Fender 6G4-A Super: simple device with elusive problem

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soapfoot

Well-known member
Joined
Dec 27, 2010
Messages
364
I've had a 1961 Fender 6G4-A Super-Amp for over fifteen years. I've always done routine maintenance--electrolytic caps, basic troubeshooting, etc--to keep it in good working order. Nothing about the design of this amp is (or should be) at all mysterious, and yet it's recently developed a bizarre intermittent symptom whose cause I can't seem to trace.

When plugged into the "normal" channel, once in awhile the amp will start playing much louder than it should. In the fault state, the following conditions are true:
  • Turning the "normal" channel's volume control to zero doesn't change anything--it keeps playing loudly as long as there is input signal.
  • If the tremolo ("vibrato") intensity is turned up on the "vibrato" channel, a tremolo will be audible, even though the instrument is plugged into the "normal" channel only. This is only true when the "abrupt increase in loudness" fault occurs
  • Nevertheless, the "vibrato" channel's volume control doesn't seem to have an impact either.
An enthusiastic "whack" on the top of the cabinet usually sets things right... sometimes for weeks or even months(!) This makes it difficult to troubleshoot. I've taken the following steps so far:
  • Cleaned and re-tensioned all tube sockets
  • Applied pressure to every single solder joint in the amp with a chopstick to try and excite a fault
  • Applied chopstick pressure to every single passive component in the amp to try and excite a fault
  • Ensured continuity to ground for most ground points, particularly those around/near front panel potentiometers and jacks
  • Cleaned and inspected all front and rear panel jacks
  • Checked all schematic voltages to verify that they're nominally close to spec
None of the above has revealed anything at all. The only thing I can think of at this point would be to remove the eyelet board and check for anything stuck there that might cause an intermittent short-circuit (but that would be a considerable undertaking with lots of desoldering and rework). Before I try that, I thought I'd check here in case someone has seen something like this before and has more-specific ideas? Schematic here:





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soapfoot

Well-known member
Joined
Dec 27, 2010
Messages
364
Check the volume controls have good grounds. If the ground end is not connected you could have full volume.
Thanks! This is good advice, and indeed was the very first thing I checked.

After verifying grounds with the meter, I also reflowed all connections to the pot and tightened the nut on the front. It didn't have any effect on the symptom, unfortunately.
 

Disco Volante

Member
Joined
Jun 20, 2021
Messages
12
Location
Sweden
I had the exact same fault on a Quad 33 preamp, until I swapped out the volume pot for a very expensive attenuator, only to have it come back 6 months later. Turns out the threads in the cast body (where the clicking happens) were badly made, which made the tiny screws holding the wafers together come loose. A few fiddly hours and some loctite later, and 33 quit yelling at me;-)

Replace the potmeter, is my suggestion...
 

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