Blatting noise when adjusting bias

[Tage]

So I have finished building my orange graphic clone amp and when I went to dial up my idle current I get a wicked blatting noise. 2 of my tubes I can check the idle current on with my "Amp Head Dual" but when I try to turn them up or if I try to check the other 2 I get a really bad blatting noise from my speakers. The idle current with my Bias pot at minimum is about 29-31ma on the 2 that didn't go nuts. I'm using KT88's into a 2.5k load with about 500V on the plates once the amp warms up. I used the marshall style bias supply with the values for 6550's and I get a negative voltage range of -46V to -62V. I am only going for an output in the 120watt range. I hope I have provided enough info for some one to help me out. Oh and if you need a schematic I used the '72 OR120 and a marshall JMP (6550) bias supply.

Another note when I tried playing it at it's lowest idle setting it blats out with large signal and I think one power tube was arcing internaly

[Steve Conner]

Try disconnecting the power amp NFB. If that makes it stop, you probably have the phase wrong and the feedback is positive.

If not, it's probably a layout issue making the thing oscillate violently. Try grid stoppers on the power tubes mounted right at the sockets, etc. Or a wiring mistake, something is just hooked up completely wrong.

[pdf64]

Try double checking the actual voltages on the tube sockets; confirm that each pin is connected where it's supposed to be. Pete.

[Tage]

Well the grid stoppers did it I must have been picking something up from the B+ wiring or the OT primary wires.

Thanks for the help again