Amp blowing speaker cabs??

[Tom Phillips]

In my one experience with the Marshall Major mentioned in post #10, when the HF oscillation was fixed the customer's speaker blowing problem was solved. That could have been a coincidence. My main point in post #10 was that I’d check out the head before hooking it to another set of speakers. We may not have all the facts about what happened in the OP’s case. If someone approached me, told that story and then asked if he could hook his amp head to my speaker cab to “see what happens” I would not be inclined to lend my equipment to the experiment.

Based on the information provided I don’t have any other ideas at this time except that I’d also Ohm out the speaker cabs and do the battery test as already described above. I’m hoping that RDK chimes back in with an update.

Tom

[woodyc]

I made a graphic:

So I think we agree a guitar amp can't do the first one?

The second is a more interesting question. My argument is kind of hand-waving, but the reason I think its improbable is this... the oscillations I've seen in amps involve small stray capacitances, leakage inductances and generally high impedances. All you need to do to change the frequency is move your hand near the power stage. So even if you can measure large voltage swings, I don't think these things carry any power. If you put a load on them, they will just disappear. So I would guess that even if by some combination of self capacitance and interwinding capacitance in your output transformer you did have exactly the right conditions for involving the voice coil in a series resonance, the load of the voice coil would just kill the oscillation.

[Enzo]

I defer to greater knowledge of the speakers.

I know this was a tube amp here, but drop the zobel off some SS power amp, and watch it produce full power at above audio freqs. AN unstable amp gone to oscillation is a different thing from parasitics and glitches, wasn't thinking of those. I had in mind an amp that had gone unstable, they do do that. A fairly common repair is replacing the burnt up resistor in a zobel network.

[woodyc]

I defer to greater knowledge of the speakers.

I know this was a tube amp here, but drop the zobel off some SS power amp, and watch it produce full power at above audio freqs. AN unstable amp gone to oscillation is a different thing from parasitics and glitches, wasn't thinking of those. I had in mind an amp that had gone unstable, they do do that. A fairly common repair is replacing the burnt up resistor in a zobel network.

Part of what you bring up is relevant. The sole purpose of a Boucherot cell is to give the amp a load at high frequencies. This is exactly because you can't get any current into the speaker, which is the same as having no load, which would leave a lot of solid state designs susceptible to instability. The same reasoning is behind the improbabilty that RF has ever been responsible for a blown a guitar speaker.