FWIW, on 4-250As and similar, (same filament structure as the 3-500Z), not being
careful about cooling the filament seals has led to trouble in my experience.
Including one military rig where it was considered usual to change tubes after
as little as 250 hours of filament life because solder came off the pins.
I use a muffin fan blowing on each tube socket, and a large fan 'upstairs' for
the envelopes and plate seals: touch wood, no problems. Rich is quite right
about the sockets, too.
73
Peter G3RZP
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