af-67 ON 20M

[NR5A]

On 20m my AF-67 pegs the meter, you can't dip or load it according to the meter on the rig yet by watcing the meter on a ant tuner you can tune it up to full power. I'm lost on this one, what should I do?

Jerry - NR5A

[WQ9E]

Jerry,

A big problem with the stock AF-67 is the only way to control drive is by de-tuning the driver stage.  Can you drop the plate current by slightly de-tuning the driver?  6146 tubes are easily damaged by too much control grid current; 3.5 mils is about the safe maximum and a healthy AF-67 will create a lot more grid drive than what is safe.

The minimum loading (maximum capacitance too low) available is generally too high on 20 meters into a normal antenna (realize this was primarily designed for mobile service).  There are modifications on the web to increase the minimum loading capacitance available.

The combination of excessive drive and too much loading are probably responsible for the excessive plate current on 20 meters.  Be careful with your testing until you cure the problem lest you burn out components in the power supply and final sections.