Miller Audio

Miller Audio LLC

Module AB-4 for 4 tubes, PP & PPP amps, requires 6.3VAC & bias supply from the amps circuit, with TTL error signal output.


The model AB-4 auto bias module is a first generation design and is designed to optimize the bias for four triode or pentode output tubes for Push Pull (PP) that have their own negative bias supply circuit.
The AB-4 provides an ERROR output for an external safety circuit, via a TTL compatible open collector output. LED’s are used for start up, fault and ‘bias reached’ conditions.
Suitable for class A, AB, or B tube amplifiers.
Bias is controlled by maintaining a constant voltage drop on the output tube cathode resistors, irresepective of driving signal.
The module automatically optimizes the bias settings and maintains a strict symmetry of currents through all tubes 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, DC or AC.
It is important to note that the AB-4 WILL NOT operate if the filament supply has any reference to ground, such as a center tap.
For that scenario, use the auto bias module models ABF-Q or ABS-Q+ or provide your own small power transformer of around 2VA with, for example, a 9V secondary.
– PCB dimensions are 74mm x 42mm high / 3″ x 1.65″
– power supply requirement is 6.3 to 12VAC
– negative DC bias supply between -50V to -250V
Price for one AB-4 module: US$96.00       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.


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