I’m really going crazy with this now.
Can’t be a tube issue, because I’ve got 7 12ax7’s of different makes and it behaves the same with all of them. Output tubes – I have at least 2 each of each type I’ve tried, and my previous post explains how they behave – only now I can’t get 6V6s to be squeal free – always a little squeal when volume is between 0-2.
Lead Dress? I’m not sure what else I can do. poking wires around doesn’t change the squealing. I’ve redone the heater circuit and it is as tightly wound and cleanly attached to the tube sockets as I can do and it’s certainly on par or cleaner than any images of similar amps I’ve seen online with the possible exception of the truly beautiful work done by Robin and similar builders.
I’ve shortened wire runs wherever it is practical.
I even took the grounded pin off the output tube and the shield of the input wire coming into the 12ax7 (which DH had you ground directly to the chassis via ring terminals) and directly wired these to my star ground. we’re nearing splitting hairs, but the little hum the amp still had seems to have lessened a bit after doing this. why not a star grounding scheme for this amp in the book?
By the way, I recorded voltages today and I have 145 and 150V on the plates of the 12ax7. I have 375V on the plate of the 6V6 and 345V on the grid of the 6V6 – pretty hot but not crazy.
All the testing i’ve done points to some sort of OT or impedance issue. I have 4 speakers at my disposal – a 12″ 8ohm, a 10″ 8 ohm, and 2 different 8″ 16ohms.
i cannot get the amp not to squeal when using any single speaker, except in some cases the squealing stops between about 5-9 on the volume knob.
with any 6v6, and the amp plugged into the 2 16ohm 8″s in parallel (8 ohms), i get the best sound out of the 16ohm OT tap. the 8 ohm tap squeals on many settings and seems thin or tinny.
with any 6k6, the 16ohm OT tap and the 2 8’s in parallel is the only combination that is squeal free and sounds good through all knob settings. if i have the 6k6 in and switch to a single speaker (any of my 4) things start squealing again.
sorry for the long post. I’m desperate to not shell out the money to take this to a tech, it’s seemingly such a simple amp. as always thanks in advance for any further thoughts.
One more weird note – i was poking around with DMM measuring resistors and the negative feedback resistor, which should be 68k, measures 1.48k and i measured it 5 different times. It’s a carbon film resistor. it clearly has the right color bars for a 68k resistor. also, the 68k going from the input jack to the board, where i used a metal oxide resistor that should have a tolerance of either 1% or 5% – that one comes in around 59-60k. could either of these (neg feedback in particular) be a cause of this? i didn’t have the time to try a swap tonight.