The idea is to build a small box which would connect to the bias measurement points (1 ohm resistors) on an OddWatt style class-A push-pull tube amp. See schematic for one channel of a typical design.

The resistors R20/R21 sense the bias current which is in the 60-90 mA range, floating about 40V from GND. It would be helpful to measure and display graphically the difference between R20 and R21 for adjustment, along with informational displays of the total current (R20+R21) and the offset voltage from GND.

Circuit ideas:

  • Divide all bias voltages 5:1 (so 50V -> 10V) then feed to INA128 (INA828) with 100X gain.
  • Measure common-mode voltage at 2 points using unity gain buffer and further divider
  • LM358 to buffer common-mode measurements


  • Disable digital inputs on analog pins on AVR (DIDRx registers)
  • If above is done, analog inputs advertise 100M input resistance

Parts ideas:

  • INA126 instrumentation amp powered on +/-12V WRT local GND
  • 3.3V Arduino mini
  • 16x2 LCD display e.g.
  • Power:
    • 5V or 3.3V floating converter for Arduino and logic
    • PDM1-S5-D12-S
    • PDM1-S3-D12-S
      • +/-12V DC/DC converter (5V or 3.3V input) for INAs.
  • couple of buttons?


INAs would measure the four bias voltages. Additional channels would measure the DC offset from GND of the cathode circuits on each side.

Last modified 6 months ago Last modified on Jan 2, 2020, 5:21:52 PM