[Amps] TL-922 Troubles

[W0OXJ]

I have been running a balanced pair of 3-500ZG's for 3 years.Recently the
amp made a loud pop and began arcing while at idle.I turned it back on after
a period of time and noticed a loud hum and the filament transformaer was
getting warm.

One tube has fresh paint on it and the other had faded paint which seemed
strange.The tube with fresh paint does not look used at all.The tube with
faded paint had a red plate after 45 seconds of excitation under low drive
and a loaded final.I swapped out tubes with a spare Eimac 3-500 and the tube
with fresh paint proved to be causing the heavy draw on the filament
transformer.I also notice no idle current where I used to have 150 ma.

The amp seems normal,it loads but I didn't try to load it fully.The relay
seems to operate OK,the 3-500ZG plate stills glows faintly red.

In the past I have noticed one tube would turn red after a period of keydown
but was not concerned as the amp has delivered 800 watts out for three years
.

Now I plan on replacing the defective 3-500ZG but would like to know what
could be causing the imbalance? and is this the reason for the failed
tube.Will a new tube be matched enough to give good service with the
existing tube?Why no idle current?
Both tubes check out with an ohm meter,no shorts between the three elements.

I can't do any voltage checks until I replace the 3-500ZG and will follow
the instructions in the service manual.They mention a bad zener,bad relay
and a bad tube as causes for this condition.


Any advice or experiences would sure help as I would like to insure a few
more years of reliable operation.

Many thanks and 73,

Bob
W0OXJ