ruffrecords
Well-known member
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
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