Cause of distortion pedal fizz on certain amps?

[MickeyB]

I have 4 different amps that i built, a Champ, Princeton reverb, Vibroverb and JTM45. My tubescreamer sounds good in all except the Princeton. Its very fizzy sounding through it. I swapped around different preamp tubes, tried 6L6's, nothing changes the fizzyness. It has the same exact Weber speaker as the Champ. Could it be the OT?

[kleuck]

Add a 1M5 resistor as a grid stopper on the concertina tube.

[MickeyB]

I did not know about the cathodyne grid stopper deal with this amp until now. Thank you.

[kleuck]

I gave you an all rounder reply, i don't own this amp, but i suppose that using a pedal generates high voltages on grids, and concertina PIs don't like to be overdriven, unless you use a large grid stopper (some on this forum had the same issue with Orange Tiny terror, and a 1M5 grid stopper cured it). Without a concertina, the issue can occur too, when some triode(s) do not have any grid stopper (usually the first has one, only because of RF issues, but very often the other triodes do not have a grid stopper, which is always a great idea when you want to push them)) , adding a 33K one usually works well.

[MickeyB]

I appreciate the suggestions. Thank you.

[pdf64]

Does the fizz happen at all volume levels?

[MickeyB]

Yes. Well, I play it between 1 and 4 on the volume pot and its always a buzz like distortion.

[pdf64]

In that case, I can't see that a grid stopper on the cathodyne will have any impact.
The root cause is more likely to be poor lead dress, resulting in capacitive interaction between amp stages, which may give a high frequency boost (due to bypassing the tone stack/vol control, or positive feedback.
Pete

[Jazz P Bass]

As far as 'poor lead dress' goes, it is helpful to cross parallel running wires at a right angle somewhere in the run.
And keep any power wires away from signal wires.

[kleuck]

In that case, I can't see that a grid stopper on the cathodyne will have any impact. The root cause is more likely to be poor lead dress, resulting in capacitive interaction between amp stages, which may give a high frequency boost (due to bypassing the tone stack/vol control, or positive feedback. Pete

You're right, my bad, i was supposing an overdriven situation only as MickeyB encounter this issue only with his TS. Anyway, a TS CAN be a very fizzy pedal (i don't like stock TS i must say) in any amp that is too much on the "Hi-Fi side" (ie with a large bandpass in the highs) only because the amp is able to reproduce the true fizzy character of a TS (mo only, don't bash me you all TS lovers)

[kleuck]

Depends on the way you use your TS too : remember that it's a powerful tool to push an already pushed amp/preamp, but alone, a very poor overdrive (mo too of course)

[daz]

I use mine as a clean boost with the distortion know all the way down and the volume between full up and about 10:00 depending on the amp settings. I never got that fizziness till recently, but it's not always so i think it's right on the edge and therefore only does it occasionally. I DID change the input grid R to 10k a while back from 68k, could that be it?

[kleuck]

Yep, i changed grid resistor on inputs from 68 to 10K too (as i used a cap across the plate resistor, but the result is not the same) and came back to 33K at least (WITH a cap across the plate resistor).