[quote name='Beer of the Bass' timestamp='1388699362' post='2325142']
I'd expect some sort of SS stuff to drive the meter, but there just aren't enough preamp valves in there for a push-pull amp with a tone stack if it's going to have enough gain. Having said that, well designed SS stages aren't necessarily a deal-breaker. It could be single ended (i.e. no phase inverter needed), but then the 15 watt rating would be rather fanciful.
I wonder if the unused valve space on the board is there because it shares a board with one of the Hayden guitar amps and they just populate them differently for the two models? Certainly the tick boxes at the top of the picture would suggest this, since I think they have one built in the same size chassis.
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Very good point about the Hayden connection, well spotted Sir. They have a 'Mofo' range - and that word can be clearly seen on the tickbox at the top of the picture.
EDIT: And having had a quick Google, the Hayden Mofo 15W clearly shows another valve between the two EL84s, which would explain the spare location in the pictures above. Also, the Mofo 15 can be switched to 2W mode - perhaps this additional valve does this, taking the EL84s out of circuit.