I've been working on a Garnet "Raven" amp. http://www.bossareaforum.com/Forum/view ... 791#p11958
It is now working, but with masses of hum and noise. So I've been analysing the circuit - there isn't a schematic, so it has taken a bunch of researching. This beast has a pair of 50C5 output tubes - a type I've never seen before.
I did not know that the number at the front of a tube designator is the heater voltage. 12AX7 is 12.6VAC heater (two 6.3V heaters connected at a center-tap). the 50C5 has a 50VAC heater.
So all the "12" series North American tubes (12AX7, 12AU7, 12AY7, 12AT7) are 6.3V heaters because we use the center tap. A 6L6GC has a single 6.3VAC heater. And so on.
In Europe, it is the "E" in front of EL34, EL 84 that designates the 6.3V heater.
As for the Raven ... I can't find an out-of-spec component, but when I swap the two output tubes the hum and noise completely change character. Interestingly, both tubes test OK on my vintage tube tester, but clearly there is a problem with one of them under full voltage and load. They are no longer made so I've ordered a NOS replacement pair (not cheap!).