In article , tony sayer

wrote:
In any case, the speaker itself was protected, because triacs
invariably fail short.


Quite the case!.

Erm... I have the feeling that people may be reading too much into an
anecdote. The conversation I reported was a casual one we had at a show.
Not a serious analysis we'd have presented at an engineering meeting. I
just thought it amusing as a recollection. :-)

That said, I suspect most users would have concluded that the speaker was
damaged if the actual failure was a welded-short triac that then stopped
the speaker from functioning.

The amp fuses would probably have blown before the output devices.
However...

You have set me wondering, though, what then would happen when/if the amp
went on driving very high steady currents into such a near short. But it
isn't an experiment I'm volunteering to try. I've certainly blown up power
transistors during bench tests when the power was continued after secondary
breadown. The case tended to fly off. Just a matter of how much current,
for how long. ;-

Jim

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