Bias adjustment on solid state amplifiers

Depending on where the feedback device is located, there may be a considerable time lag to the thermal compensation device. Consequently, many designs are under biased to preclude thermal runaway.

Transistors with on-die bias diode offer better performance at low signal levels, just where most of the music resides.

If your amplifier is in for service, the bias is a thing that should be checked by the service technician (and its very likley that the technician will have to check it anyway if the service has anything to do with a damaged output section). Its not hard to do, but isn't for the faint of heart as one slip with the probe of the meter and the entire output section could be destroyed.


If the amp has not been serviced and is 20 years or more old, I would have it tested by someone that knows what they are doing. While semiconductors are much more stable in terms of bias drift as opposed to tubes, its a simple fact that they do change over time and 20 years is plenty for that. In addition to the bias the DC offset should be checked as well but any competent technician will check that too.

I am joining this Forum because I need to set the bias on my vacuum tube amplifier. It appears I need a miliamp meter reading up to 500 mA.  I don’t know where to buy one. Amazon does not have any. Can anyone of you guys give me some ideas? Thanks, Richard

Just get a Digital Voltmeter with a current scale. Most have a scale with a range of milli-amps and also up to 2 amps and work fine for that.