I have a Heath SB-221 that had the filament go out years ago on one
of the tubes.
It turns out, with the Eimac version, that there was no direct contact between
the filament leads coming down from inside the tube and the base pin if the
solder
melts.
In other words, the filament current goes through solder from the base pins
to the filament leads on all of the Eimac tubes I had.
The Heath has a portion of the fan blade below the chassis but since the pins
are in line relative to the air flow, the rear pin gets hotter than the front
one
does.
First, I stuffed small pieces of hookup wire inside the pin to make direct
physical
contact with the filament leads and checked it with an ohmmeter. Than I filled
the pin again with solder.
Then, I rotated the sockets 90 degrees so that the pins are perpendicular to the
air flow.
Then, I put in a bigger fan to provide more air flow.
While this solved the problem, I later decided to use 3-500Z's other than
the Eimac which had a better designed, and more reliable, filament lead to pin
connection.
Jim, K1PX
K1PX@msn.com<mailto:K1PX@msn.com>
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