Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: ChubeRules on November 25, 2019, 04:18:02 pm
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I haven't seen any other posts with this particular issue, forgive me if it's here but I missed it.
I've got a '74 Fender Twin Reverb on my bench with the following problem. (BTW the MV has been removed from the circuit).
With the amp fully powered up, not in standby, and with the volume all the way down, the amp is quiet except for the usual slight hum/hiss coming from the speakers.
Turning either or both channel volumes up to just about one on the dial an aggressive buzz is introduced. The higher the level, the louder the buzz. The tone controls add/subtract the expected tone contours to the buzz. Sounds much like when the cable is unplugged from the guitar. I've checked the input jack wiring and can't find any grounding issues.
Inside the chassis chopsticking doesn't give any clues, but if I pinch one of the shielded input cables, that run from the input jacks to the preamp tubes, either of the two connected to each channel, OR, touch with my finger either of the coupling caps immediately exiting either channel's preamp tube, the buzz is completely squelched. Touching either channel's coupling cap removes the buzz from both channels.
A guitar can be plugged in and the amp sounds great.
Any thoughts would be much appreciated.
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the buzz is completely squelched. Touching either channel's coupling cap removes the buzz from both channels.
repair work is a time vs cost, replace n see, lots less TIME which is the $$ saved by a happy customer
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> if I pinch one of the shielded input cables, that run from the input jacks to the preamp tubes, either of the two connected to each channel, OR, touch with my finger either of the coupling caps immediately exiting either channel's preamp tube, the buzz is completely squelched. Touching either channel's coupling cap removes the buzz from both channels.
Missing/broken ground.
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Buzz may be a symptom of the heater circuit being unbalanced / unreferenced to circuit common. eg blown 100 ohm resistor/s usually from a power tube short.
A quick simple check to rule that out.