"Ron(UK)" wrote in message
...
Karl Uppiano wrote:


I already said "I am sure there are situations where this can happen".
What part of that says I don't believe it happens? What I am saying is,
in my own experience, I have never seen this happen even when tube amps
are unloaded as a part of normal operation, even with large signals
applied. In at least one case, I had an oscilloscope attached, and the
output signal changed imperceptibly when the speaker was removed. If the
amp doesn't have feedback around the output stage, then I guess it could
be removed from the gene pool when you disconnect the speaker.

The solution is quite simple... Either, telephone Marshall or Fender or
Hiwatt or any other manufacturer of high powered valve amps, and ask
them, or better still, go out and buy 'say' a 100watt Marshall amp head,
take it home, plug it in without speakers crank it up to 11 and play a few
chords. If it blows up, you can telephone the manufacturers and tell them
that their design is rubbish and demand your money back.

Report back with your findings

I reported my findings with a variety of tube amps used in my experience.
None of them had a problem running without a load, even with large signals
applied. If musical instrument amps have a problem with this, so be it. If
that is the case, then I would expect them to put words to that effect in
their warranty agreement.

Sorry to have wasted so much of your time.

Karl Uppiano wrote:

"Ron(UK)" wrote in message
...
Karl Uppiano wrote:


I already said "I am sure there are situations where this can happen".
What part of that says I don't believe it happens? What I am saying is,
in my own experience, I have never seen this happen even when tube amps
are unloaded as a part of normal operation, even with large signals
applied. In at least one case, I had an oscilloscope attached, and the
output signal changed imperceptibly when the speaker was removed. If the
amp doesn't have feedback around the output stage, then I guess it could
be removed from the gene pool when you disconnect the speaker.

The solution is quite simple... Either, telephone Marshall or Fender or
Hiwatt or any other manufacturer of high powered valve amps, and ask
them, or better still, go out and buy 'say' a 100watt Marshall amp head,
take it home, plug it in without speakers crank it up to 11 and play a few
chords. If it blows up, you can telephone the manufacturers and tell them
that their design is rubbish and demand your money back.

Report back with your findings

I reported my findings with a variety of tube amps used in my experience.

And us likewise.

None of them had a problem running without a load, even with large signals
applied.

How about clipped, when feedback ceases to operate ?

If musical instrument amps have a problem with this, so be it. If
that is the case, then I would expect them to put words to that effect in
their warranty agreement.

But they don't. I expecttthe owner's manual has something though.

Sorry to have wasted so much of your time.

Not wasted as ssuch.

Graham

Karl Uppiano wrote:
"jakdedert" wrote in message
. ..
Karl Uppiano wrote:
"Pooh Bear" wrote in message
...
Karl Uppiano wrote:

"Pooh Bear" wrote in message
...
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.

I'm not an engineer but the guy who had fireworks in his Vox AC10 power
tubes. Here's what I know...

A chassis screw was touching the speaker binding strip, I'm really not
sure if it was shorting one of the speaker taps or if it was causing an
open, but it was pressing up against it very hard. This is how I
diagnosed the problem... when the chassis was removed everything worked
fine, when I screwed it back together I had a problem. I had to do
this 2 or 3 times before I discovered the too-long screw. FYI the AC10
has no negative feedback and is cathode fixed biased push/pull. The
sparks occurred when any input signal was introduced, no sound from the
speakers other than a 60hz hum and distortion crackle, (scary because
quiescent hum is usually 120Hz on this kind of rectifier). The fact
that there was sound probably indicates a short of either the 8 or 16
(outside) ohm taps, and not an open. In either case the secondary was
reflecting some pretty high voltages, I'm not sure if it was shorting
an inside tap or the outside. Normal plate voltage on this amp is low
(around 300v).

This amp still has the death cap installed at the primary but it does
have a grounded plug and the outlet tests correct. I have to get
around to snipping this death cap, someone added a grounded cord at
some point but left the death cap in place. If it was a short to
ground then I suspect that AC on the chassis (via death cap) was
induced into the OPT secondary, effectively making it a step-up
transformer to the EL84 plates. But this is just a guess.

I hope this helps. If you need a schematic I can Email you one, just
email me and I'll send it to you.

In any case, with tubes it's best to have a proper load on OPT
secondary, most every tube amp comes with this warning in the manual.