Bassman 5b6

[Matt D]

Maybe decouple the first stage and the PI giving each it's own cap? Since my last post on this one I've tried all kinds of shielded cables and grid stoppers, a capacitor across the plates of the PI, and changing the PI to a long tailed pair. I changed the filtering to: capacitor-choke-capacitor(OT center-tap node)-resistor-capacitor(screen grids)-resistor- capacitor(PI and Preamp). I've decided this amp is possessed and it needs to be disassembled and have it's parts scattered to the four corners of the earth, or maybe thrown into a volcano.

[Matt D]

I'm no expert but what you have sounds like classic 'motorboating.'

[Blackburn]

Yeah that's what a friend said about my preamp cap... but, I had this amp built before, perfectly functional using the same cap setup, same tubes and same everything except the pi cathode caps and input filter (now it's a 30uf instead of two 16uf in parallel). However, I now have boards with eyelets whereas before it was a ptp rat's nest and I could have the chassis directly on the speaker cabinet playing with it around 3 o clock and all was well.

Just got the bug to rebuild it cleaner and with boards and my longer chassis to fit my head cabinets. I did order more Allen Bradleys to replace the clipped lead dirty ones from before. Pretty much same values.