Looking for opinions on the subject of 8877 filament voltage...
Due to high AC line voltage here (~ 127/254 VAC), my 8877 filament sees
approximately 5.75 Vrms (as measured on a Fluke 189 True RMS DVM). The
8877 is rated by Eimac to run at 5.0v (+/- 0.25V). Eimac specs filament
current at 10A.
Since the filament current is significantly less than it would be on a
directly-heated cathode, I am considering bringing the filament voltage down
0.75V by using a series 0.075-ohm @ 20W wirewound resistor with integral
chassis-mount heatsink on the secondary side . The resistor would bring
the filament down to 5.0V and dissipate about 7.5 watts of heat in the
process. However, if I can find a nearby mounting hole to add an angled
aluminum mounting bracket, I could use a 0.1-ohm @ 25W rheostat.
I suppose yet another option involves series limiting the common HV/filament
primary side to bring the line voltage back down to 235VAC or so. Whatever
I use, I need to make the mod reversible without hacking things up and
affecting the re-sale value of the 77Dx amp.
Paul, W9AC
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