A very interesting thread. AFAIK when a tube amp is powered with the
speakers unplugged, the output tubes are in fact presented an infinite load.
Consequently, the primary voltage is forced to swing up to very dangerous
levels and ultimately the insulating enamel will crack.
If the output is shorted, the opposite will occur: an enormous current surge
that will ultimately trash the tubes.
I use to provide a 10R, 1/4W cathode resistor, but it only protects from
over-currents (shorted output). HF oscillations can be ruled out connecting
a R-C filter across the tranny's connections to the anodes of the PP pair,
but still the case when a speaker is unplugged is not covered.
I suppose that a good resistor in parallel with the speaker is the easiest
and cheapest solution, which means it is the safest (all safety devices
should be intrinsically simple and failure safe). Now the question is, will
it affect the amp's performance? Would it be a nice idea to use an R-C
bridge tuned at some frequency? I don't think so.
I suppose a plain 9W, 150R wirewound resistor is fine for any amp up to some
100W (about 5W would be dissipated through the R with 8 Ohms boxes). A
bigger one (say 25W thick film type) would be safer, but more difficult to
install.

I'm running to "retrofit" all my units with this "safety device".

Ciao

Fabio


ha scritto nel messaggio
...
Why is it that running an output transformer into an open load at high
power levels will destroy it, where as running it into a short circuit
load won't ? Many thanks in Advance.