Twin Reverb PI issue, or?? Is caused by guitar input, reset by master vol.

[Travis88]

Hey guys,

I'm trying to figure out an issue in a Twin Reverb (master volume).

When I plug the guitar in and don't play, it's fine, but as soon as I strum a chord, it freaks out and doesn't stop until I turn down the master volume all the way, then turn it back up. The Master resets the issue.

When the issue happens, the tubes flash to the sound pretty intensely, almost like motorboating but really loud. More like a vibrato sounding noise.

I pulled the chassis and did some measure. Voltages appear fine around the power tubes and phase, but as soon as you inject signal (guitar strum), the voltages go haywire on all pins of the phase inverter (except heaters). This happens with power tubes out, etc. The voltages re-stabilize when Master is zeroed, then brought back up, until you strum again.

Any ideas on what to look at? Doesn't seem to be a tube issue. PI and power tubes are good.

Thanks!
-Travis

[Tom Phillips]

...When I plug the guitar in and don't play, it's fine, but as soon as I strum a chord, it freaks out and doesn't stop until I turn down the master volume all the way, then turn it back up. The Master resets the issue...

It's strange that the Master resets the issue. Anyway, my first thought is that there is a bad solder joint, tube socket connection or something inside a component that is sensitive to vibration and once it is set off it is self sustaining. Does the amp freak out if you just thump on the cabinet with your fist? Does it ever do it again after you "re-set" it with the master volume? (Thump the cab again to try to force the failure) I would be giving the circuit a good visual inspection and poking around with a chopstick with the amp turned on to try to locate the problem. CAUTION: Only do this if you are sure you know how to work safely on a powered up circuit.

[Enzo]

A little history first. Has anything been done to this amp? A new output transformer perhaps? Just for fun, disconnect the negative feedback wire from the output jack. ANy change?


How do your main power supply filter caps look? Any bulging ends? Any white goo trying to ooze out the vent holes on the ends?


Don't play through it, but set it up to play, turn the reverb off. Ball up your fist and whack the top of the amp. Does that start it freaking out? Normally it should ignore that. If it started, make it stop and set it up again. Now go in back and flick each tube smartly with your fingernail - like you were flicking a bug off your sleeve. Did thwacking any tubes like that make it happen?

[soundguruman]

Hey guys,

I'm trying to figure out an issue in a Twin Reverb (master volume).

When I plug the guitar in and don't play, it's fine, but as soon as I strum a chord, it freaks out and doesn't stop until I turn down the master volume all the way, then turn it back up. The Master resets the issue.

When the issue happens, the tubes flash to the sound pretty intensely, almost like motorboating but really loud. More like a vibrato sounding noise.

I pulled the chassis and did some measure. Voltages appear fine around the power tubes and phase, but as soon as you inject signal (guitar strum), the voltages go haywire on all pins of the phase inverter (except heaters). This happens with power tubes out, etc. The voltages re-stabilize when Master is zeroed, then brought back up, until you strum again.

Any ideas on what to look at? Doesn't seem to be a tube issue. PI and power tubes are good.

Thanks!
-Travis

When power supply oscillation occurs like that

Probably that one of more filter capacitors have gone bad. I might Replace all the ones in the B+ supplies. Or at least look for the ones that failed.

You could also have bad preamp tubes...dirty jacks and controls.

Capacitors go bad with age or just defects to start with.

You can look at them, under the metal cover (tube side of chassis), and see if one cap popped open.
These things will pop open and leak goo when they get old. That's a visible sign the life cycle has ended.

In newer Fender models, there is still two caps in series for B+ BUT one of the caps is lower voltage.
This I guess was supposed to save money.
But it failed frequently. The lower voltage cap blows, and can cause an oscillation like this....or other problems.
If you fix one of these models, use 2X high voltage caps instead of the 1X low voltage one. Make it like old style twin B+ instead, with 2X same high voltage caps in series...

[Travis88]

Sorry for the month late reply, things have been hectic and haven't been online/working on this amp much.

New discovery is that this noise only presents itself if the Bass pot on the normal channel is turned up to 10... it starts to make the oscillating type of noise around 8 or so, then goes out of control until you turn it all the way back down and 'reset' it. You have to strum the guitar to get it to freak out, and it won't stop until you turn the pot back down.

I tried swapping in the pre tube from the other channel and it didn't change it. The noise also won't happen when that normal channel pre isn't in the amp.

The amp is really only ever used on the Vibrato channel, so the pot being up to 10 on the normal channel wasn't really something that was being paid attention to since it's 'not in use.' When playing on the Vibrato channel, turning the bass pot up on the normal channel causes it to freak out, just in case that wasn't clear. Very strange to me. I replaced the 4 caps in this section (the two .047's, .1, and the 250pF), since I had a bunch of Orange Drops and Silver Mica's around. Those didn't change anything though. All the filter caps/electrolytics look good, all replaced within the past 5 years or so. Everything is very neat, board is clean, LDR was replaced last year because it wasn't working anymore.

Thanks for any input and thanks for all the responses!

[Chuck H]

It could be a cold solder joint. Just use some flux core solder and re flow all the solder joints for that channel. It doesn't take long and it may help.