"Phil Allison" wrote in message
...

"David Farber"

"François Yves Le Gal"

Judging from the schematics, the output jacks are short to ground types
in
series. And yes, the wiring leads to a secondary short if no jack is
inserted.


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.



....... Phil








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.

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.So I replaced the shorted output and driver transistors and some small
signal diodes. I turned it up slowly on my variac through my series 200 watt
lamp and all seemed well. Then I turned up the input signal and the light
started to glow. I hadn't turned up that much signal to draw a lot of
current. In fact, there wasn't even a load on the amp. To make a long story
short, I discovered the problem was the new speaker jack. If there were no
plug in it, it shorted the signal to ground. Now I know to watch out for two
types of 1/4" jacks. The ones that short, and the ones that don't. I told
him the jack was defective but I guess that only applies to a transistor
amplifier.

Thanks very much for your reply.
--
David Farber
David Farber's Service Center
L.A., CA