by 6L6/807 » Thu Jul 10, 2008 5:51 pm
Greetings to all,
"Class A" is a definition of the operating point of any valve whether large signal or small signal, and whether single ended or push-pull. It means that throughout the entire intended cycle of operation the valve(s) is never driven into the cutoff region. This is achieved by setting the bias to one half of the cutoff value. All single ended amplifiers must be operated in class A as distortion will be the result of operation in any other class. The limiting factor on power output will, of course, be anode dissipation limits. It is possible to construct a class A push-pull amplifier which is basically two single ended class A amplifiers strapped together in such a way that each valve is driven out of phase with the other but their outputs are additive. In this case, the push-pull amplifier will deliver twice the output power of a single ended amplifier and if one output valve is pulled from its socket the amplifier will still function very well at about half the normal power output.
For all practical purposes, single ended 6L6 family and EL34 valves are capable of about 10 or 11 watts of output so any push-pull amplifier that delivers more than 20 to 22 watts of power is probably not operating in class A.
Since the bias requirements of 6L6 family and EL34 are different, it will be difficult to make an amplifier that can operate either valve in class A without bias adjustment.
Class A - 50% of cutoff, no grid current, single ended or push-pull
Class AB1 - 66% of cutoff, no grid current, push-pull only
Class AB2 - 80% of cutoff, grid current on peaks, push-pull only
Class B - 95% of cutoff, grid current at all times, push-pull only
Class C - RF only, not used in audio
Best,
Wade