Bias Adjustment

The original TL-922 used a 7.5 volt Zener and had no way to adjust the bias.  Don Kessler sorted this in his kit.  He provides a simple arrangement where you move a shorting link over a string of pins to select the optimum idle current.  It is very neat and tidy. The kit is an easy install and his instructions are superb.

Just a safety note: Adjustment is never to be made while powered “ON”. The tubes must ALWAYS be vertical, base up or down but NEVER horizontal with the power on. Even if it is just the filaments.

Across the bottom of the control PCB you will see diodes D11 to D18. Above the string of diodes are the 2 pin headers. You can see a 2 pin shorting lug is covering the pair 2nd from the left.

The “ZSAC” under the jumpers stands for “Zero Signal Anode Current” The closer to the left that the jumper sits, the lower the idling current. As you move the jumper to the right the idle current increases. You have 8 possible set points in this string. No jumper at all, is option 1. Then you can jumper or bypass D11 first and progress right bypassing them cumulatively. REMEMBER. Do not adjust with the amp on its side like this while powered up.