The model AB-Qi-ST70-ST120 auto bias module is designed to fit the Dynaco ST70 chassis and can be used in an original Dynaco ST-70 upgraded with a VTA driver circuit or the new generation Bob Latino ST-70 and ST-120 stereo power amps.

It comes with a 1:1 isolation transformer so that the module can safely use one of the PA-060 power transformer 6.3VAC filament windings for its power.

The AB-Qi-ST70-ST120 dynamically controls the bias in four 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 has a 1:1 12V isolation transformer for the 6.3VAC supply, making it suitable to use in an amp where the 6.3VAC filament supply is referenced to ground!

– PCB dimensions are 100mm x 57mm / 4″ x 2.25″

– Suitable for Cathode Feedback amps

– power supply requirement is 6.3VAC from one of the PA-060 filament windings

– negative DC bias supply between -55V to -90V

Price for one AB-Qi-ST70/ST120 module: US$85.00       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.