noulou

Re: RCA BA-6A problem
« Reply #20 on: July 12, 2018, 07:27:01 AM »
What about it?   I'm having trouble imagining a transformer problem that wouldn't also present as an obvious response problem.

No idea. It sounded fine when not compressing.  Just impossible to get 2 tubes to match. I had tried around 20 different 6sk7s and their combinations.
I have that interstage transformer here somewhere. I have been planning to test it when I have the time.



scott2000

Re: RCA BA-6A problem
« Reply #21 on: July 12, 2018, 08:22:24 AM »
Make sure that the voltage regulator tube is operating properly (OD3) It sets the 'threshold' voltage at the GR diodes. I mention it only because it wasn't mentioned earlier, and it's important to the GR process.

This makes sense???

« Last Edit: July 12, 2018, 10:08:17 AM by scott2000 »
"Give me a 25-stack of 9V batts, I'll monitor my truck's spark or fuel injection on the magic eye." PRR

Re: RCA BA-6A problem
« Reply #22 on: July 12, 2018, 01:49:15 PM »
Yes the 6SK7s balance up fine. I could even get them to match almost spot on with DMMs.

Interesting about the interstage. This unit also passes signal fine below threshold. I will try 500Hz and 1KHz when I get to it today. I also have 2 new NOS pairs of 6SK7s to try out.

I tried several OD3s (VR-150s) and no luck.

Re: RCA BA-6A problem
« Reply #23 on: July 12, 2018, 01:51:52 PM »
I'm also going to look into C20.

Re: RCA BA-6A problem
« Reply #24 on: July 12, 2018, 01:53:05 PM »
I wonder if any of the connected heater/hum circuit has any bearing here?

Re: RCA BA-6A problem
« Reply #25 on: July 12, 2018, 01:56:13 PM »
I'm also going to take some DCR measurements of the interstage and compare windings.

Re: RCA BA-6A problem
« Reply #26 on: July 12, 2018, 02:03:31 PM »
On the advice of another tech friend who's helping on this, I added .47uF to C1 and it still clamped down hard, but after that, it was reducing gain a few dB. If I clicked another input level, it dumped again but came back and seemed to provide proper GR. I am thinking about trying the 1M pot on after the 6H6 anodes and see if it can be dialed down.

Re: RCA BA-6A problem
« Reply #27 on: July 12, 2018, 02:55:44 PM »
New 6SK7s, no improvement.

David Kulka

Re: RCA BA-6A problem
« Reply #28 on: July 12, 2018, 03:32:52 PM »
Sometimes when working on these type problems I'll disable the sidechain section, connect a DC bench supply in its place, and  try using that to control audio level through the vari mu stage. Doing that may tell you whether your issue is in the sidechain or in the vari mu section.

CJ

Re: RCA BA-6A problem
« Reply #29 on: July 12, 2018, 04:33:45 PM »
send the transformer and we will "figure it out",  :D
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html


Re: RCA BA-6A problem
« Reply #30 on: July 13, 2018, 02:30:21 PM »
Thanks David. That is a good next step.

Interestingly enough, it is now partially working. It still thumps down hard, but it returns and provides GR. If I click some more input, it shuts down to zero current (according to meter on V1, V2) but comes back to more compression,  and the output signal is staying limited. When I come back down, it un compresses as you would expect until the signal drops below threshold.

2 things changed yesterday - I put in new pair of 6SK7 and calibrated up with a DMM and I rearranged  the wiring a little thinking lead dress oscillation. Something has changed and it might not even be related to those two things, as experience will me. I need to look more into this and go over voltages again and such. Also I have a couple more 6SK7 combos I can try. One of the NOS ones I got was shorted. :( Back for refund.

I did some measuring of the IT primary and it looks good at about 1.2K a side and 2.4K total.

My father taught me a lot about persistence ;)

Thanks everyone for your time.

EmRR

Re: RCA BA-6A problem
« Reply #31 on: July 13, 2018, 02:41:27 PM »
Schem not in front of me, but recall input Daven between trans and grids, bias dc across it.  Worth disconnecting and measuring at all positions, compare side to side.  Likewise check that input secondary is all there. 
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde


 

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