I've been reading the old posts on this pre-amp and looking at the tube charts and something doesn't make sense.
The bias point of the 6C4 seems very unusual, from the spec sheet posted by Doug here:-
The current draw for both amps is 8mA, there will be about 0.8mA going to each 6AQ6 which will bias each tube at ~1.4V which is what we would expect. The problem is with the 6C4 which will have ~3.2mA/tube making the bias point on the early model 150 x 0.0032 = 0.48V, or the later model 220 x 0.0032 = 0.7V. These bias points are way too low and will overload with the slightest voltage from the 6AQ6.
Incidently, the 6AQ6 is not half a 12AX7 it is half a 6SC7 or 6SL7.
Any ideas?
best
DaveP
The bias point of the 6C4 seems very unusual, from the spec sheet posted by Doug here:-
emrr said:1956 catalog:
The current draw for both amps is 8mA, there will be about 0.8mA going to each 6AQ6 which will bias each tube at ~1.4V which is what we would expect. The problem is with the 6C4 which will have ~3.2mA/tube making the bias point on the early model 150 x 0.0032 = 0.48V, or the later model 220 x 0.0032 = 0.7V. These bias points are way too low and will overload with the slightest voltage from the 6AQ6.
Incidently, the 6AQ6 is not half a 12AX7 it is half a 6SC7 or 6SL7.
Any ideas?
best
DaveP