Biasing the MK III Amplifier

How to set, check and adjust the bias on a Dynaco MK III

For amplifiers with Standard Bias Only


A voltmeter/multimeter that can display DC voltage is needed to perform the bias check and make proper adjustment.


Before plugging the amplifier in


Install the tubes


Set the Bias before using the amplifier: (set before first use!)

Bias is related to the current flowing through the power tubes. The bias will change over time as hours of use (age) affect the tubes and other components. Although the amplifier ships with the bias already set, the bias needs checked and /adjusted as necessary before first use. A voltmeter that can display DC voltage is needed to perform the bias check and make proper adjustment.

Bias will be set with the amplifier powered on and warmed up but do not play music through the amplifier during this process. If your preamp is connected to the amplifier, simply do not power it on during the bias procedure. Again be reminded, you must have a proper load (speakers) attached to the amplifier, if it is to be powered on.



It is suggested to keep the bias just slightly below the factory suggested setting of 1.56 DC volts. A setting of 1.50VDC to 1.54VDC (DC volts) is friendlier to your tubes and allows for some drift or changes in the AC supplied to the mains.


Check the bias again at about 40 – 50 hours of use, then again at about every 100 – 120 hour intervals thereafter. 

Keep the bias slightly below 1.56 DC volts for best results, never higher though. A setting of 1.50VDC to 1.54VDC (DC volts) is friendlier to your tubes and allows for some drift or changes in the AC supplied to the mains.

When or if the bias adjustment becomes “maxed out”, meaning there is no more adjustment possible, it is usually time to replace the power tubes or possibly the rectifier tube. You should have the tubes tested to be certain. If new tubes do not resolve the issue, do not continue to operate the unit, seek qualified service for the amplifier.
The rectifier tube will eventually wear out and need replaced, but usually the rectifier (5AR4/GZ34) will last much longer than the output/power tubes. Test the rectifier for strength when in question. Most guitar shops or shops that repair vintage equipment can test vacuum tubes.