Distortion in Twin Reverb

[überfuzz]

I'm trying to locate a gnarly in an amplifier. It's a Twin Reverb that sounds great, but the Vibrato channel has something fishy going on.

Status

1. Normal channel is doing just dandy.
2. Sparky distortion on Vibro channel if volume is over 7.
3. Swopping around the preamp tubes doesn't help
4. Vibro and reverb OFF or ON don't make any difference. Nor do the pots; reverb, speed, intensity.

I'm going to gut it open to morrow and see what I can find. I'd really appreciate help in narrowing it down a bit. Any ideas?

[pdf64]

Which TR model is it?
Does it help with the distortion if V3 reverb driver tube is removed?

[überfuzz]

It's a ab763. Is it possible to just remove the tube and run the amp without it?

[pdf64]

Yes, it doesn't cause any problems / put additional stress onto anything that I can think of.
Also try swapping V1 with V4.
Is this a real BF TR, or a clone?

[überfuzz]

OK, I'll try that. Before I do, and forgive me for being nosy, what conclusions can I make from the result. I really like to know more about Fender circuits... Well this particular amp seems to be a clone, there's no serial number, PT has mojo serial number, etc. But the circuit is ab763.

I haven't really started with this amp as of yet. Well I've done all the standard stuff, like you suggested, swapping preamp tubes.

The first thing I came to think of was a bad solder, but then again it feels strange that it only appears at 7 to MAX on the volume and only one of the channels. Well that is why I address it here. Furthermore, I haven't got a oscilloscope, so I thought I'd just pass it on. I'm helping a friend here and the dude, local tech, with the oscilloscope is bitching and mourning about all the work he has to do...

[pdf64]

The main thing with BF / SF Fenders is that the effects channel has an extra gain stage over the normal channel. It's used for mixing the reverb and dry signals, one of the V4 sections, the other V4 section being used for amplifying the signal from the reverb tank output.
So problems generally appear in the effects channel, as there's more gain.
See
Fender Amp Preamp Tube Layout and recommendations | kcanostubes.com
As there's a lot of gain, lead dress can be a problem. If it's a clone then the lead dress may not be ideal.
I'm wondering if you got parasitics oscillation - if the volume is >7 but treble is low, does that get rid or reduce the problem?

[überfuzz]

I don't get the part were you talk about lead dress. Are we talking about ground shielding all the signal wires..?

I'll try the treble stunt first thing in the morning. Nightfall in Europe.

[überfuzz]

The treble pot didn't do anything for the sparky distortion. However...

I started to poke around in the tremol/reverb. Some groundings weren't great so I resoldered a bit here and there. I took the opportunity to separate leads and signal wires. (Listening to the amp with the volume at 8 - 10 I heard that the distortion appeared after a short while. This made me draw the conclusion that conductivity might have a part in it. Not sure but now the signal wires have more breading air around them...)

Result: The amp plays well at high output. With guitars sending STRONG signals the amp distort in a sparky way, very faint but still, in one of the inputs.

[pdf64]

One possibility is positive feedback around the B+ if the B+ cap/s aren't good (B+ is ground as well, for ac) - try tacking additional caps across them to see if that helps.

I'm sure that you would gain massive benefit from a scope - maybe even a software one. As we're only working in audio, even the most basic spec is fine, checkout the seconhand options. If you have a decent DMM with frequency etc, the scope doesn't need to be accurate / in calibration, all it needs to do is show you the waveform.

[überfuzz]

You're definitely right about the scope. It's like fumbling around in the dark with out it. I'll put it on my list to Santa.