Sawtooth output problem

I have a problem with one half a a twin line amp (TLA) tube preamp. The PCB as usual contains two identical preamps. One works fine. The other when fed with a sine wave input produces a lower level saw tooth output. Visually the saw tooth rise time looks longer than the fall time. Here is the schematic:



The only difference between the schematic and the PCB in this case is the positions of the 100uF cap and the variable resistor are swapped (so the gain setting resistor is ground referenced) and the variable resistor is replace by a 1Meg resistor. So the overall can should be 6dB. I am feeding a +8dBu 1KHz sine wave into the 12AX7. I see about 24V pp at the 12AX7 anode and just under 6V pp at its cathode both of which look right for the stage with no NFB. The output of the 6922 is a 2V pp saw tooth.

My first thought was there is no NFB but I checked all the components and tracking and it is definitely connected. I have swapped tubes but it made no difference. It is definitely a capacitive effect because as you reduce frequency the saw tooth amplitude increases. I have also checked the 12AX7 plate signal reaches the grid of the bottom half of the 6922. I have removed the output cap in case there was an accidental short to ground but again no difference.

Thoughts?

Cheers

Ian

I would think you have checked all the usual suspects, shorts, contaminated sockets, swapped tubes...
Are the DC voltages correct?

I finally found it. Disconnected NFB and output stage worked fine. Turned out to be wrong value NFB resistor (470K instead of 47K). Works fine now.

Cheers

Ian

I once put a 220 ohm resistor in place of a 220pf cap ,took lots of headscratching before I realised what was wrong .

That is one of the benefits of debugging production builds that later failed. They are likely to have correct values.

Back in the 70's/80's I offered flat fee repair service for my kits, and saw some remarkable errors. One customer inserted the parts into the wrong side of the PCB. I sent him a new bare PCB with new parts and told him to try again.  8)

JR