Gents,
Thanks for all the good information in your replies
concerning the correct filament voltage for the
77Sx clone.
As told to me, the 3CPX1500A7 triode is a
3CX1500A7 that's been to the gymnasium,
and even though it may be rated for 5.50 volts on the
heater pins, that rating is for pulse service and
using that voltage will still trash the tubes earlier.
Using a 150 watt rheostat at 35 ohms in the 240 volt
primary winding of the filament transformer it seems
that 17.7 ohms provides 5.008 to 5.102 volts on the
pins of the tube as the line jumps around. Jerry at Economy
recently sent me some Dale 225 watt 17.6 ohm wirewound
resistors so it was easy enough to squeeze another piece
in under the filament transformer.
New tubes require filling out a mortgage application and if
keeping them happy with a reading under the 5% tolerance
means a few months longer service it was worth the effort.
"Cheers," as the man says.
Hal Mandel
W4HBM
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