[gary o]

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

[gary o]

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.

[EmRR]

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.

[gary o]

Hi Doug yeah thanks thats what Im thinking...Iv soldered it up now just gonna check it and power up

[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.

[JohnRoberts]

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

[EmRR]

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.

[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 ??

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

[EmRR]

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.

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

Both amps are single ended....?

right

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

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.

[gary o]

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

 

[gary o]

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

[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.

[gary o]

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

[EmRR]

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.

[gary o]

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.

[EmRR]

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.

[gary o]

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

[tardishead]

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???

[tardishead]

bump

[gary o]

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.

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