General Audio Research BA-2C build -- PT spec question

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Shattersignal

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Hi there!  I'm building a few of the new General Audio Research BA-2C clone preamps and I seem to have run into a question that I can't answer on my own.  The build calls for 200vac off the PT, along with a 6.3vac filament voltage.  I can't seem to spec a PT that provides both of these voltages.  Is anyone building off the same PCB and can steer me in the right direction, please?  Also, is it possible that I should spec a higher voltage than 200vac off that PT?  I've used the Hammond 269ax in other builds that were looking for 250vac off the PT, for instance--and that transformer is readily available.

Another question, too--I am also looking to build a stereo unit (in addition to a mono unit) which will have double the current draw.  I think that will put the power rating at 70va.  Can anyone recommend a PT?  Or will I need to use two separate PTs for safe operation?

Thanks a ton!

Here's a link to the build and the BOM is attached:

https://www.general-audio-research.com/ba-2c-pcb/
 

Attachments

  • BA-2C BOM v1.pdf
    1 MB · Views: 50
I'm guessing they've gone to SS diodes and thus the 60VAC reduction in PT secondary voltage.  You can use whatever in that range, and vary the power resistor values to drop the appropriate voltage. 

Note the recommended audio transformer ratios are wrong, and will alter the gain and performance of the preamp.  To be accurate should technically be 200:50K and 50K:500.  You won't find that output ratio available to day, but definitely in your interest to use something higher than a 10K primary. 
 
Yes, it has an SS rectifier--good call on the voltage difference. 

Is it possible that the 10k pri on the OT is designed to reflect the higher input impedance of our modern equipment?  If not--dang!  I already ordered Cinemag iron with those ratios.  What sort of problems will it create if the output stage is loaded too heavily?  I might just build it as spec'd and see how it sounds.  What sort of "issue" should I be watchful for?  Reduced headroom?

Thanks a ton, Doug!
 
emrr said:

I was also planning on using a pair of 1930s UTC A10s on the stereo unit's inputs.  Do you think it's better to use the 200/50k taps or the 50/50k taps, in that case, Doug?  I was figuring I would be using the 50ohm pri winding--are you saying that the 200ohm winding will be better for the BA-2C?

Thanks again, Doug!
 
The output secondary is still going to have to be loaded properly, thus the primary is still 10K to the tube.  Triode 6J7 has a plate resistance of about 10-11K, so it's a matching load which reduces output voltage and increases distortion.  Standard rule is at least 2x plate resistance for load.  BUT.  Without hearing the real thing, you may never know the difference.  Gain will probably be about the same because voltage gain trades off fairly equally with turns ratio changes.  I think Cinemag has a 30K primary SE output in their line-up, that'd be the most similar spec to the original. 

You don't want to use the 50 ohm input winding for mics, it will not be excited properly by 200 ohm mics.  The mic dictates the relationship here. 
 
Ahhh, okay.  I tend to regard transformers mostly in terms of ratios for reflected impedances--but I guess in this regard the "nominal" impedance is to be regarded.  Maybe I'll add a switch at some point, to select the input pri winding per microphone.

Thanks again, Doug.  This is very helpful.
 
emrr said:
Cinemag has a 30K primary SE output in their line-up, that'd be the most similar spec to the original. 

The build specifies use of a 1uf output cap--clearly because there is DC on the plate of V2 and most builders will be spec'ing a non-gapped OT.  The original RCA design specifies a custom OT with a gapped choke (I think) for DC blocking, hence no output capacitor.  Can I eliminate the output cap if I use the gapped Cinemag OT that you referenced?  It's designed for SE output, so that means that it can tolerate the DC, right?  If I can eliminate the output cap, I assume I'll gain some very nice audio fidelity.  Does my thinking here seem correct?  Or am I better off with the output cap and a non-gapped OT? 
 
Ah, they aren't clear about that at all in the BOM or on the site. No schematic shown. Cap coupled will have much lower maximum output. That's not a BA-2 at all. A transformer that can handle the required current with good response will give much greater output level and get much closer to what an RCA BA-2 actually does. There is also a Lundahl output or two that will do the job. You can get away with a UTC A-25; even with the lower ratio the DC coupled version will be superior.
 
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The BA-2 is actually a 1930's circuit, not a 1950's circuit as they state. 

RCA didn't make a tube preamp with resistive output B+ feed and cap decoupling from the transformer that I can think of, other than the OP-6 which is choke parallel feed. 

If you use cap decoupling because of the output transformer you have, you could use the cheap Hammond choke for parallel feed instead of whatever resistor they are suggesting, and get higher output level back.  Would require tuning the coupling cap to the transformer/choke combo with some response tests. 
 
emrr said:
The BA-2 is actually a 1930's circuit, not a 1950's circuit as they state. 

RCA didn't make a tube preamp with resistive output B+ feed and cap decoupling from the transformer that I can think of, other than the OP-6 which is choke parallel feed. 

If I forego the output cap and go with the gapped Cinemag OT, should I also look at changing a resistor value, too?  I know this is difficult without a schematic--I don't even have the PCBs in hand yet, either.  When the PCB arrives, should I look to see if there's also a resistor on the output (part of an RC HPF network), and eliminate that as well as the output cap?  Is that what you mean about a resistor--that the RC network would rob some level from the circuit?

I would definitely like to build the circuit as close to the original RCA design as possible.  I've already contacted Cinemag to see if I can substitute the SE OT in my order--it seems like it will suit the original RCA design much better, as you've said. 

Thanks a ton, Doug!
 
if DC doesn't get to the tube through the transformer, it gets there through a resistor with a cap blocking the path to the transformer.  Probably 50K or so.  You use one, or the other. 
 
emrr said:
if DC doesn't get to the tube through the transformer, it gets there through a resistor with a cap blocking the path to the transformer.  Probably 50K or so.  You use one, or the other.

Ahh, I think I get it now.  I haven't built an amp with a DC coupled OT before.  So I may need to cut a trace and deliver the B+ to one pole of the OT Pri, if the PCB is designed for an output cap.  Will I need an additional resistor?  Or does the DCR of the OT Pri take care of that?  I guess I might need to see a voltage map for an original BA-2c in order to modify this build correctly? 

Thanks again, Doug!
 
Is the Name "g.a.'research" misleading? That circuit could be done point to point-- without term strips or pcbs,so whats the point?
Sorry for trolling... Again
 
OTOH, I agree about the point to point part.  If a project is going to go so far as become product and have a sales site, it should at least be truthful in representation.  This all seems pretty half-cocked, even misleading.
 
Well I wouldn't have tackled the build without a PCB--and since deciding the build the amp, I've learned a lot about the design.  None of that would have happened without this PCB.  All of that seems like an excellent reason for the product.  I suppose they should say that it's based on a BA-2C and perhaps ought to be more forthright about the specific departures from the original design.  The PCB is only $30 and the custom metalwork for me to do a p2p build with chassis-mounted tubes would have added much more than $30 to the BOM, so I still think it's great to have the PCB available.  Clearly, the turn to SS rectification and to the capacitor-coupled OT are cost saving measures for the DIY builder--all nice considerations as long as they would have been clearly stated as such.
 
Was not trying to diss anybody... Only to point out this is a diy forum and that is pretty basic as far as diy circuits go... And the pcb  is lacking support as well as research.
 
I'm trying to build one of these point to point off an old chassis I got for a dollar with a power supply containing  a stancor power transformer and filament transformer and a 5U4 rectifier tube. Wasn't planning on using turret board... it probably isn't necessary?

The circuit actually seems very simple, but I've never actually taken it upon myself to  learn about matching impedances where it concerns transformers and tubes... I'm pretty lost as to deciding a proper input and output audio transformer, but looking at this thread, would the Jensen 115-k-e do for the input or is 150Ω still too little impedance for, say, a U-47?

If anyone has any hard suggestions for the output transformer it would be much appreciated. Don't really care to compromise. I'm willing to spend a bit.
 

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