No squealing with no guitar, but squeals at 4-6 with guitar

[titser_marco]

So I thought I had already sorted out the parasitic oscillation problem, but apparently I haven't. There wasn't any more squealing when controls were full on last night with NOTHING plugged so I thought it was OK.

So tonight I decided to try and dime it with a guitar on and to my surprise, it was squealing at around 4-5 on the MV with everything else (incl OD trim) dimed. I tried moving wires around to no avail. What could be the problem?

[Stevem]

Does it still do that with the guitars volume control down?

[titser_marco]

Does it still do that with the guitars volume control down?

Hmm, am out of the house now but earlier I remember fiddling with the control knob and it was changing the squeal's frequency. I'll try again later. Is this parasitic oscillaion if this is indeed the case?

[Stevem]

Very likely , and if so plugging the guitar in pins down the first gain stage as being one of the two stages involved in the issue.

[Leo_Gnardo]

Does it still do that with the guitars volume control down?

Hmm, am out of the house now but earlier I remember fiddling with the control knob and it was changing the squeal's frequency. I'll try again later. Is this parasitic oscillaion if this is indeed the case?

Your guitar's pickups may also be at fault. Loose windings, metal covers, they all like to squeek & whistle at volume. Having the pickups facing right at the speaker can bring it on when volumes are dialed up. If you simply turn around with guitar facing away from speaker, or press finger on the pickup cover and squeals stop, there's your problem.

[titser_marco]

So, I plugged it in and it was there. Did some preamp tube rolling and now the squeal is gone even with the guitar plugged in.

I think this should be OK for now except for some hiss on the clean (and ergo on the OD section) when controls are maxed out. I guess my question now is whether parasitic oscillation could be caused by bad tubes or is it strictly a lead dress issue?

[Firestorm]

That sounds more like a microphonic tube. In my experience, parasitics don't squeal as much as make other bad sounds.

[pdf64]

Yes, squeal is free running oscillation. Parasitic oscillation is a term generally used to describe oscillation that only manifests when a signal, possibly of certain amplitude / frequency, is present.

[titser_marco]

That sounds more like a microphonic tube. In my experience, parasitics don't squeal as much as make other bad sounds."

I see. What sort of "bad sounds" are these?

Yes, squeal is free running oscillation. Parasitic oscillation is a term generally used to describe oscillation that only manifests when a signal, possibly of certain amplitude / frequency, is present.

So wait, do you mean to say that the amp indeed has parasitic oscillation since it only squeals with a guitar on (and therefore has at least some signal going through it?

[pdf64]

do you mean to say that the amp indeed has parasitic oscillation since it only squeals with a guitar on (and therefore has at least some signal going through it?

No, amplify anything enough and a tiny residual signal may become apparent, eg EM line hum, radiation from the big bang.
It seems to be stretching things beyond what's reasonable, eg it may be a contradiction in terms to regard background noise as signal.

When the relative expected background noise has been accounted for, if there's an output wave apparent without there being an input wave, then there's free running oscillation.

[Stevem]

Poor connections from tube pin to tube socket grippers make for high resistance , and high resistance a act like a antenna to kick off osscilation issues.
Note that on many new preamp tubes they cheap out and the pins are smaller in diameter then days of yore, so if your socket grippers are not in tip top shape you will have potencial issues like this even with the shinny new pins of a new tube, no less older tubes with dull tarnished over pins !

Hows that for a run on sentence?

[Andy Le Blanc]

a flakey tube socket and connection can induce parasitic capacitance, high frequency coupling.

[titser_marco]

Crap, I thought I had solved this. Apparently not.

I tried diming everything again except the master volume with a guitar on and even at 2-3 the amp still squeals in OD mode, with the guitar volume and clean channel controls affecting the pitch.

I'm starting to suspect that it might be the preamp tube sockets. How do I determine if that is indeed the problem?

[pdf64]

Anything is possible but unless there's reason to suspect the tube sockets, it's way more likely to be input-output coupling due to eg imperfect layout, lead dress.
Schematic and photos of its implementation?

[Firestorm]

What I always have to do is go through the layout and mentally label each lead as "0" or "1" to confirm their phase relationships. Phase isn't "absolute" but it's a place to start. A high signal lead in phase and in close proximity with a prior low signal lead is usually trouble. What constitutes close proximity varies, of course.