[AMRadio] Oscillator Filament Voltage change and frequency drift

Jim candela jcandela at prodigy.net
Thu May 11 00:19:38 EDT 2006

Don K4KYV said "Even a slight change in line voltage, < 1v, will cause the
PTO to drift frequency, due to the change
in filament voltage."


   I was experiencing similar filament voltage induced oscillator drift in
the Command Transmitter VFO (BC-458) for my Central Electronics 20A which
uses a 1626 with unregulated DC on the filament. I found it interesting that
for a given change in voltage, say 10-12 volts, different 1626's shift
frequency of varying amounts. Then after trying 3 or 4 tubes, I tried
another that was dead stable when varying the filament between 9-14 volts.
Apparently the oxide cathode was very stable with this tube. I didn't need a
Sola here. The point here is that if you drift when going key down due to a
drop in the line voltage, just maybe a different oscillator tube will
perform better.

   I once took a Heath VF-1 (Varying Frequency number 1)and resorted to DC
on the filament  with a 7805 five volt 3 terminal regulator, and a silicon
diode on the common pin to ground to provide a regulated 5.6 volts DC. That
took out the 60 HZ hum, and line voltage induced changes in filament
voltage. I later experienced band switch contact issues with that VF-1, and
it was more like a spread spectrum exciter. Off to the dump that POS


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