Oscillation problem

[catstrat]

Before I try to post a schematic I have a general question. What will cause a 12ax7 tube to oscillate on it's own? The oscillation shows up at pin one plate of V 1 of an Engl power ball. I call it V 1 it's the tube closest to the input Jack. I scoped it and there is no signal at the grid 2 but the oscillation is present at pin one. It's only on channel two high gain it does this. Tubes are not micro phonic that I can see. I've already swapped them out.Nothing is plugged into the input Jack. Thanks.

[52 Bill]

In really general terms, almost any tube will oscillate when a positive output signal re-enters the input. This can be through the power supply lines or through the air at radio frequencies or through a bad component, just to name a few things.

So at the high gain levels of an input tube, bad lead dress (routing of the wires), a bad filter cap, a poorly designed pc board, an off value or leaking component, etc, etc.

[Enzo]

Ground the grid, does it still osc? Scope the B+ atop the plate resistor, is the oscilation riding on the B+?

It doesn't oscillate on its own, it is in a circuit. I bet if you disconnect the coupling cap to the next stage it will stop.

[Gingertube]

Wisdom of the ages:
From way back when (also in recent Morgan Jones "Valve Amplifiers" 4th Edition):
Every triode should have a grid stop resistor with the resistor body hard up against the tube socket pin, despite the fact that a lot of guitar amps ommit them .
The value of the grid stop resistor for a triode should be at least 8 divided by the triodes gm.
For 12AX7:
Book Value gm is 1,6mA/V
Worn out tube replacement gm is <70% of new gm so 1.1mA/V
8 / 0.0011 = 7.14K
So really every 12AX7 triode should have a grid stop of 7K5 minimum although I've been using the 8/0.0016 = 5K calculation and fitting 5K1 grid stops. Why? COZ modern new 12AX7 typically have gm of 2.2mA/V not the 1.6mA/V book value.

If the grid stop does not fix the oscillation then there is another problem. Enzo has indicated what to check.

For info to show how these things can be a bear to find/fix.
I recently has a Fender SuperSonic which whistled at me when either Gain1 and Gain2 (Burn Channel) was turned passed 80%. Fitting 10K grid stops to the 3 triodes in the "Burn" Channel (amd tidying the wiring) made a rodents hind quarters difference so I went looking for other problems. It's problem turned at to be bad design. It has 3 cascaded gain stages running off a single power supply node. This was causing a whistle which in this case was a form of motorboating (postive feedback via the power supply line) rather than parasitic oscillation. Fixed by adding a 470nF/630V polypropylene cap across that power supply node. Well it did'nt fix the design fault but fixed the whistling.

Cheers,
Ian

[catstrat]

Oscillation

Thanks for the advice. Since the oscillation is occurring at pin one plate of the circuit,I'm looking at the filter cap. There is no signal at all entering the tube through the grid. This amp has a ten micro fared 450 volt filter cap for that plate only. It has other filter caps in the power supply. I have never seen though a faulty filter cap cause anything other than a hum. According to the read out on my scope,if I remember, the noise is like a 500 hertz signal. I did temperaraly connect an alternate cap to the plate and the noise went away. I have the correct values in stock. This amp has the board where all components are on the under side so you have to remove the board to get to the component side. If I replaced the 10 micro with a 22 would that affect the tone? But again, I may be barking up the wrong tree altogether. If this doesn't fix it then I'll look elsewhere. Thanks.

[pdf64]

Since the oscillation is occurring at pin one plate of the circuit,.. If I replaced the 10 micro with a 22 would that affect the tone?

You've moved from the general to the specific, so it would be helpful to know what the amp / schematic / layout is.

[52 Bill]

I have never seen though a faulty filter cap cause anything other than a hum.

Filter caps are used in the early stages of the power supply for smoothing/filtering (hum removal). By the time the power supply is reduced and filtered to the preamp stages, there is little hum left to remove. The main purpose of the power supply caps at that point is for decoupling.

[catstrat]

Oscillation

I replaced the filter cap for that stage and that fixed it. I used a 12 mf instead of the 10 mf. The amp itself is an Engl Powerball. I'll say one thing, on hi gain channel it's one hissy son of a gun. Most hi gain amps are hissy anyway but this one,I'll have to look into that. May be just the design. My Oranges are pretty quite. Thank everyone.