[Amps] 3-500z - intermittent filament

[Peter Chadwick]

The Mullard data sheet for the 4-250 and 4-400 equivalents (which have the same 5 volt, 14 amp filament as the 3-500Z) specifically call for a certain amount of cooling air over the socket pins. If you ignore this, you get long term reliability problems.
It may be worthwhile looking into putting in a small fan (or even 2 if there's room) to cool the pins.
73
Peter G3RZP