New valve amp build - 30w 6146 AB2 PP.

[Diabolical Artificer]

Good points there Paul, your analysis spot. I ended way over complicating it going down several rabbit holes. Going to the nub is the HT bouncing, yes, under certain conditions. I'm pretty certain what's happening is that the front end is going unstable, which causes the whole amp to go unstable. So we get a LF pulse at the front end, this causes the whole amp to breath like a 70's comedy man in a brown coat in a telephone box, the OP valves start pulling heavy current, bias and g2 supplies start bouncing as does the HT.

So, two problems - unstable front end & HT PSU resonant at 3.8hz. I'm not sure why the front is going unstable, more investigation needing. The PSU as suggested can be damped with a 100r ish resistor across the choke, and according to my calc a 3400u cap. I do have a 4700u @ 400v,not sure there's the room though.

Thanks for your help, Andy.