Really makes it sound as if turning up the plate current with the bias pot is kicking the tube into oscillating.
That would explain the toasted parasitic.
Next question would be why. The parasitic choke on the tube's anode connection is a big deal. Doesn't have to be the hairpin-style the factory used, just needs to have enough inductance to prevent it from oscillating. I found that just substituting larger resistors on the 'hairpin' widened the spacing and reduced the inductance enough to cause trouble. Got in the habit of inserting the resistors' wire leads from the outside of the hairpin, leaving the too-large resistor alongside, rather than inside the hairpin. This allowed using the original spacing across the inside of the hairpin.
The tiny parasitic choke wound on a half-Watt resistor on the tube's grid terminal is equally important.
Ground connections always come to mind when an amplifier oscillates. You did the 'suicide intervention' with the output-side coax cable, didn't you?
The factory used push-on pin contacts for the coax between the Load control and the output relay. They always come loose.
If those pins have not been replaced by a soldered connection, now is a good time.
More than once I have seen the rivets come loose that secure the input and output coax connectors to the rear panel. That can cause odd behavior until the rivets are drilled out and replaced with a machine screw and suitable tooth washers.
The relay can create odd symptoms, but usually gives itself away by arcing on the contact points, or making an area of the clear plastic cover dark near the arc.
The layout in the Pride has a good record of being stable, so long as everything is nailed down and grounded properly.
Cool upgrade to the factory Low-voltage board. More or less what we were doing back in the day. The foil traces get pretty fragile on that one. Biggest reason we started making our own was foil traces falling off the factory board.
Here's hoping the stability problem has a simple cause.
73
