"Phil Allison" wrote in message
...

"Arfa Daily"
"Phil Allison" "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.


Agreed, but I did say "or whatever".


** Fine - but you also had no idea what was importantly different about a
GZ34 / 5AR4 compared to other rectifiers.


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.


** Many times, such analysis pays off in the long term.

Cos failure mechanisms that are not dealt with by the service tech are
bound to be repeated.


For the most part, that's accepted, and if a component has failed in a way
that it obviously shouldn't have, then of course the reason behind the
failure should be investigated and understood. But in the case of valves
with an obvious internal mechanical failure, it's just that - a dud valve
....




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.


** Seen that scenario many times myself - especially with Chink valves.

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.

** Valves are generally not upright when servicing is going on - and a
great many amps have the power valves mounted horizontally or inverted in
normal use.


Agreed, but that does not make it 'right' in terms of minimising the stress
on the valve's internal structure. I guess that upside down is better than
on their sides, but I still prefer to see power output valves and
rectifiers, being run upright.



Owners of same can operate the amp any damn way up they like - and do.


Popped fuse - new fuse - clouted valve - fireworks - new valve - new
fuse - soak test - write bill - phone customer. Job done. Cash in bank.

** As posted by me earlier - the best way to fix recurring problems with
crappy GZ34s is called a 1N4007.

Works a treat in Quad Mk2s, long as you get rid of those dreadful oil
filled filter caps.



... Phil