"Phil Allison" wrote in message
...

"Arfa Daily"


I've had a number of GZ34s fail short circuit, or suffer from serious
flashover, and knock out fuses over the years, but I've always put this
down to saggy filaments or whatever, where the valves are mounted
horizontally, or upside down.


** GZ34s have an indirectly heated cathode - just like most other
valves.

Sooo, the cathode is a solid tube of metal, with a heater wire inside it.

Ergo - it simply cannot sag.

Only directly heated cathode types do that

- like 5Y3s, 5U4s, 5AS4s etc.



.... Phil




Agreed, but I did say "or whatever". I've not taken that much notice of the
actual failure mechanism. I think you can spend too much time in a
commercial world, agonising over exactly what went wrong. I have a roof to
keep over my head, and I don't have time to analyze how every fault
occurred, and why and how every component expired in the way it did,
interesting as that may be.

Suffice to say that in 'modern' amps that use a GZ34, I have seen more than
a few with a vapourised fuse and no obvious short causing it. Often, a
replacement fuse holds up, until you clout the valve with the butt end of a
screwdriver, whereupon a miniature fireworks display is launched inside, and
the fuse goes out again. Sometimes, it's spontaneous, and only needs the
voltage to be present. More often than not, it seems to occur when the
valves are not upright. This would imply that gravity acting on the
structure has something to do with it. But what is touching what or why - I
really don't care. Popped fuse - new fuse - clouted valve - fireworks - new
valve - new fuse - soak test - write bill - phone customer. Job done. Cash
in bank.

Arfa