[glyphmon]

MrFixr55,

No, the plate of the 6F6 is not fluctuating that I've observed.  Places that I've observed fluctuation are:
Output side of field coil (white wire) at the field coil terminal, 269V - 278V
Junction of white wire above with C60, 6F6 screen grid, 265V - 270V
Junction of R33 to 2nd IF Transformer, 264V - 272V
Plate of IF 6K7, 166V - 180V

Of note, I'm not seeing fluctuation on the input side of the field coil, so I feel like the power coming from the rectifier is stable, at least (settled at 349V).

All of this testing has been with the volume control all the way down and in that state there is still some intermittent/random static-like noise (think wind blowing across a microphone) from the speaker.

All caps non-mica have been replaced, though I didn't replace the electrolytics myself.  I'm wondering if I should replace them again.  They have a few years on them now, and admittedly, that's not a few years with any kind of regular use.  Bakelite blocks were excavated and reused with new caps.  Electrolytics were not restuffed, but originals are disconnected from the circuitry.  Everything else was cleanly removed before replacing.  Any out-of-tolerance resistors, in fact, I think all resistors, barring the voltage divider have been replaced.

I'll dial the Isotap down to 115V for the next round of testing, and see what I can do following your suggestions further.  Not sure what compatible speakers I may have around.  You say another radio that puts all its B+ through the field coil, but is that the case in this radio?  What about the branch that goes up to R64?  That doesn't seem to go through, or I'm misunderstanding.

Thank you for your patience.

Best wishes,
Roy