Probably not the fault of relays. Most likely it's oscillating.
Sounds like maybe missing parasitic chokes. A lot of these were built with a black ferrite bead slid over a bare wire leading to the plate-cap clip. The bead would get hot, crack and fall off. Usually around the first time the amplifier got pushed a little too hard. Seems like all of them used this on the four final tubes.
Unless someone with sharp eyes found the fragments loose in the bottom of the amp there is no trace they were ever installed. Just four bare wires, one to each plate cap.
A two-Watt resistor with four turns of wire around it is what we install on each plate-cap clip in place of the bare wire. to make this model stable. The resistance value is not critical, between 47 and 100 ohms more or less. Just so long as all four have the same resistance value.
The two driver tubes were sometimes found with a resistor-type choke from the factory. Others would have the ferrite bead on each driver tube. It was a lot less common to see these fail on the driver stage. Those two tubes don't work as hard, and won't get the beads as hot.
There are other things that can make this amplifier unstable. Tube sockets mounted on circuit boards depend on the mount screws for a ground connection. If plastic spacers over the screws get hot and shrink, the ground connection comes loose.
Internal coax connections should have the shield grounded. If someone did a sloppy job replacing old coax inside the amp, that's a way to create instability.
