Re: Two amps, one set of speakers ...


"David Nebenzahl" <nobody@xxxxxxxxxxxxxxx> wrote in message
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Karl Uppiano spake thus:

"jakdedert" <jakdedert@xxxxxxxxxxxxx> wrote in message
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Karl Uppiano wrote:

"Pooh Bear" <rabbitsfriendsandrelations@xxxxxxxxxxx> wrote in message
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Karl Uppiano wrote:

"Pooh Bear" <rabbitsfriendsandrelations@xxxxxxxxxxx> wrote in message
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Karl Uppiano wrote:

I cannot recall a single failure that was attributable to
unloaded operation.

These were tube amps.

In which case that's relatively unusual. There can be circuitry
to detect the issue though and prevent it becoming a problem.

That's only because OTL amplifiers behave entirely
differently. If a solid state amp has an output tranny it
would hate unloaded outputs too.

Hint. An unloaded transformer is simply a huge inductor.

An unloaded transformer still has feedback windings that tell
the amplifier what the output is doing.

Not *always* by any means and even so, one that I know
certainly does have this feedback was nevertherless instantly
trashed when someone accidentally disconnected the speaker.

Ok, what is the failure mode then? Because frankly, I'm just not seeing
it.

Listen, you've gotten some pretty good advice already. Either take it or
not. If you don't I think you'll get to see some pretty sparks, or even
get to watch the smoke get let out of your tube amp.

If you do, you won't. Take your choice. IMO, it's like the agnostic who
said that he didn't know if God existed, but he wasn't taking any
chances....

Perhaps you've confused me for the original poster. I am an engineer
trying to understand what mysterious mechanism would cause this problem,
coupled with many examples in my real life experience of it not
happening -- in harsh industrial settings, no less.

My calculations show that the phase shift in the transformer never
exceeds 90 degrees due to impedance changes, and that the overvolt
situation for the ends of a center-tapped primary only approaches 2 x B+.
True, I might be missing something.

If this is a known phenomenon, then there should be articles and white
papers about this at the Audio Engineering Society. If anyone knows of
any such authoritative and scientific analyses, please point me to them.

So far, my money's on you: your real-world empirical experience trumps the
usual Usenet speculation. (Although I do wonder about that other poster
who experienced real-world arcing in the output tubes of his guitar amp
when the speaker got disconnected ...)

I am sure there are situations where this can happen for some reason or
another -- a poor design, for example, with tubes rated for barely more than
B+ (either by design, or replaced with the wrong types). Or an amp that is
only marginally stable under favorable conditions and begins to oscillate
when unloaded. BTW, the "flyback" behavior of an unloaded transformer might
come into play if there is high frequency oscillation.

A well-designed, unconditionally stable amplifier with a proper feedback
design should automatically reduce its drive so that the unloaded output
voltage is practically the same as the loaded output voltage. That leaves
only the poorly designed, unstable amps to blow chunks when unloaded, IMHO.
Sort of Darwinian justice, isn't it?


.

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