I’m having an issue with my Gibson Scout amp. The reverb is very weak to non-existent. I’m trying too trouble shoot the issue and figured I’d take some voltage reading first. The reverb is driven by a 12AX7. Just a not, the attached schematic shows the V1 tube as 6EU7, however, the installed tube is a 12AX7 and the pin diagram on the schematic is also for a 12AX7. The schematic shows plate voltages for pins 1 and 6 as 190- and 273-volts, respectively. I measure 165-volts and 242-volts at pins 1 and 6. I also measure a voltage drop of 1.48 volts for the cathode bias resistor from pin 8, but get 0 volts drop across the cathode bias resistor for pin 3. I have another reverb tank from a Hammond organ that does not change the anything. I’ve also swapped out the tube for one that I know is good. I get signal (AC voltage) when I strum the strings on the secondary side of the reverb transformer T-3. Any thoughts?
Something is quite afoul here. The 6EU7 and 12AX7 tubes are NOT pin-compatible at all! Since the rest of the tubes are 6_series, I'd suggest that the pinout designations on the schematic are wrong; as you mentioned, the schematic pin-outs follow a 12AX7. I'd put a 6EU7 in there.
> 190- and 273-volts, respectively. I measure 165-volts and 242-volts
Same-as for tube-work.
> get 0 volts drop
Probable short or mis-wire. Snip that cap, does voltage come back? However the recovery stage will work zero-bias, just distorty.
The thing I do not see: if you put your finger on the pin to the recovery stage grid, you should get a BIG buzz from the speaker. That is a HIGH gain path, will amplify all the buzz in the air. Likely failure is C9. Also S3 (pedal?) and R15.
Same-as for tube-work.
> get 0 volts drop
Probable short or mis-wire. Snip that cap, does voltage come back? However the recovery stage will work zero-bias, just distorty.
The thing I do not see: if you put your finger on the pin to the recovery stage grid, you should get a BIG buzz from the speaker. That is a HIGH gain path, will amplify all the buzz in the air. Likely failure is C9. Also S3 (pedal?) and R15.
Sorry if I wasn't clear. The text in the schematic says V1 is a 6EU7, but the pin out is for a 12AX7, which is the tube installed in the amp. I agree, if you plugged a 6EU7, bad things would probably happen.
When i pull out the RCA input jack from the tank and touch the male end, I get buzz, Doing the same to the output, I get nothing, which makes sense. With everything connected, when you depress the foot pedal switch, you can hear reverb engage, I get some hum (thinking bad ground) and shaking the tank makes noise, but, when you twist the knob, from min to max, you hear very little change.
> I get signal (AC voltage) when I strum the strings on the secondary side of the reverb transformer T-3.
> when you twist the knob, from min to max, you hear very little change.
So when you twist the knob, does the secondary signal change?
Turn the main Volume full-down. Adapt a speaker to T3 secondary. This "is" a speaker driver. Very puny, like 0.1 Watts. But it should speak clearly at conversational volume, and should turn-down smoothly.
> when you twist the knob, from min to max, you hear very little change.
So when you twist the knob, does the secondary signal change?
Turn the main Volume full-down. Adapt a speaker to T3 secondary. This "is" a speaker driver. Very puny, like 0.1 Watts. But it should speak clearly at conversational volume, and should turn-down smoothly.
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