Bad hum on initial power up/first bias setting

[Tage]

So just fired up my latest build, although it's been under way for some time. I'm having an issue when I start to bring my bias up on the power tubes, i get a slight hum that then takes off into a super loud buzz from the speakers and i have to shut things down. I suspect I have some sort of oscillation going on, but it takes off as soon as I have the power tubes conducting as little as 2ma.

The amp is a Bassmanish preamp running into mosfet drivers and then onto the power tubes, 4 KT90's, 600V on the plates, about 580V on the screens, the screen voltage comes from a mosfet capacitor multiplier. I know none of this is much good without a schematic so I will be drawing a good one up right away. But any shots in the dark would be appreciated, maybe some other similar experiences? I would try to monitor the signal circuit, but I am without a scope at the moment, no AC voltage with my meter though.

I'll post back as soon as i have a readable schematic together.

Thanks, Tage.

[Tage]

ugh pretty sure i just have my Output primaries backwards.

[Jazz P Bass]

Keep us posted wiwth an update.

[Tage]

Yup that's all it was just had the primary mixed up. swapped them and no more positive feedback blaring noise at me. Glad I powered up with speakers not a dummy load...

Now i can get to tuning things. Thanks for the interest JPB, hopefully I can get some tracks up soon once the customer comes over to demo it and tune with his cab.