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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