AM-6155 and AM-6154 Grid Bias Circuit

Trying to use a voltage divider for the negative control bias provides for poor regulation in UHF applications. If you are using an AM6155 amplifier (or AM6154) converted to 432MHz, this is evident from the increasing negative voltage as RF drive increases. The tube just goes into more cutoff and RF output is low. As a result the tube ends up being overdriven (trying to increase drive to get more power output) and causes distortion.

 

Here’s a circuit I use based on the G3SEK bias arrangement. There are more complicated versions of this circuit, but this one works extremely well. The grid voltages are based on the Svetlana 4CX400A tube. If you are using the stock Eimac 8930, then the 33V zener may need to be changed to provide enough voltage adjustment. Mount the 1k pot on the rear panel for easy idle adjustment.

All resistors are 1% metal film types. Do not use carbon composition resistors. This is for DC control, not an RF application. Digikey and Mouser have all these parts. The diode bridge and filter cap are shown for clarity only. These are already installed in the amp.

 

This circuit also improves 144MHz and 222MHz conversions and is highly recommended.