Although much credence is given to the notion that hand-wired amps are inherently superior to other methods of construction, this is certainly not always the case.
Here’s the inside of a Fender Tonemaster amp, which as you can see is a total rat’s nest of wires. I dread to think what Harry Joyce would have thought of this!
The amp needed new valves, but was also oscillating at higher volumes.
The problem was the length of wire from the phase inverter output to the power valves.
Normally grid stopper resistors are attached to the valve control grid to prevent oscillation, however although there were grid stopper resistors in the amp they were attach to the far end of the the grid wires, and NOT directly to the valve sockets.
Adding 1k5 grid stops to power valves (under the heat shrink in the bottom photo) cured the problem.