Fender 65 Twin RI Vibro problem

[Aphid]

Hi, I have a 65 twin reissue and there is a nasty problem with the vibro where it cuts in when loud notes are struck then stops as the note dies.

Just to be clear, the amp is set with reverb and vibro OFF. When you strike a note quietly everything is fine. When you strike a note hard the vibro cuts in but then as the note dies it turns itself off.

This problem happens if the footswitch is plugged in or not. Also, I have THREE different footswitches, all of which work perfectly well in a seperate amp.

The speed of the vibro is consistent with the speed set on the amp, e.g. if it's set at 1 and 1 then its not noticable, but when its at 10, 10 its a nightmare. This leads me to believe that its some sort of switching issue.

Tubes: All of the preamp tubes have been replaced.

Help!

Thanks for looking.

[52 Bill]

While anything is possible, I can't imagine anything that could cause the symptoms that you describe.

If you remove the tube that is used by the vibrato circuit, it would make the circuit itself non-operational under any circumstance. So try that and see if the problem remains. If the problem continues after removing the tube, then the problem is somewhere else in the amp. If if goes away, then you will need to investigate further.

[Aphid]

Ok, in the meantime, I stripped the amp and had a look for any dry joints etc... nothing obvious. Put it all back together turned it on... problem solved!

Maybe Bill has a point, perhaps it's just the tube used by the vibro circuit - removing it and then putting it back in has done the trick perhaps. Strangely though the very last thing I did before posting here was to re-seat all of the tubes.

On the one hand I'm relieved that there is no longer a problem. On the other, I'm frustrated becuase I've spent the best part of three hours doing absolutely nothing.

There you go, life can be comical!

I'll see if the problem re-emerges, but in the meantime, thanks for your help!

incidentaly, which tube is responsible for the vibro circuit?

[52 Bill]

...incidentaly, which tube is responsible for the vibro circuit?

The vibrato tube will probably be the second small tube away from the power tubes.

[Enzo]

I don't see how it could be that tubes fault. All the tube does is oscillate.

I would suspect the feature switching circuits are reacting to a high current surge when you hit the loud chord.

That you took the amp apart and reassembled it, curing the problem leaves me further thinking a loss of solid ground somewhere in the circuit. The resistance from circuit to ground allowed some voltage drop to form under current peaks.

That would be my working theory.