Audiophile HD-500 head cuts volume

[MrDinsdale]

Hey,

 

Got given this Audiophile HD-500 amp a while back, I've not seen much talk about these but it's a pretty great sounding head. Unfortunately after a few minutes of playing the volume cuts completely, the only thing that brings it back to life (temporarily) is playing a loud note by strumming aggressively. Tried both channels and just about every combination of settings I could think of but doesn't seem to have any impact.

 

I've tried it with 3 different tubes and had a friend investigate it but had no luck fixing it, just wondering if anyone has any ideas?

 

The person who gave it to me passed away a few years a go so would be really nice to get it back in working condition if possible.

Cheers!