Question: "Why do you think the regulated dc on the filaments makes such a
difference? Closer to the original battery powered situation? Would that be
true for any
old tube transmitter? Or because these particularly were originally designed
to run on 28 volt DC."

The oscillator obtains feedback by tapping the cathode up on the coil. This
allows the ac on the filaments to modulate the oscillator slightly. A
filament winding was added to reduce this effect, but it does not. Some
ARC-5s have hum modulation - especially when multiplying to 10 meters.

Also, a change in filament voltage causes drift of the vfo. When we were
converting these rigs 50 years ago, it was difficult to get 24 volts
regulated. Today it is a snap. It may be easier to get a regulated 13.8
volts (or 12 volts or anything in between).

In my early hamshack, the test for stability was to switch on the heater and
listen for drift about 30 seconds later. Even the heralded SX-101 would
drift mightily on 10 meters when I did that trick - until I regulated the
filament voltage on the oscillator using the separate transformer that
always kept the heater on.

73, Colin K7FM










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