RCA Ba2A Pre amp qestions & answers

[gary o]

Not there...but found, its a 9040 5:1 ratio thats all I can find at moment...il try again later

 

[emrr]

should work fine, but is somewhat overkill. I'd save an output with that sort of level handling for a better suited project.

 

[rafafredd]

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?

 

[tardishead]

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??

 

[gary o]

Rafa

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

 

[emrr]

[quote author="tardishead"]

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.
[/quote]
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.

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.

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.

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

 

[tardishead]

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??

 

[emrr]

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

 

[tardishead]

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?

 

[gary o]

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

 

[emrr]

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.

 

[tardishead]

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.

 

[tardishead]

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.

 

[emrr]

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.

 

[tardishead]

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.

 

[emrr]

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.

 

[tardishead]

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.

 

[emrr]

running the B+ down is easier that re-biasing the cathode, no? Answer in a few seconds. keep us posted.

 

[tardishead]

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?

 

[tardishead]

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.