I've owned a few Guitar Mates and they too went
through tubes. They ran the EL84s in Class A, so
adjusting the bias wasn't as easy and not a good idea
(you'll see why later). When I removed the two
conductor AC cord I used the ground switch as a Class
A/Class AB switch. Worked nice. But on MY Guitar
Mates, the screen supply was decoupled by only a
22-ohm resistor, and the screen B+ was HIGHER than
the plates. This is why you can't play with the bias too
much. I changed the 22-ohm resistor to a 1K and
everything was fine.