I went ahead and tried it, "smoke test" style, and the change worked
quite well. I moved the HV winding transformer tap from "0V" to "240V"
(there was a misprint on the schematic, but looking at the transformer
itself it was easy to see what to do) --- the (receive-mode) plate
voltage went from 880V to 500V. I then cut a wire to the second PA tube
filament, disabling it. The bias control was slightly different
setting, and I didn't change screen voltage or try to re-neutralize. A
quick tune-up test shows the rig now produces about 100W max on 80, 40
and 20 (my unit isn't functional on 15-10M due to alignment or other
issues). This is close enough to what I expected.

wb0gaz wrote:
I'm looking at the schematic for the Yaesu FTdx560 and see that the
HV
winding is tapped in several places (0/340/460/600) --- I have in
mind
to pick a different tap for transmit HV to reduce max power output.
The
rig puts out 300W on lower bands - this is far too much for my QTH
considering proximity of neighbors - I'd actually like to have it run
at something in the 25-50 watt range (I like the rig otherwise, just
don't want to run that power output level here.) Any thoughts on
consequences of selecting a different tap for the plate voltage?
Other
changes should I consider (screen voltage and/or grid bias to restore
the tube's operating point, drive level, etc.?) It seems given this
choice of taps I could even connect between the 600 and 340 point for
260VAC, or 600 and 460 for 140VAC if 340VAC proves excessive.

73 Dave WB0GAZ