Digital Servobias v4

Many years ago, i made an vacuum tube amplifier. All was fine, for few months, then i was hearing something, that was not there (maybe its just me, but ears are ears).  After i made some new measurements i find that bias for final tube was anything but not where it suppose to be, where it was. I made the bias setup, and all was fine again for another period. Everybody know that vacuum tubes aging is not the same, even at the beginning all tubes are perfectly matched, after some time (depend on tube quality) tubes bias drift and amplifier need to be rebiased.

This project solve this problem and extend tubes life. Set the tube bias and is keep that way till the end of tube life. 

An little history. All starting in 2013, with first version, and testing the idea, all was only on breadboard, bias voltage was controlled via PWM LPF. On version 1 the concept and basic firmware was tested, but PWM with LPF was not an good idea. In 2015 i have tested the version 2, with 12bit DAC and more complex output circuit. This version was more reliable that v1, also v2 was on breadboard only. In 2018 version 3 was finally released on PCB, firmware more refined and new feature added. This year on covid-19 pandemic was the time to refine even more and that come out the V4, the most complete version of servovias. 


  • work with SE, PSE and PP class A with 4 tubes
  • size 100mm X 49mm X 15mm (W*L*H) for 1-4 tubes and 81mm X 49mm for 1-2 tubes
  • hole size 3mm
  • power input 6.3Vac - 12.6Vac  ~ 28mA
  • grid voltage -20V ... -120V (module can handle up to -200V)  ~ 3.5mA per tube
  • can control an external relay for HV delay (delay can be adjusted, min 5s to 1000s in one second step) to connect/disconnect HT
  • small OLED display to show information and setup the module (can work without after initial setup)
  • rotary encoder with push button (can work without after initial setup)
  • max voltage read from tubes cathod (K1...K4 inputs) 1V
  • can detect valve current runaway, in this case all valve will be maxim negativated and HT relay is decoupled
  • with single click on rotary encoder Servobias go in "half mode" PDA (to preserve valve life)