I’m not 100% sure what did it, but I did the following:
– moved the ground wire from the AC mains off the terminal strip and onto it’s own dedicated ground (my chassis has a welded on bolt next to the PT mounts for exactly this purpose, i think.
– with the now freed up 3rd lug on the terminal strip, i ran a 16 ga insulated copper wire to the back of the tone pot. ran the same wire type from back of the tone pot to the ground lug of the vol pot. ran yet another from there to the ground lug of the input jack.
– i took the ground wire from the cathode bias resistor/filter caps off the tone pot and put it onto the terminal strip ground.
Voila! a *tiny* amount of hum, about what i expected there would be from the onset – it’s pretty quiet. There are 3-4 leads in the amp i can shorten if I try, and i’ll do that over the next couple weeks while I build the cabinet.
I also switched to the 125V tap and that brought the plate voltage down to about 369-370 from 389-390.
That made me ready to try a NOS 6k6 in there, which worked great, although it was not as big a volume drop from 6v6 as I expected (I have a small PP amp that will take all the same tubes as this one, and the difference between 2 6V6s and 2 6K6s is noticeable.
between that and a NOS British made 12ax7 (mullard? who knows) I bought for it this thing is ready to go.
ps – anyone tried a pentode/triode switch on this? that’s the first modification i’ll make. i want this thing to be as low volume as i can make it…
pps – Robin, what’s that black box with 2 knobs and a jack on the back of your red two stroke?