"David Farber"
"Phil Allison"

And this secondary short won't cause any damage with the signal going
through?


** You don't get to see too many tube guitar amps, do you David ?

Nearly all Fender tube amps all have the same precaution - ie no
speaker
plugged in = shorted output.

The fact is that it is more hazardous for a tube amp to be operated into
an open circuit than a shorted one. Tubes have inherent current limiting,
unlike transistors, so do not immediately self destruct when presented
with such a load condition.

However, operation into a open circuit can result is larger than normal
voltage appearing on the tube plates and consequent possible internal
arcing of the electrodes, socket bases or even the output transformer.

You may note that "Music Man" amps take an additional precaution of
having
high voltage diode stacks wired from output tube plates to ground. This
prevents the plate voltage from exceeding twice the DC supply voltage
under overdrive conditions - which it otherwise will when presented with
a
typical inductive speaker load.



Do I see many tube amps? How'd you know? lol. I have been doing business
lately with the proprietor of a music store who sells guitars,
amplifiers,
keyboards, etc. He likes vintage tube equipment so I'm getting an
education.



** My "education" with commercial guitar amps began back in 1971, while at
uni doing engineering. Became my main job a few years later and I'm still at
it - along with all manner of stage and PA gear.



Your explanation though solves another riddle I had a while back. I was
repairing another amp (this one was solid state) for the same music store.
He said all he did was add a speaker jack and then it started blowing
fuses.


** Customer carried out mods often prove fatal - but only to the amp
unfortunately !

SS combo amps that have no ext. speaker jack are invariably NOT short
circuit safe. If an owner wants such a jack fitted, then use a plastic one
and wire it so it connects the ext. speaker in *series* with the existing
one. This way it is safe even if some crappy jack to jack lead shorts out.



........ Phil