Modifying valve RCA (UK) tape recorder play back amp to mic pre advice wanted.

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Rob Flinn

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Jun 3, 2004
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Between Sussex, UK & Aude, France.
Hello folks,

I acquired a few of these RCA made in the UK Tape recorder amps.  The one I have been testing so far with my bench PSU seems to function OK, but has limited F resp due to the eq curve which is handled by 2 feedback loops. 

I'm planning to try subbing in some Gardners 1:4 for the input transformer, so the Z isn't quite so low.

Looking at the first loop round V1 could I just remove it altogether since any recommended resistors in the filter network are pretty big values, or should I try a 1M there.

The second filter loopp from the tertiary winding on the o/p transformerlooks like it needs to be there.    Do you think just bypassing the caps & connecting R17 straight to V2 pin 8 would work
 
Nice, that's the first official doc I've seen on anything from that line, I've drawn several out by hand. 

Notice it's more or less a BA-21A with additional front end gain stages before a pot, with lots of value differences and equalized feedback differences.  There are multiple examples of this basic 3 tube variation in various packages, some with a higher Z input primary, some with the same transformer set as a BA-21A. 

I'd start tailoring the second loop more closely toward the BA-21A which is 20dB NFB @ 1K.  You might squeeze another 10dB out of that if overall gain proves too high. 

R4, R9, are sized to maximize gain, they could be lower in value with corresponding enlargement of coupling caps.  C7 is an unnecessary gain boost as well.  R1 won't be needed. 

V1 gain overall is excessive before the gain control.  Mod will be dependent on what you want to achieve.  You can tailor the NFB loop to minimize gain, but you'll build a clean machine.  If you want something with some forgiving breakup, you might want to bypass a stage of V1 entirely.  I would expect parallel V1 stages could exhibit too much Miller effect, though you might get some top back with a small hi boost cathode cap at that same stage. 
 
Rob Flinn said:
Looking at the first loop round V1 could I just remove it altogether since any recommended resistors in the filter network are pretty big values, or should I try a 1M there.
Looks to be some NFB at dc due to R7. Just leaving that in will result in maximum gain which will be too much. I suggest you wire a 68K in series with a 1uG capacitor so its gain is similar to the second stage.
The second filter loopp from the tertiary winding on the o/p transformerlooks like it needs to be there.    Do you think just bypassing the caps & connecting R17 straight to V2 pin 8 would work

I think that would work for stage 2.

Cheers

Ian
 
ruffrecords said:
Looks to be some NFB at dc due to R7. Just leaving that in will result in maximum gain which will be too much. I suggest you wire a 68K in series with a 1uG capacitor so its gain is similar to the second stage.
Cheers

Ian

Just to clarify.  a 1u cap in series with a 68 between pins 1 & 8 of V1 ?
 
emrr said:
Notice it's more or less a BA-21A with additional front end gain stages before a pot, with lots of value differences and equalized feedback differences.  There are multiple examples of this basic 3 tube variation in various packages, some with a higher Z input primary, some with the same transformer set as a BA-21A. 

I think I have some of the BA21A style preamps as well.    I have the attached diagram which I got at the same time which is remarkably close to a BA21A.

I'd start tailoring the second loop more closely toward the BA-21A which is 20dB NFB @ 1K.  You might squeeze another 10dB out of that if overall gain proves too high. 

R4, R9, are sized to maximize gain, they could be lower in value with corresponding enlargement of coupling caps.  C7 is an unnecessary gain boost as well.  R1 won't be needed. 

V1 gain overall is excessive before the gain control.  Mod will be dependent on what you want to achieve.  You can tailor the NFB loop to minimize gain, but you'll build a clean machine.  If you want something with some forgiving breakup, you might want to bypass a stage of V1 entirely.  I would expect parallel V1 stages could exhibit too much Miller effect, though you might get some top back with a small hi boost cathode cap at that same stage.

Thanks Doug I'll give these options a try.  It certainly does have quite a bit of gain in hand ......
 
interesting, another I haven't seen.  Pics of units?  I've seen one other UK built LMI- rack that might be the same product line. 
 
I think this thing is way more gain than you want. From the 30 Ohm input through three cascaded gain-stages and hi-gain power stage.

As you know, the tape play EQ is all slope up to ~~2KHz, flattening above. The input section NFB does that with caps and the 10K trimmers. So we have maybe gain of 5 on top to maybe 200 down low. The second section is, I think, gain of 68K:2.7K, with some caps to trim the last octave up/down, so gain of 26 to the tertiary winding. We do not know the ratios in the OT, there could be gain from the tertiary to the 600/250 outputs.

Here is a sketch of several gains. The lowest gain will have trouble with 27K loading on 12AY7+100K, input overload well under 1V, maybe 0.2V. May be under 10mV at the input winding. The highest gains may be all hiss.
 
The "P/S1 9.17/1" corresponds to 50.5K to 600r primary to secondary.

I would then ass-ume "P/S2 23.4/1" is really the tertiary ratio.

This gives tertiary voltage of 1/2.55 of the voltage at the 600r output. Or 600r winding runs 8dB higher than 600r winding.

Assuming 30r:grid input iron is 30dB voltage gain, total gain may be 116dB!! True, 30r will load-down most mikes, maybe even 20dB, so less than 100dB gain.

If we increase NF around the fist duo, cool, until we get NFB resistor smaller than plate node impedance, where it distorts easy. You should probably increase V1 B+ feed from 75V(!) to ~~200V.

I think you have 60dB from mike to Gain pot at the 85K NFB point.

The output stage probably could be NFB-reduced to near unity, but the bias on V2a would be upset without further changes.

The 30 Ohm input is a big piece of the problem. You hate to throw-out RCA-sourced iron, but this is not a transformer for modern mikes. While the secondary is unloaded and there's no actual resistive Ohms here, the inductance may be less than 150 Ohms reactance until up near 100Hz.
 
PRR said:
The "P/S1 9.17/1" corresponds to 50.5K to 600r primary to secondary.

I would then ass-ume "P/S2 23.4/1" is really the tertiary ratio.

This gives tertiary voltage of 1/2.55 of the voltage at the 600r output. Or 600r winding runs 8dB higher than 600r winding.

Assuming 30r:grid input iron is 30dB voltage gain, total gain may be 116dB!! True, 30r will load-down most mikes, maybe even 20dB, so less than 100dB gain.

If we increase NF around the fist duo, cool, until we get NFB resistor smaller than plate node impedance, where it distorts easy. You should probably increase V1 B+ feed from 75V(!) to ~~200V.

I think you have 60dB from mike to Gain pot at the 85K NFB point.

The output stage probably could be NFB-reduced to near unity, but the bias on V2a would be upset without further changes.

The 30 Ohm input is a big piece of the problem. You hate to throw-out RCA-sourced iron, but this is not a transformer for modern mikes. While the secondary is unloaded and there's no actual resistive Ohms here, the inductance may be less than 150 Ohms reactance until up near 100Hz.

I tried using a 0.3uF with a 56k on the v1 feedback loop, & I slugged the 68k with a 4k7 on the V2 V3 loop.    That definitely tamed the beast a bit.    The F response on the first stage from 100Hz to 20kHz is ....alright(ish), fluctuates by around 3dB total.  But on the output I lose around 14dB by the time we hit 20kHz.  I was looking at the signal caps whcih look like metalmites (not sure if they were around in the States).  They don't look too great so I'm going to try subbing them when I get an opportunity.

I was also thinking that my sig gen may not be massively happy driving the input transformer as is has a balanced 600R output. SO will try subbing a 1:4 transformer in the front end.
 
> my sig gen may not be massively happy driving the input transformer as is has a balanced 600R

That will not be happy, and does pencil to like 100Hz.

As you have way too much level available, pad-out with say 270r+56r+270r to get a ~~50r source at ~ -20dB.
 

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