Squeal in JMP 1987 build - help needed

[Roe]

redplating might indicate oscillation. try installing a 47k on v3's grid (besides the grid/swamp resistors on the power tubes)

[sniffin]

I did re-bias the amp today, it was hitting 43-45mA again so I had to set the bias pot back again to max but it wouldn't go further than 39-41mA. I had already changed out the 47k bias resistor to a 56k yesterday and that seemed to get the bias voltage up a bit atleast for a while, I'll try fitting in a 68 k and see if that changes anything, if I find that bias is still unstable then I'll try fitting a new trim pot and bias caps too.

thanks

[Roe]

worst case scenario with marshalls is cold bias at idle and redplating when playing :-/
this is possible since the primary is low and voltages high

[sniffin]

well the tube redplates slightly and only at one point where plates are pinched together, and redplates even when not playing, after about 3 or 4 minutes after hitting the standby switch.

[Roe]

bias is off or you got oscillation. try installing those grid resistors! and check lead dress!