Amp Classification - What's the difference?


We often get asked questions regarding tube amp 'classification'. Is Class A superior to Class B? What are the differences? Here are a few simple tips to help demistify the wonderful world of tubes!

Amp Classification The 'Class' rating system for amplifiers refers to when each system was invented (Class A being the oldest).

Class A

  • Runs hottest - like controlling a cars speed with the brakes, Class A amps run at full power constantly and thus will wear cause the most amount of wear on the power tubes.
  • Least efficient use of power
  • Sounds warm, smooth & natural even when pushed to clipping
  • The oldest, simplest and most traditional way to wire a tube amplifier


Class B

  • Uses two transistors to amplify the signal.
  • Runs fairly 'cool' - when not being pushed, very little wear is being put on the tubes
  • Very efficient use of power
  • Produces 'objectional crossover' distortion


Class AB

  • Comprises elements of both the Class A and Class B amplifier
  • More efficient use of power
  • Produces some distortion but not 'high gain' from power amp stage
  • Produce a tight, firm and 'punchy' sound
  • More power and greater volume capabilities
  • Many of the most influential and premium tube amps run a Class AB design (Fender '59 Bassman, Ulbrick Arena 50)

Class C

  • Not used for audio

Class D (solid state)

  • New technology that uses width modulation to amplify the signal
  • Extremely efficient - are quite small in size and produce high output
  • Runs very cool
  • Heatsinks are much smaller, enabling a smaller, lighter and cooler amp
  • Used in Fender Passports, Bass amplification and other lightweight devices