[Diabolical Artificer]

Input has a 1M on the grid to ground, have tested input, PS and driver stage, all ok, but swapped valves out. After further investigation am sure the OP stage is to blame.

You make good points Joe, my amp follows your convention, IE input left, OP right. I put a lot of thought into layout of the amp, but the amp is temporary lashed up to the PSU with long wires, not good.

Yesterday I tried making a video to demonstrate the fault, however my editing software OPENSHOT is a pile of s*** , making video editing a nightmare. That aside whilst filming the video I was poking it with a stick and playing and thought to give the CCS a tweak. Adjusting the CCS output altered the "oscillation" frequency, then it disappeared. Fault gone. No rejoicing though as now two EL34's are redplating, no input, input grid grounded. This is with a dummy load and speaker load. There is an oscillation on the redplating valves, but before I jump to conclusions I need to do more testing, but there's instability somewhere.

I have a few thought's and improvement's in mind, (ferrite beads on g2 and anodes is obvious) that I'll run past you all later. First I need to do a revised schematic and take some pics.

Havn't forgot your post Al, also Joe's comment is pertinent. The sig gen OP has four OP 4mm jacks. Two red ones are the balanced signal, in between these are two black jacks marked ground, however I was using the other black one, which is shown on the panel as being the middle of the two balanced signals. Think the middle of a potential divider, OP sigs on the ends. Using the ground jack seems a better choice in retrospect.

Whatever the cause, the amp was fine, now it isn't, something broke or changed.

Thanks for your input, pics and more info later, Andy.