Mono-Block Tube Amplifier



  1. Mono-block tube amplifier

  2. 5 tubes

  3. Class AB (‘ish, not quiet class AB, closer to class B)

  4. EL34 outputs

  5. approx. 50 Watts RMS output power

  6. 10 Hz to 25 KHz -3 dB bandwidth

  7. 0.65 % THD

  8. Direct grid bias on output tubes

  9. The following information is provided:

  10. Summary

  11. Photos

  12. Engineering notes, schematics, and plots


This mono-block tube amplifier is one of 5 that were developed for the vacuum tube home theater system.  It is the same design as is shown in the quad tube amplifier with the exception that it utilizes a smaller power supply.   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 10 Hz to 25 KHz and a THD of 0.65 % while achieving a peak output power of approximately 50 watts RMS.

Engineering Notes, Schematics, and Plots

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.


General power amplifier block diagram

Mono-block power amplifier schematic

Mono-block power amplifier power supply schematic

Mono-block amplifier notes

Mono-block power supply notes

Mono-block amplifier design notes

Biasing notes and schematics

Bode plot analysis and feedback loop compensation notes

Bode plot, %THD, and peak-power data from the mono-block tube amplifier

Bode plots in MATLAB for the mono-block amplifier.  Before and after loop compensation., including closed loop bode plots.

Bode plot of open-loop transfer function before loop compensation circuitry was added.

Bode plot of open-loop transfer function after loop compensation circuitry was added.

Closed-loop transfer function of compensated amplifier.


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.