Twin reverb sparky distortion

[überfuzz]

I have a Twin reverb that distorts in a bad way, it sounds sparky, if the settings are as follows:

Treble higher that 0 - 1 and the volume close to max, somewhere over 9.5.

I thought I'd check the groundings around and on the treble pot. Is there something else I should consider checking? I'm well aware of the fact that a Twin Reverb seldom needs to be pushed to max. I just like to fix this.

[Euthymia]

Not sure what "sparky" sounds like.

Could it be grid blocking? Does it happen on both the Normal and Vibrato channels?

[andyjazz]

You run a Fender Twin with the volume at 9.5?

[überfuzz]

You run a Fender Twin with the volume at 9.5?

Well, mine doesn't go to 11. :-(

I'm well aware of the fact that a Twin Reverb seldom needs to be pushed to max. I just like to fix this.

[Chuck H]

Very different amps... But looking at the scope of full OD in most of my builds it's clear that there are high voltage spikes to a greater or lesser degree on the tailing end of each cycle. This can be audible or not depending on the OT and speakers used. When audible it sounds like spikey treble, especially on note attack.

I expect this is the case with most any amp and could probably be considered "normal". The phenomenon wasn't that audible in my amps. But I did fine it up a bit with a filter across the OT primary consisting of a big resistor (capable of high voltage) rated for about half the amps wattage at 1.3 X the primary impedance in value in series with a 1kV 1500pf cap. For a Twin Reverb that would be a 50 watt resistor of about 5.5k. Probably one of those aluminum housed jobbies since I think they're rated for 1kV.

It is more likely that at full tilt the amp has a minor parasitic oscillation. You might try moving some leads around. Possibly the ground scheme. And as a last ditch effort some small value caps to remove the very high end component from circuits where it seems to help.

Chuck

[überfuzz]

Chuck - Thank you for a fleshy answer. My first bet was the ground scheme and (or) capacitance between wires - chassis. It seemed to help a bit, so it might be the case. I'll try to poke around a bit more to see if I can nail it.

However, I just noticed that if I run my own guitars with moderate output through it the unpleasant distortion is gone, (Rickenbacker 330). But with a higher output it's there, (Hagstrom Viking).

[Chuck H]

Anything at the input gets amplified. More in = more out. Perhaps only a more square wave but it still applies. Make sure the NFB lead is pushed away from preamp leads, OT primary/output leads and grid leads are as short as practical and far away from each other. Make sure any OT leads that run inside the chassis are twisted together. Try to raise grid leads off the chassis where possible. Make sure the preamp grounds aren't on the same spot as the output and PI. As a last ditch effort you can try isolating the input and output jacks from the chassis. As a last. last ditch effort you can put a 4.7pf/500V+ cap on each input triode from the plate pin to the grid pin. Barely audible but almost guaranteed to stop oscillations.

Chuck