Magnatone (now very intermittent) crackle

[mhuss]

I have a custom 120 that's got an intermittent crackle. These are 'rat's nest' point-to-point builds.

Crackles with all volumes down. When it was acting up, I did a bunch of chop-sticking and o-scope tracing, and chased it down to one of 6CG7 sections -- then it stopped crackling! I had moved some parts around for better spacing, etc., so that may have played a part. Last night I had it on for ~45min, and it made one brief crackle after about 30 minutes.

The owner is two states away, and I don't want to send it back unsure if it's really 'fixed.' Any suggestions?

This is mostly correct:

[mikeydee77]

Carbon resistors, valves... but when all ideas are running out leaky valve sockets or old tag strip or pcb. Sometimes AM radio can pick up airborne noise and give a clue.

[mhuss]

Did swap out all tubes already. Also tried the 'turn out the lights and look for flashes" bit.

[Jon Snell]

I would replace the anode load resistors in the part of the circuit you mentioned went quiet when you tapped it and check the 100n/820p/2n2 capacitors.
If it was in my shop, I would replace the resistors and capacitors to be sure of fixing it, assuming it isn't a noisy valve of course. Valves should be removed, one at a time to pinpoint where the noise stops, thereby revealing the section with the issue.

[mhuss]

I replaced all of the plate (and larger cathode resistors in the vib section), and now it makes the (very loud) noise about once an hour. I'm loathe to just keep 'shotgunning' all the parts in this amp, but it's very difficult to trouble shootsomething that is so intermittent (but 100% inevitable while sitting on, waiting, always out of reach at the moment it occurs!)

[trobbins]

You could ground the pin 7 grid that connects to 270k and 10M, and if there is no noise after x hours then that may well confirm that the noise comes from the VIB modulation stages (given that you said the noise occurs when the vol pots are at min setting).

Have you checked that that the noise occurs when the VIB foot switch is off (ie. noise is not related to the oscillator circuitry) ?

You could temporarily bypass one or other of the VIB modulation stages (the stage with the varistor pair).

If you had the fault localised to a VIB stage, then maybe freezer spray on individual parts may be a quick faultfinding step to take, as well as contact cleaner on the valve sockets, and swapping valves between stages.

[Pedro Vecino]

When I think that heat is involved in the problem I cover the chassis with a plate. This accelerates the onset of symptoms.

[mhuss]

Following trobbins idea, I moved backwards from the junction he cited, either grounding 0v points, or 'AC grounding' higher voltage points using a 22uf 450v cap.

I found if I ac-ground the anode or cathode of the rightmost 6CG7 triode (on the schematic), the noise still exists, but if I ground the junction of the two varistors connected to those two points, no more noise. Has anyone seen antique varistors make random loud noise like a bad carbon comp? Problem is, I can't just swap out these 'rare as hens teeth' parts with spares I have on hand to check this theory. :\ These are the tubular varistors that look like old-school power resistors; the so-called replacements I've seen online look more like antique MOVs, and at $25/pop I'm not game to "just try it and see."

(By random chance, I have a small drawer full of unlabeled old-school disk varistors, and have no idea how to characterize them.)

Does anybody have a suggestion?

[mhuss]

I replaced the varistors with ones I had on hand, no difference.

I pulled both 6CG7s - the noise is still there. I've replaced all the passive components except the 820p and 2n2 (neither of which I have on hand).

The noise is most pure, waveform-wise, at the point marked A2. Note that there is no input from previous signal stages when the tubes are pulled.

Is it possible the tube socket is the problem? I've seen bad contacts, etc. (and I've tried cleaning and re-tensioning here), but never 'leaky' bakelite or whatever.