Cannot Bias amp with Mosfet follower drivers...

[Tage]

So my 4 kt88 bass amp with mosfet drivers is giving me some trouble since I changed to a dual rail power supply set up. When I try to bias the power amp I get around 30mA per tube, -49V at the mosfet gates, -52V at the kt88 grids, and the amp starts to "pump." I was monitoring the plate supply rail voltage on a multimeter when this occurred and noticed it swing up to around 2x it's normal voltage of 690V. if I back off the current through the tubes to about 28mA I can use it just fine and there are no problems, just not the same level of power it had when running the single voltage rail. Here is the schematic of my driver stage, the preamp is pretty standard Tweed bassman, although with adjusted feed back values for the different power amp.



One other thing I have noticed thus far is that if I touch the protection zener diodes with my multimeter test lead I can cause the same condition, I suppose I am putting the meters input resistance, 1M, in parallel right at the gate of the mosfet maybe causing the mosfet to oscillate or saturate?

Some insight from those who have been here before would be greatly appreciated.

[Mick Bailey]

Swinging to 2x HT supply suggests oscillation to me. The fact that it can be induced by touching the test probe onto the zener is also a good indication. You'd really need to scope it to be certain. Did you alter the lead dress?

Just an observation, but the devices already have zener protection built in. What's your actual layout regarding the zeners and stoppers? My thought would be to take off the external protection and ensure that the grid stopper is located right on the gate terminal as close as you can get it.