Weak Reverb in a Gibson Scout GA 17 RVT - diyAudio
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Old 10th December 2016, 10:24 PM   #1
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Default Weak Reverb in a Gibson Scout GA 17 RVT

Iím having an issue with my Gibson Scout amp. The reverb is very weak to non-existent. Iím trying too trouble shoot the issue and figured Iíd 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. Iíve 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?
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File Type: pdf Gibson ga17rvt Scout schematic.pdf (80.8 KB, 11 views)
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Old 10th December 2016, 11:42 PM   #2
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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.
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Old 11th December 2016, 12:46 AM   #3
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> 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.
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Old 11th December 2016, 01:15 AM   #4
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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.
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Old 11th December 2016, 01:27 AM   #5
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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.
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Old 11th December 2016, 01:46 AM   #6
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> 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.
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Old 13th December 2016, 01:25 AM   #7
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Kind of a strange side effect, when I turn the reverb knob to max, the sound gets distorted.
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Old 13th December 2016, 04:44 AM   #8
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> turn the reverb knob to max, the sound gets distorted.

Replace V1a, or the cathode bias under it.

However it is possible this is WHY they put the reverb pot where it is-- if it distorts, don't turn it all the way up.
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