It's essentially the first stage of a Vox AC30 running starved plate (60v in this case). It's a pentode rather than a triode so don't stick a 12ax7 in there, it won't work ;) It works really nice as a valve boost. It's probably most like a smoother and gruntier SHO. We use a 6BR7 as it's essentially an anti-microphonic EF86.
We do a triode based one also, that's different again (it's actually a heavily modified Valvecaster and sounds nothing like one), more of a smooth treble booster. That could use a 12ax7, but we run them with 12AU7A RCA cleartops as they were a great sound with low current draw.
We can do the pentode boost using a sub-mini missile guidance valve ;)
I'm a big fan of both. They really add to my low wattage single channel amps.
Good to see that it was snagged by another forumite.
