Thanks robin. here’s a follow up after a bunch of work tonight.
I rewired the heater wiring between the output tube and the preamp tube. i also greatly shortenen one of the connectors between output jacks. see photo.
it didn’t fix the hum problem but it helped the squeal alot.
– the squeal is completely gone at all settings.
– the hum is still there, just as loud as before. BUT, when you crank volume all the way up to 12, it almost completely goes away, down to the point I would expect in a quiet amp.
– if i crank the amp to 12, and put the guitar volume down to about 1, I can get a nice clean sound with almost no hum and just a little hiss that you might expect from an amp that is all the way up.
– i tried running a test lead between the grounding terminal strip and the tone control pot body(which is where i am also grounding the filter caps and the 220kohm/500ohm resistor junction). this cuts the hum by ~30-50% but it’s still pretty obnoxious.
robin, you say “The filter caps for the power stage should not ground to the preamp side.” I think what you mean by preamp side is the brass plate? meaning the power side grounding is the terminal strip bolted to PT bolt? But the DH book instructs you to solder both the filter cap ground and 220kohm/500ohm ground to the brass plate. so by doing away with the brass plate and simply putting both of those onto the tone pot, what is the difference? anyway, i tried taking that grounding wire off the pot and putting it on the power ground terminal strip and it didn’t change anything.
other notes i forgot to mention:
– another couple differences between my amp an the spec’d parts list – I used plastic element pots (expensive!) because i thought i’d get lower noise. should i try just regular pots?
also, I added a filter cap drain resistor (220k, 3 watt) in parallel with the big 40uF cap. it drains the caps down to millivolts in seconds after turning amp off. why isn’t this part of the amp design? (hopefully that’s not the cause of my noise…).
Finally, i measured the ac lines coming into the amp and I get 123.6V. I guess I should switch over to the blue 125V tap on the PT? that might lower my high tube voltages, right?