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In article , Nick Gorham
wrote:
Jim Lesurf wrote:
In article , Nick Gorham
wrote:

Rob wrote:


It strikes me that the valve amp is the problem here - SS amps don't
need a dummy load.


Don't bother if the amp has triodes on the output, its only Pentodes
that will get upset with no load.


Can you explain why? Afraid I don't know why an amp built with triodes
would be inherently incapable of becoming unstable when unloaded.


No reason why it shouldn't become unstable,

OK...

but the fact remains that a triode is happy with a choke load, which the
transformer (ignoring OTL's) will become when unloaded. The pentode will
not however like that load and you can end up with high dv/dt that could
cause problems in the output TX.

Again, I am not sure I understand. I'd expect a real valve power amplifier
not to consist of just a triode or a pentode, and may well have global
and/or local feedback. Hence I'd expect either kind of design to perhaps
show instability with an output o/c. I'd also expect in either case that
this - if it occurs - might lead to some kind of failure or other problem.
Thus I'd assume this is a matter of the detail of design and construction,
for both a design using triodes and one using pentodes. Is this not the
case?

My curiousity is to why we can assume if the amp uses triodes, and know
nothing else about it, we can assume it has no o/c load problems, but a
pentode based design might.

Slainte,

Jim

--
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Barbirolli Soc. http://www.st-and.demon.co.uk/JBSoc/JBSoc.html

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Nick Gorham wrote:
Keith G wrote:
wrote in message
ups.com...

Nick Gorham wrote:

Jim Lesurf wrote:

In article , Nick Gorham
wrote:


Rob wrote:


It strikes me that the valve amp is the problem here - SS amps don't
need a dummy load. Really, your suggestion is fine. I'm just
trying to
factor in certain extreme scenarios - say, someone else using the
stereo.




Don't bother if the amp has triodes on the output, its only Pentodes
that will get upset with no load.


Can you explain why? Afraid I don't know why an amp built with triodes
would be inherently incapable of becoming unstable when unloaded.


No reason why it shouldn't become unstable, but the fact remains that a
triode is happy with a choke load, which the transformer (ignoring
OTL's) will become when unloaded. The pentode will not however like
that
load and you can end up with high dv/dt that could cause problems in
the
output TX.


Phew! Sorry Nick/Jim, didn't get much of that. Do you think this amp:

http://www.beardaudio.com/p100-505broc.pdf

(the power amp only, obviously) would be safe switched on and not
connected to speakers?


No, I would not trust that without a load, I would guess its a pentode
or UL output stage.


Thanks K&N - good, I won't do it! I think dangly wires or ye olde
switchboard/banana plugs for me :-)

Rob

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Jim Lesurf wrote:
In article , Nick Gorham
wrote:

Jim Lesurf wrote:

In article , Nick Gorham
wrote:


Rob wrote:


It strikes me that the valve amp is the problem here - SS amps don't
need a dummy load.


Don't bother if the amp has triodes on the output, its only Pentodes
that will get upset with no load.


Can you explain why? Afraid I don't know why an amp built with triodes
would be inherently incapable of becoming unstable when unloaded.



No reason why it shouldn't become unstable,


OK...


but the fact remains that a triode is happy with a choke load, which the
transformer (ignoring OTL's) will become when unloaded. The pentode will
not however like that load and you can end up with high dv/dt that could
cause problems in the output TX.


Again, I am not sure I understand. I'd expect a real valve power amplifier
not to consist of just a triode or a pentode, and may well have global
and/or local feedback. Hence I'd expect either kind of design to perhaps
show instability with an output o/c. I'd also expect in either case that
this - if it occurs - might lead to some kind of failure or other problem.
Thus I'd assume this is a matter of the detail of design and construction,
for both a design using triodes and one using pentodes. Is this not the
case?

OK, so we start by assuming the design is of good quality, and that its
not going to break just because its being asked to deliver full power
output (which I would suggest is the worst case that instability could
provide). Given that, my point was purly that a triode is happy working
into a high load impedance, a horizontal load line, in fact its at its
best in some ways like this, whereas a pentode will be far less happy in
these conditions and more likely to result in problems.

At the end of the day, its just a view, I won't be looseing sleep if I
am wrong, and in reality (other than debating points on a newsgroup), I
would be happy to bet that what I say was functionally correct.


My curiousity is to why we can assume if the amp uses triodes, and know
nothing else about it, we can assume it has no o/c load problems, but a
pentode based design might.


Note that I am talking about the output devices, not if the voltage
amplifier is a triode or pentode.

--
Nick