motorboating?

[drewl]

Got an old ~ '66 Ampeg Reverborocket that is making a "thumping" sound like a really strong tremolo, except the trem is off!
Pulled the Phase inverter and the problem goes away.

I suppose this is what you'd call motoboating?

Think it's the PI output coupling caps leaking?

[ron vogel]

I have a similar issue, from the research I have done, it is either a coupling cap or a filter cap that is bad or a poor connection.

[drewl]

Yeah, possibly a filter cap.
Amp does work, no audible hum but there does look like some ripple on the output tube screen grids.

[g1]

If it is caused by a filter cap, it is likely one for the preamp stages and will probably not show up as ripple. When the problem disappeared with the phase splitter out, it showed that the problem was somewhere before the power tubes, may be the phase splitter, but could also be anywhere before the phase splitter.
Temporarily clipping a good filter cap across the suspects one at a time may pinpoint one of them.

[Jazz P Bass]

That schematic above is not coming up.
Here are two.
(Enzo's favorite) J Piazza's version & the Ampeg original.

[drewl]

Strange, it isn't showing up.
It's Piazzo's redraw.
I haven't dug into it yet, just a quick look for an estimate, but all the preamp stages looked good, only started oscillating at the PI stage, swapped in a good tube and got the same result.

I'll post wha I find.

[drewl]

Well this is turning into a pain.
I swapped the phase inverter coupling caps, no change.
Jumpered in filter cap to power nodes, no change.

Only if I lift the cap going to the PI input pin 4 does the problem go away.
I changed that cap but it doesn't fix the problem.
I tried another 330pf snubber on pin 4 to ground, no good either.
If I lower pin 4's resistance to ground to 47K it kills the thump but alot of the gain for that side.
Any resistance higher just changes the speed of the thump.

That stage has become an oscillator.
All of the associated resistors in the PI circuit are fairly close to what they should be with slight drift upwards in value.

Any other ideas?

[Jazz P Bass]

One at a time can you pull the preamp tubes?
I would like to know if any tubes before the PI are causing.

Make sure the Intensity control is full off.
I would go so far as to verify that the wiper is grounded when the control is off
And double check the 220K - 220K connection from the wiper. Is that grounded too.

[drewl]

I had done all that.
The problem turned out to be the 510K and 470K coming off the plates feeding the PI pin 4 input.
They drifted high, but were still within 20% tolerance.
I jumpered across the 510K with a 1Meg and the oscillation stopped, so I replaced both and it's working great now.

[Jazz P Bass]

Cool fix.
Glad that you got it.

[drewl]

That's why we post the results, to help others, or ouselves if we forget.