Quote Originally Posted by TS808
Tung

The Blackheart Handsome Devil is described as "Class A, push-pull". Could you explain that to me? My Bandmaster VM is Class AB, push pull. I'm totally confused!!
This is one of my pet peeves: the vast majority of push pull amps are Class AB. The "Class A" is marketing hype. Alessandro makes a true Class A push pull amp which puts out about 10W or so from a pair of EL84s. In Class AB a pair of EL84 puts out about 18-20W.

Tot has it right, class of operation, output topology and biasing are all independent of each other. A single ended amp can only be run in Class A for audio. The ham radio guys have single ended amps for transmitters and they run them in Class B or Class C, but they are not audio amplifiers. Most single ended amps are cathode bias because it is less expensive to configure them that way (takes fewer parts and less complicated circuitry.) I have run single ended amps using fixed bias circuitry though, so it can be done.

A push pull amp can be Class A or Class AB for audio use. Most are Class AB as it is most efficient use of the power tubes. A push pull amp can be biased using fixed bias or cathode bias. Again cathode bias can be used to simplify the circuitry of the amp. Cathode bias is also a tad less efficient than fixed bias. To get the maximum efficiency out of a push pull amp, most are configured as Class AB using fixed bias.

tung