Power Tube Life Question

J. Forster jfor at QUIK.COM
Tue Mar 16 20:43:13 EDT 2010

This relates to the 500 Watt audio power amplifier for the 1MC PA system
on the Massachusetts again.

The amp uses 4 type 838 tubes, push-pull and parallel connected, with
directly heated cathodes (flaments) presumably Thoriated Tungsten,
although the RCA data sheet does not say specifically.

Since the tubes are somewhat precious, I'd like to run them at reduced
filament voltage (maybe 5-10% less) to extend their life. However, I'm
concerned about loss of Gm and/or emission from prolonged operation with
the reduced voltage.

The amp has separate plate and filament transformers, BTW.

Can anyone provide some guidance, please?




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