Ampeg B18X oscillation. Puzzled by the PI cathode circuit

[pula58]

I am working on an Ampeg B-18X. For all but the smallest output levels the amp breaks into oscillation. It does so even if the 10K negative feedback resistor from the output is removed. Once the oscillation kicks-in even grounding the grid (pin 7) of the phase inverter pentode will not stop it.

what WILL stop the oscillation:
1) big cap on the plate of the pentode in the PI (pin2 of V3), (the low side of the cap connected ot the chassis).

obviously this renders the amp useless, but, it is just an experiment.

2) Big cap (20uF) to the node labeled "F" stops the oscillation

3) Grounding (to chassis) the node labeled "F" fixes the problem.

This node, labeled "F" by Ampeg, I have not seen this before. Why the 0.05uF cap to ground instead of connecting F straight to ground? The way they've got this wired, it looks like the low side of the OT seconday is DC floated.

[VacuumVoodoo]

It's not floated, it's grounded via speaker plug which also shorts that 0.05uF cap - from what I could see on my fuzzy schematic copy. Unplugging the speaker lifts the cathode off ground, puts the triode in cut-off, cuts off any signal that might be present. Sort off preventive measure against blowing the OT in case speaker gets disconnected or blown. I need a clearer schematic copy...
Oscillation: maybe speaker socket & plug need cleaning?

[martin manning]

...

[pula58]

to me it looks as if "F" is grounded only if an ext speaker is used. But the amp should not oscillate if driving only its internal speaker, right? That is, it should not need an extra speaker to operate correctly?

[martin manning]

I believe the intent is that F is lifted from ground if no speaker of any kind is plugged in to protect the output transformer. This likely has nothing to do with the oscillation you are experiencing. How are the filters? If they are very old or one has failed the stages may not be decoupled.

[pula58]

I believe the intent is that F is lifted from ground if no speaker of any kind is plugged in to protect the output transformer. This likely has nothing to do with the oscillation you are experiencing. How are the filters? If they are very old or one has failed the stages may not be decoupled.

Thank sMartin...but..The ampeg (it's a flip top style ampeg) uses a 4-conductor cable that comes out of the back of the amp chassis (it is hardwired internal to the chassis), with a 4-pin connector (looks like an XLR jack, but it has 4 pins) that plugs-into the back of the speaker cab. So ,there is no "main" output jack. Thus, the amp has no way of "knowing" that a speaker is connected or not at the main speaker output. Now, at the ext jack , true, it floats there unless a speaker 1/4" plug is inserted. But, in terms of the amp operating normally, without an ext speaker, using the speaker in the amps cabinet, it looks , to me, that the "F" node is floating.

If I clip "F" to chassis ground the amp works [perfectly, oscillation stopped.

[VacuumVoodoo]

On schematic. Jack is extra speaker socket. The 4 pin XLR-like socket or the 4 wire cable you mention are main speaker connections. It can be external cab or internal one in a combo. Doesn't matter. Now, connect all "F" circles. Connect corresponding numbers 1-1, 2-2 and so on. Without speaker connected F floats. With speaker it's grounded through 2-3 jumper at speaker end.
If that jumper is broken, missing and connector oxidized than all sorts of nasties are invited. Unless yours isn't wired in accordance with schematic.

[martin manning]

In short, F is floating unless the 4-pin plug with the pin2-pin3 jumper is inserted. Accordingly, a speaker plugged into the extension jack will not work unless the 4-pin plug is in. Notice also that the extension speaker jack switches to the 8-ohm OT tap when a speaker is connected there.

When you clip F to ground and the amp works correctly, do you have the 4-pin plug in? If so then I think the issue might indeed be a bad contact or solder connection somewhere in that circuit.

The vibrato circuit is interesting- I can see the major pieces. an LFO and two phase splitters, but I don't know what's in VM-1. Varistors? Anybody know the details?