The model AB-D-MKIII auto bias module is designed to fit the Dynaco MKIII amp chassis.

It optimizes the bias for two triode or pentode output tubes in Push-Push (PP) amplifiers that have their own negative bias supply circuit.

Suitable for class A, AB, or B tube amplifiers.

The AB-D-MKIII dynamically controls the bias in two tubes.

The module automatically optimizes the bias settings and maintains a strict symmetry of currents through both tube branches in the output stage. It will also stop any DC current flowing through the output transformers, all with the minimum of current loading on the anode of a tube. 

DC current saturates an output transformer which results in increased low frequency distortions.

The bias level is kept at the preset value irrespective of the input signal.

This module may be powered by the filament supply of an amp, with a range from 6.3V to 12.6V AC.

It is important to note that the AB-D-MKIII MUST use an isolation transformer, as the Dynaco MKIII’s filament supply is referenced to ground. We sell this isolation transformer as a separate unit, 

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– PCB dimensions are 65mm x 57mm / 2.55″ x 2.25″

– Suitable for single ended amps

– power supply requirement is 6.3 to 12VAC

– negative DC bias supply between -70V to -120V

Price for one AB-D-MKIII module: US$62.00      in stock

Shipping of $11.00 for the lower 48 will be added when you click on the PayPal button.

For shipping outside the lower 48, please contact us first, thank you.

NOTE: Hardware such as spacers and screws to mount the module is NOT included.