Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: LooseChange on May 29, 2012, 04:57:26 am
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Got a Super Reverb (AB763) that's kicking my butt.
The problem is that when the volume is pushed over 4 the output "farts". Not a stutter. Sounds like a bad speaker or even a bad output transformer. Both confirmed to not be the issue. Checked all tubes and voltages and all is good. Voltages read good and tubes are good.
I'm looking for any ideas.
Thanks!
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Sounds like a lead dress issue to me.
There are two wires that are lifted above the circuit board and run parallel to the board. One of them is longer and terminates at the two resistors where the Normal and Reverb channels are mixed.
That wire, if it has been moved around and gotten bent back down on top of other stuff on the board can cause symptoms exactly as you describe.
Make sure it is where it is supposed to be and if that's not it, start moving other wires around with a chop stick.
Dave
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Is the problem on either channel? Even with reverb off?
Any scratchy pots? I'm wondering if a leaky coupling cap could be the problem.
Dunb question: did you clean and retention the tube sockets? Another: clean & check tension on speaker jack plus plug from speaker wire?
Seems like sometimes a bad solder joint of other connection shows up more with higher current sometimes.
Good luck!
Chip
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Both channels - yes.
No leaky caps anywhere.
All sockets are good to go.
Reflowed many joints.
On my bench through my shop speakers, same issue.
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Hi Dan--maybe somebody modded the feedback circuit--likely too much feedfack. Is the driver tube actually a 12AT7? Good grid stoppers? What happens if you sub a 12AU7 driver for fun.... Jim
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While I am a novice to most tube amps, supers I am very familiar with. You can disconnect the NFB and tape it off. The 2 wires Dave speaks of, I have had that problem. Usually caused when someone is attempting to add trem and reverb to the normal channel. Dave is correct in them having to be up in the air and twisted. If they are in any way causing the problem, if you touch them they will make that farty bass sound.
If that is eliminated and you know they are connected correctly, then I would focus on the filter caps. I have had many that look fine, not to be the case. Question is, did it start all at once or did the amp gradually begin to lose bass control? If it happened all at once, I would be willing to bet someone tried to connect the verb and trem to the normal channel. Also, are you getting any volume increase when turning up the bass as you should. If you jump the channels, are the bass pots independent or does one overtake the other?
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I wonder if a "signal injector" would help you diagnose this?
What I'm thinking is having a source of test signal that you could inject at, say, the phase inverter input to test everything in the amp after that point. If all is good, move the injection point to mid-point of the previous circuit stages and test again; repeat until the trouble location is found.
It seems like a sine oscillator may not have transients that would help "fartiness" be obvious. So it seems like a "perfect" injector would be 2 triode stages of amplification, with an alligator clip-lead for signal and ground (which could be center conductor and shield of the output cable). You'd like to be able to have your signal cable to be tapping a signal after either one or both stages, to vary the signal level. Add a volume control for finer level setting and an input jack to play your guitar through, and you're all set.