Quad Tube Amplifier



  1. Quad channel vacuum tube amplifier

  2. 14 tubes total

  3. 4 class AB (‘ish) tube audio amplifiers in one chassis

  4. EL34 output tubes

  5. Approx. 80 watts RMS output power per channel

  6. 20 Hz to 25 KHz -3 dB closed-loop bandwidth

  7. 0.45% THD

  8. Single power supply for all 4 amplifiers

  9. The following information is provided:

  10. Summary

  11. Photos

  12. Engineering notes and schematics


This is the quad tube audio amplifier portion of my tube home theater system.  In designing this amplifier I have attempted to squeeze as much power as I can out of a pair of EL34 pentodes while still maintaining high fidelity performance resulting in a bandwidth of 20 Hz to 25 KHz and a THD of 0.45 % while achieving a peak output power of approximately 80 watts RMS per channel.

Engineering Notes and Schematics

Caution!  High voltage.  Do not attempt to build anything shown here unless you have taken proper high voltage training.  The high voltages present in these tube designs are deadly.

Amplifier notes

Quad amplifier notes

Biasing notes and schematics

General power amplifier block diagram

Quad power amplifier schematic

Quad power amplifier power supply

Transient response before of old fall ’05 amplifier design before latest loop compensation:


Open-loop Bode plot before compensation:







Bode plot analysis and feedback loop compensation notes

Bode plots and loop compensation network calculations using MATLAB

Open-loop Bode plot after compensation networks:




All power amplifiers are capable of 20 Hz to 25 KHz -3 dB closed-loop bandwidth.  Closed-loop Bode plot after compensation is shown here:


0.45 % measured Total Harmonic Distortion.  Total Harmonic Distortion (THD) tests and analysis notes


G. L. Charvat, “Vacuum tube home theater system (part 2), vacuum tube power amps for home theater,” Audio Express Magazine, June 2012.

  1. Make Magazine Blog, ‘How to designa  vacuum tube amplifier.’

G. L. Charvat.  “Design and fabrication of high-fidelity vacuum tube audio-frequency power amplifiers,”  MIT Haystack Observatory, February 3, 2010.