Hi David from Ken in the UK,

This is a common fault with these valves ( tubes ).
It's due to heat built-up over time.
The only solution that I know of is to blow cold air up around the valve
pins.
One final point, to extend the life of the '500Z' is to lower the heater
voltage to around 4.75v. This will save the valve from premature filament
failure.

Hope this helps, cheers, Ken, G4KIR.

"David Thompson" wrote in message
nk.net...
I have a LK 500NTC (no tune) amp that ran well for several years. Now one
of the 3-500Z's (both Eimac)
filament pins came unsoldered.