FUQ (eff -yu - cue) - FAQ's with an EDGE

Tagged As: Push Pull Amp

Question:
by accident I ran my Seymour Duncan Convertible with only one power tube. I was surprised that it worked! A little hummy but it worked. Doesn't a push pull amp need one tube to push and the other to pull?

Answer:
A push-pull amp operated in class B (which is fairly elusive - only the Music Man comes close), where the output devices (be they tubes or transistors) are used alternately for exactly 180 degrees of each 360-degree cycle will fit your definition. However, the vast majority of push-pull guitar amplifiers operate in class AB, where the output devices conduct for more than 180 degrees, giving an overlap centered around the zero crossing point. This is a fairly ambiguous definition, because the drive level has much to do with how long the output devices stay on; if the output waveform is small in relation to the amp's full output and the amp is biased to a fairly hot level - say, 300 degrees for each phase - at lower outputs, neither device will be idle for any portion of the waveform. This fits the definition of class A, wherein the output devices remain on for 360 degrees of each cycle. Most class AB amps operate in class A for smaller signal levels, going into class AB when driven harder.