #s in the 80 series. 

No reason not to build a BA-2 with a single 6SN7, really. 
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: vari-mu on October 15, 2015, 02:48:23 PM
No reason not to build a BA-2 with a single 6SN7, really.

I use in my preamps, similar RCA BA-2,
low voltage sub-miniature military tubes 6N28B-V
(Mini 6SN7, I think) :

(http://i59.tinypic.com/dwfaps.jpg)

 Have made   30-40 products different configurations and constructions, mainly amplifier + optical compressor.
Here a two-channel preamp into 1RU :

(http://i58.tinypic.com/t71ngi.jpg)

(http://i62.tinypic.com/ete1jo.jpg)

My basic scheme (similar RCA BA-2) :

(http://i57.tinypic.com/15zqy5i.jpg)

Examples sounding guitar (2009) :
http://moemesto.ru/vari-mu/file/9283267/1%20Pre%20El%20Gtr%20.wav

   Vladimir P.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: studiosteve on December 10, 2015, 03:33:21 PM
Vari-mu
Wow. . . A beautiful build. Have you tried it on vocals yet? I tried to listen to the link you put up of guitar sounds, but it wouldn't play.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on September 08, 2017, 01:13:03 PM
This is the original thread with all answers!  It ain't dead!  If you see that stupid warning about no posts in 120 days, ignore it!  The warning is that the thread needs reviving, not that it should be avoided!
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: burdoto on September 17, 2017, 11:45:15 PM
I built a Ba6a circuit with some not-ideal transformers and the I have pretty large bass roll off. I think the roll off starts around 1k and is eventually -10db at 60Hz. I've bypassed the input transformer and I've bypassed the first tube stage but no change in the rolloff. It could be the output tranny which is a single ended edcor job( GXSE5-15K which I've since read is not great for this purpose).  Any ideas? Output tranny? I also built the op6 circuit similar to the preservation sound guy, which has different output tranny setup, and same bass roll off. What have I done wrong? Any help is appreciated.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on September 18, 2017, 07:51:56 AM
The Edcor spec sounds fine, but I haven't used one.   Have a 620 load R on the output?  Overall it sounds like an output transformer problem in both. 
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: lassoharp on September 18, 2017, 02:44:13 PM
That Edcor looks like it's intended for SE 6BQ5 or 6V6 and just right for flea watt guitar amp duties but the sec is given as a 600r only (no speaker taps). Interesting.

You can possibly get better bottom response from running the output tube at much higher current to lower the source impedance.  Running it at 10 -12 ma may knock it down to 8-9K which is better than the 12-14K it normally runs at. 

You'd probably be looking at a rebuild or separate project but that Edcor OT would be just right for running 6BQ5 or 6V6 triode strapped.  Dave P designed a SE 6BQ5 mic pre and I think the thread and schematic are still up in the archives.

Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: burdoto on September 18, 2017, 04:04:11 PM
Output is strapped with 620 resistor. So it sounds like 6j7 and that OT are sucking my bottom end down?

So I should try a different OT? You know off the top of your head of an affordable option to swap one out?

One thing I'm confused about is preservation sound dude built the OP6 with an BA2A first stage and used an edcor wsm 15k:600 matching transformer and didn't seem to have problems like I'm having. As far I know I duplicated that same circuit and used the same OT. So I built bthe BA2A circuit with the SE OT I mentioned earlier and the hybrid OP6 circuit which the wsm OT, but both circuits exhibit the same low end loss. Makes me think I'm missing something else in the circuit. Is there something else I should be looking for?


Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: lassoharp on September 18, 2017, 05:15:55 PM
I'm pretty sure Cinemag has made at least one SE OT that aspires to work well for a direct coupled output like the BA2.  I would email Cinemag and tell David what you're looking for and see what he has available.   About 4 years ago he was looking into doing a replica with a nickel core but I don't know if they completed that project or not. 
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: kosi on January 02, 2019, 10:08:23 AM
Small question, as I'm in the process of building one:

What's the purpose of the cathode resistors in series ? On the joints between the resistors are connections to the  MI-11599 plug, pin 7&8.

Thanks for any info !

Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on January 02, 2019, 10:11:11 AM
Small question, as I'm in the process of building one:

What's the purpose of the cathode resistors in series ? On the joints between the resistors are connections to the  MI-11599 plug, pin 7&8.

Thanks for any info !

External tube check meter.  You can combine into one R if you like. 
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: kosi on January 05, 2019, 05:39:25 PM
Thanks  EmRR
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: hylaphone on November 12, 2019, 08:54:01 AM
May I bump the thread to ask about this Doug? What does this secondary-cathode connection accomplish?
Thanks --

Not much left to do, beyond using an input transformer closer to the original ratio. Cinemag cmqee-3440a is most appropriate in their line.  50k instead of 15K, buys a few more dB.   

 Bypass the eq (r11).

 Connect the input transformer secondary to v1 cathode through 0.5 mfd or more, rather than ground.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on November 12, 2019, 09:04:15 AM
May I bump the thread to ask about this Doug? What does this secondary-cathode connection accomplish?
Thanks --

Not sure I recall the full context.  Since the cathode is bypassed it would not seem effective.   
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: hylaphone on November 12, 2019, 12:54:44 PM
Thanks --

Has anyone here compared the Cinemag CMMI-10C with that CMQEE-3440A?
I know the turns ratio is closer in the latter..
But I wonder if the former is a better fit for this build, at the expense of a few dB?
Obviously subjective, just looking for some advice..
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: Winston O'Boogie on November 12, 2019, 01:57:27 PM
Has anyone here compared the Cinemag CMMI-10C with that CMQEE-3440A?

Not in an direct comparison.  But, fwiw, bigger core, more signal level handling before saturation, wider bandwidth with the CMMI-10C.    Not quite the same difference as, say,  between a UTC 0-1 and an HA-10 but, you get the idea.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: hylaphone on November 13, 2019, 12:44:42 PM
Thanks guys. Does anyone know the operating level spec (or ballpark) for the original xt-3635 input?
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on November 13, 2019, 12:59:17 PM
Not published and I’ve never made a test, but they weren’t imagining anything over -30dbm at the time.  It surely does much more.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: hylaphone on November 13, 2019, 01:12:40 PM
Would you say that transformer saturation is part of the sound of an original unit?
Or should I be looking at higher headroom iron?

I'm planning a build, and hoping to learn a bit about transformers
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on November 13, 2019, 01:33:59 PM
Doubtful.  The amp distortion is the main signature. 
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: dmp on November 15, 2019, 03:28:35 PM
Would you say that transformer saturation is part of the sound of an original unit?
Or should I be looking at higher headroom iron?

I'm planning a build, and hoping to learn a bit about transformers

The original transformers are big. I don't think they saturated in use.  If you are going to use this as a Studio preamp (i.e. max +4 dB) then headroom isn't a major concern.
Are you asking if the original transformers are important to the sound of the original unit? Probably
The circuit is a no feedback design and has a lot of character. But good transformers are surely in play as well.
If I were to recommend an 'obtainable' input transformer it would be UTC LS-10, as UTC was making RCA transformers, but they are expensive. Size wise physically it is similar. But any 10:1 will work.
The output is more difficult since it has DC. UTC A-25, LS-27, or switch the topology to not have DC in the OPT.
   
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: IanB on May 08, 2020, 06:21:05 AM
About to start a 2 channel build. Gonna stick the 6x4 PSU in another box. I think I’m ready thanks to you guys / this thread. Thank you!
I’ve got 2 Simpson meters I was going to run off a jlm buffer board strapped across the output xlr’s. Sound like a reasonable plan?
I’m using A10s for inputs and the gapped version of the 4:1 Carnhills Colin had made for LA2A builds. I’m going to put a 3 way output cap selector on a Tele switch for fun.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: rockinrob86 on May 13, 2020, 12:09:34 PM
I'm building one at the moment with a Carnhill VTB2290 10k:600 gapped OT.  Hopefully it works out!
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: IanB on May 17, 2020, 10:11:18 AM
I'm building one at the moment with a Carnhill VTB2290 10k:600 gapped OT.  Hopefully it works out!

I think it will work. I used the ungapped Carnhill backwards as input on the B/Z STA build and it sounds awesome! So it should work & sound great here.
Well I’m racing you - ordering cases from Frank this week.
Keep us posted!
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: rockinrob86 on May 21, 2020, 11:38:32 AM
I think it will work. I used the ungapped Carnhill backwards as input on the B/Z STA build and it sounds awesome! So it should work & sound great here.
Well I’m racing you - ordering cases from Frank this week.
Keep us posted!

The race is on!  I've been working on some other projects, but just need to finish wiring this up
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: rockinrob86 on May 28, 2020, 07:53:14 PM
I finished mine this afternoon, sounds great!  I ended up using 5693 tubes with a 6x5 rectifier. 

I have been fighting some hum since I turned it on, and it is currently acceptable - until I put the top on my chassis!  Hum shoots up about 20db with the cover on.  Any ideas?  I may just rack it up and see, but I kind of think it is going to hum in the rack, and worried about it causing other gear to hum as well.

Edit: racked it up.  Hums just like with the cover on :(
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: rockinrob86 on June 10, 2020, 08:54:46 AM
I added mu metal around the output transformer, and it quieted down substantially.  It was around -75db, and at that point izotope dehum takes care of the rest.  I would build it with a different pt next time, or put that one in an outside enclosure.

What I should have been expecting, but was not, was how great this thing sounds with my ribbon mics.  It should be obvious, they were made for each other and it sounds like it! 

I’m probably going to build several of these.  They’re cheap, easy to build and sound great!
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on June 10, 2020, 01:23:03 PM
I added mu metal around the output transformer, and it quieted down substantially.  It was around -75db, and at that point izotope dehum takes care of the rest.  I would build it with a different pt next time, or put that one in an outside enclosure.

What I should have been expecting, but was not, was how great this thing sounds with my ribbon mics.  It should be obvious, they were made for each other and it sounds like it! 

I’m probably going to build several of these.  They’re cheap, easy to build and sound great!


Cool!   Looks good. 

Can you not move that PT to the outside?  It'd be far quieter on the outside of the chassis.

5693 tubes:   I consider mine, and any 1603/1620's scarce resources.  They really only make a difference in the first stage, if at all, so I save mine for that position and use standard 6J7 / 6SJ7 in later slots.   FWIW. 
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: rockinrob86 on June 10, 2020, 05:26:50 PM

Cool!   Looks good. 

Can you not move that PT to the outside?  It'd be far quieter on the outside of the chassis.

5693 tubes:   I consider mine, and any 1603/1620's scarce resources.  They really only make a difference in the first stage, if at all, so I save mine for that position and use standard 6J7 / 6SJ7 in later slots.   FWIW.

I could move it.  I guess I didn't like having the connections only separated from the chassis by the little air gap, but it shouldn't be a real problem looking at it now.    I probably will eventually, but now it's nice to have it to use!

I got the 5693 for pretty cheap on ebay.  I'm considering buying a bunch of them - they work and sound great and I may start substituting 5693/6sj7 and their brethren for 12a types - they are getting expensive!
 
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: IanB on June 11, 2020, 03:48:52 AM
I added mu metal around the output transformer, and it quieted down substantially.  It was around -75db, and at that point izotope dehum takes care of the rest.  I would build it with a different pt next time, or put that one in an outside enclosure.

What I should have been expecting, but was not, was how great this thing sounds with my ribbon mics.  It should be obvious, they were made for each other and it sounds like it! 

I’m probably going to build several of these.  They’re cheap, easy to build and sound great!

Awesome! Still waiting for my cases.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: JW on June 17, 2020, 06:30:08 PM
I wonder if someone can comment on having used a mic to grid transformer like the Jensen JE 115K in this circuit? https://www.jensen-transformers.com/wp-content/uploads/2014/08/jt-115K-e1.pdf

Or similar Cinemag with not as high a step up ratio?
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: lassoharp on June 17, 2020, 08:03:54 PM
I've used the Cinemags a few times for tube pre builds.  They work fine.  Largely transparent. You lose a little gain but I wouldn't consider it a problem.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: rockinrob86 on June 17, 2020, 10:58:13 PM
I wonder if someone can comment on having used a mic to grid transformer like the Jensen JE 115K in this circuit? https://www.jensen-transformers.com/wp-content/uploads/2014/08/jt-115K-e1.pdf

Or similar Cinemag with not as high a step up ratio?

What comment do you want?  I used this one and set the front end up with the resistor like they suggested. 

It sounds great and works well.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: Bird on June 22, 2020, 02:43:23 AM
Hi
Im looking for some UTC M 8366A RCA part 205326 mic transformers for my RCA BC 3 consolette!
Anybody !!

Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: dmp on June 22, 2020, 10:55:05 AM
I added mu metal around the output transformer, and it quieted down substantially.  It was around -75db, and at that point izotope dehum takes care of the rest.  I would build it with a different pt next time, or put that one in an outside enclosure.

What I should have been expecting, but was not, was how great this thing sounds with my ribbon mics.  It should be obvious, they were made for each other and it sounds like it! 

I’m probably going to build several of these.  They’re cheap, easy to build and sound great!

I've built tube preamps with similar layouts and ended up switching the PT to a toroid. Much less hum if you have space inside. The EI PT generates a bunch of magnetic field in your enclosure. So you could move outside or just switch to a cheap Antek toroid and save some headache.
I've found using the conductive metal duct tape with mu  metal shielding works well. You want it to be continuous all around the transformer so the magnetic flux can fully travel around the interior space you are trying to isolate (i.e. Faraday cage).  Any bends should be as gentle as possible as stress reduces the permeability.


I wonder if someone can comment on having used a mic to grid transformer like the Jensen JE 115K in this circuit?

Would sound fantastic. It is a nice input transformer. I used one in a V76 build.  They sell different versions with different levels of shielding. Since this is a relatively low gain preamp you wouldn't need the higher amounts.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: rockinrob86 on June 24, 2020, 11:12:27 AM
I've built tube preamps with similar layouts and ended up switching the PT to a toroid. Much less hum if you have space inside. The EI PT generates a bunch of magnetic field in your enclosure. So you could move outside or just switch to a cheap Antek toroid and save some headache.
I've found using the conductive metal duct tape with mu  metal shielding works well. You want it to be continuous all around the transformer so the magnetic flux can fully travel around the interior space you are trying to isolate (i.e. Faraday cage).  Any bends should be as gentle as possible as stress reduces the permeability.

But I got a good deal on these EI transformers!!! $30 for a pair, and a pair of meters! 

I only spent 3 hours tinkering with the hum, haha.   I will probably move it to the back of the enclose, but for now it is good.  I've used it in some sessions with very pleasing results
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: rockinrob86 on June 29, 2020, 09:06:42 AM
So because of all of your bullying, I finally moved the Power Transformer out of the chassis and to the back.  it is now very quiet!  And it didn't take me more than 15 minutes as I was able to reuse all but one wire from before.

So note to self, keep the unshielded PT outside the chassis
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: rock soderstrom on June 29, 2020, 10:16:04 AM
So because of all of your bullying, I finally moved the Power Transformer out of the chassis and to the back.  it is now very quiet!  And it didn't take me more than 15 minutes as I was able to reuse all but one wire from before.

So note to self, keep the unshielded PT outside the chassis

Like!👍Did you measure your amp? Bandwith, noise or THD? 
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: rockinrob86 on July 02, 2020, 01:10:51 PM
Nope!

I recorded a bit of singing into an sm57, and then switched to a different preamp.  gain matched in the DAW, the noise levels were similar/acceptable, the frequency response was good. 

I compared to a peavey VMP-2.  The RCA is a bit darker/thicker, more colored, but is a very good sound. 

I don't particularly care about the specifics - it is a tool for recording, so if it works, it works! 

After using it some, I would like to build more.  have been considering building a pair in another chassis, or maybe even a quad of them.  These offer a really good value IMO and a cool color to have in the toolbox.  I really like what they do with my dynamic and ribbon mics.  Hoping to get a nice run through with the Tube LDCs soon. 
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: rock soderstrom on July 02, 2020, 01:28:02 PM
Thanks for your feedback!I guess this is my next project.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: IanB on July 12, 2020, 03:17:51 PM
Cases arrived
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: EmRR on July 12, 2020, 03:40:13 PM
Cases arrived

About to start a 2 channel build.
I’ve got 2 Simpson meters I was going to run off a jlm buffer board strapped across the output xlr’s. Sound like a reasonable plan?
I’m using A10s for inputs and the gapped version of the 4:1 Carnhills Colin had made for LA2A builds. I’m going to put a 3 way output cap selector on a Tele switch for fun.

Cases look good!

Yes on the VU plan. 

Not sure what this input attenuator is.   

 I don't think you will find need for the output attenuator either unless you plan on overdriving the whole system on purpose frequently.  I clip electric guitars with my original pretty frequently, and it never needs output control.  I haven't run it into full scale distortion on purpose though to see where it stops, I should do that. 

Are you doing that PCB version that's a modified cap coupled circuit?  If so, that WILL clip earlier, with even lower headroom.  That change to a parallel feed circuit will greatly reduce overall headroom.  It's really worth having DC through an output transformer designed for it.  You mention a gapped output, so I'm confused. 
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: IanB on July 12, 2020, 06:30:54 PM
Cases look good!

Yes on the VU plan. 

Not sure what this input attenuator is.   

 I don't think you will find need for the output attenuator either unless you plan on overdriving the whole system on purpose frequently.  I clip electric guitars with my original pretty frequently, and it never needs output control.  I haven't run it into full scale distortion on purpose though to see where it stops, I should do that. 

Are you doing that PCB version that's a modified cap coupled circuit?  If so, that WILL clip earlier, with even lower headroom.  That change to a parallel feed circuit will greatly reduce overall headroom.  It's really worth having DC through an output transformer designed for it.  You mention a gapped output, so I'm confused.
Hey Doug.
It’s p2p. I’ll post pics as I go. I read here on this thread that noise was a common problem with the power tranny proximity, hence the separate rack box for that.
Re the output tranny, carnhill make a 4:1 gapped that can take the dc as per original schematic. I’ve used this transformer in a few other builds and it works & sounds great. Should be ok here I think.
I know I will prob have the input & output attenuators dimed much of the time but I’d rather have the option if needs arise. Inline pads are a bummer.
I’ll use nice toobs for V1 and the cheaper ones for V2 as per your suggestion.
“American Beauty” is a nod to something you actually said in a thread once. No-feedback American Beauty. Just gotta get it working. Wish me luck.
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: Winston O'Boogie on July 12, 2020, 09:47:02 PM

“American Beauty” is a nod to something you actually said in a thread once. No-feedback American Beauty...

Nice, I like it. 
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: IanB on July 31, 2020, 10:29:31 AM
It flippen works!!!
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: mjrippe on July 31, 2020, 10:41:12 AM
Handsome work there!
Title: Re: RCA Ba2A Pre amp qestions & answers
Post by: IanB on August 02, 2020, 12:22:57 PM
It is dead quiet (thanks to all the amazing knowledge and advice in this thread!). I’m so stoked with it! Very grown-up sounding (my first 2 valve pres so a new thing/sound for me).
Doug is right: with a dynamic & ribbon on vocal all 3 knobs are dimed but with a condenser the attenuators might be useful.
The cheap Chinese VU buffer I got is noisy as! Just ordered the JLM one which should hopefully work a treat.
So thankful for the previous 20 pages of gold that got me over the line so smoothly. Cheers boys!
REDD47 next! PSU is built and works (6x4 & 2 x OA2s = 300vdc). Turrets are done. Just gotta wire it up.