[Engineer[_2_]]

Hi, Vacuumlanders,
The good news is that my Williamson "near-clone" (the topic of some
long posts under VLF stability, etc.) is stable and working... so it's
done, more or less... for now... I think (do we never fiddle with
stuff?!) Hum and noise is fairly low.
However, when I connected the Heathkit WA-P2 pre-amp some low level 60
Hz hum appeared... not much, but more than I expected. Adjusting the
WA-P2 hum pot (across the heater supply) yielded a hum minimum at one
end (rather than near the middle as on another WA-P2 set-up I have),
but still too high. Did some head scratching and found the answer - a
hum loop! This loop is around the B- line from the power amp, though
the WA-P2 chassis (ground) and back to the PA through the audio cable
shield. The solution: I added a 100R resistor in series with the B-
line at the PA socket that feeds power to the WA-P2. Pre-amp hum is
now very low and it now minimizes with the hum pot somewhere near the
centre of travel, as expected. I could have used an open-shield audio
cable but did not want to depend on a selected cable hook-up. Don't
even think of opening the B- feed line since removal of the audio
cable would then put the WA-P2 chassis at B+ potential ! If I pull
off the audio cable the WA-P2 chassis goes to about +0.6 volt (pre-amp
current x 100R.)
Cheers,
Roger