When we sell new tubes we are sometimes asked how to adjust the VAIC amplifiers.

We have to tell this here for for legal reasons:

Some are made by Dr. Kron, some by Mastersound, some by Mr. Vaic personally. So I just try to tell here what I know, and for legal reasons this can not be e official bias instruction. The official bias instruction can come only from the makers of the amplifier. The responsibility for the work done is the sole responsibility of the person who carries out this work, and not with JAC Music company.

  1. Do not work on the inside of a tube amplifier, when you are not qualified to do so.
  2. The VAIC amplifiers work at higher voltage than most other amplifiers, and they are most dangerous inside for that reason.
  3. There is no guarantee that this information is correct, and no guarantee that it is right for your particular amplifier.



  1. Remote control
  2. Channel selector
  3. Volume knob
  4. Motor for potentiometer
  5. ALPS Potentiometer
  6. Bias resistors. Measure bias voltage over those. Value is written on them. Here you see four in parallel
  7. Small resistors for symmetry of heater voltage. HUM POTS mounted on other side of this small PC
  8. Series Slide resistors for filament voltage, one for each tube
  9. 9 Relay board
  10. power supply board
  11. Fuse
  12. Pre amp board
  13. Power input

This is how I exchange tubes myself:


  1. You always have to re-adjust the amplifier when exchanging tubes, also when you exchange them with tubes of the same model and brand.
  2. Later models have stabilized 5V heaters, with those you need to check if the tubes run at 5V. (If a tube draws more current than the stabilizer can do, it might give not 5V. Older models have the black slide resistors, these are very nice, but it means always an adjustment when you change a tube.
  3. Originally it was said stabilized heating doesn't sound good. After changing to stabilized heaters, it was said to sound better. Already from this, you can see both is not true. I think stabilized heating is more comfortable, but it doesn't sound better or worce.
  4. Possible adjustments, depending on the type of amplifier:
    • Plate voltage
    • Filament voltages (if you have unstabilized haters). Slide resistors "8" in above picture
    • Hum potentiometer (if you have unstabilized haters) . Well hidden, at the back of two very small PCB's (nr "7" in picture).
    • Plate Current


  1. I Make some provisions for being able to put the amplifier upside down with the tubes in. I tape large pieces of thick styropor on the transformers for that.
  2. See if it works at least normally before doing anything. Does it give sound, is the bias adjustable from the inside or outside?
  3. Switch off the amplifier, take out the speaker wires, short circuit the speaker connections. Set volume to zero.
  4. Turn it upside down, open it up.
  5. Check the mains voltage settings. These are often WRONG. Use the higher voltage from two, if you have to choose. So if you have 230V in Europe, and the amp has only 220V or 240V, you must choose 240V and NOT 220V. This is extremely important. 240V or 240V is 10% difference and you blow out the tubes or transformers, and some power resistors inside if you do not look at it.
  6. After adjusting the mains voltage settings, switch on, and wait for normal operation.
  7. Are the old tubes run at correct filament voltage?
  8. Find out where and how to measure the bias.
  9. Measure the bias, and do the measured values make sense? If not, clarify this first.
  10. Write down the measured grid voltage of each tube. Grid voltage can be measured with a 1000Volts DC scale voltmeter, with the negative lead on one of the two thick pins, and with the other lead you sense the negative voltage on one of the two thin pins. It is somewhere between -70 and -150V. When is measured +600V or so, take the other thin pin.
  11. Set all bias adjustments to minimum. So the negative grid voltage is around -150V.
  12. Switch off, put in one replacement tube, switch on.
  13. Measure quickly the filament voltage. If wrong value, you must adjust it.
  14. Repeat for the other tubes
  15. So now the amp has basic function.
  16. Adjust the bias to the "must be" values for the tubes. This is the hardest question, and not simply possible to just say so here. Most 300B amps have 21Volt across the bias resistor (number "6" in above picture). However, you can look "only" just at that voltage if you really know it is the right value. In all other cases, when you have the voltage, and when you have the resistance, then wIth Ohm's law you must calculate the plate current. (There is no other way). The plate voltage is measured between one of the two thick pins, and the other lead on the thin pin that has around +500V. From this can be calculated the tube dissipation. Recommended is:
    1. 22 Watt for EML 300B-Mesh,
    2. 28 Watt for EML300B standard.
    3. 33 Watt for EML300B-XLS, AVVT 32B
    4. 34 Watt for EML320B-XLS,
    5. 39 Watt for EML520B-V2 or V3,
    6. 50 Watt for EML1605.
  17. After this, check the filament voltage again, and re-adjust if needed
  18. In case you had to re-adjust the filament voltage, you may have to re-adjust the bias again.
  19. If you know where to find the hum pots, you may need to adjust those too, because often they were adopted to the very old tubes to compensate some tube hum. This has to be corrected, when your new tubes appear to be humming.
  20. Now let the amplifier run, and check and re-adjust the bias for the next hour or so. Make sure the chassis doesn't get to hot, upside down.
  21. Turn it on normal position and let it run for one day, check the bias every hour and readjust when needed. Don't focus on fine adjustment, just correct changes of 7% or more. Micro changes will appear all of the time. That is not coming from the tubes, but coming from changes in the mains voltage
  22. At the end of the day, when still warm, do the final adjustment. Try to find a moment when the mains voltage is at 100%. That is mostly in the evenings.
  23. Let the amplifier run on your HiFi System now, and burn in the tubes.
  24. After some days check the adjustment again, and when all ok, the work is done.
  25. Make note of the final bias values somewhere inside the amplifier. Some day, somewhere, somebody will be happy with those numbers. You would have needed those at point 7) above here.

PLATE VOLTAGE SETTING. Some amplifiers have the option to select two or three plate voltages. In older models this is done by soldering off a wire from the edge of the power supply board, and connect it to the adjacent solder pad. Later models have a link that you unscrew set to another position. So it is a three position switch, but for operating it you need to unscrew the link. Take low plate voltage for 300B, or 300B-mesh. Medium is for 300B-XLS and 320B-XLS. High is for 520B and 1610. Also by this method you can operate tubes like 520B-V3 in a normal VAIC 300B. They were made for this, and it is the purpose of this switch.

CONCLUSION FROM THIS TEXT: If you have more questions after reading it than you had before, then my strong recommendation is, find a local tube specialist. The VAIC amplifiers work not so difficult, but they work by a little unusual concept, and are not really self explaining for a beginner. Also I can not change a beginner into a specialist by answering questions by email. Getting experience takes a few years time.