The TH781 tetrode PA that I am testing has one side of the filament
grounded, to one of the transmission lines in the input resonator.
Because of this, while I use 60 Hz transformer I get ripple on the
output, not a lot, but enough to be noticable and unacceptable. One
solution is to bring the control grid bias PS back to a CT of the
tranny, or to the center wiper of a low value resistor to fake CT,
causing the bias PS to have to float at 1/2 filament potential above
ground. Tube manufacturer Thales has suggested going to DC for the
filament, with a 12 pulse 3 phase supply without filters. Another
choice is to get a switcher, 10 VAC at 320 Amperes. That isn't a tall
order these days, but getting it reliable, ramping up voltage, etc
are the hurdles. I may consider it, however.
John
K5PRO
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