Dentron Clipperton-L

I got the Dentron Clipperton-L at a hamfest mid-2019. The SO-239 connectors were pretty weak. After a bit of operation, they arked. They were replaced with new connectors. Someone had added one ohm carbon composition resistors in series with the plate of each tube (between the parasitic suppressor and the RF choke). These tended to fail and arc. I found no documentation that suggested adding these, so I took them out. Various modifications are suggested by VE3NH, though I did not do any of these. The amplifier already had the KM1H modification of adding a 10 ohm 20W wire wound resistor between B+ and the plate choke. This resistor was added on the bottom of the power supply printed circuit board. The amplifier had already been modified for operation on 10 meters.

The amplifier had a voltage divider on the filament supply to try to balance the supply to ground to minimize hum. This voltage divider was replaced with a Hammond 166J6 transformer as suggested by W2XC. The primary of this transformer is unused. The secondary is used as an autotransformer to balance the filament supply.

The output network was checked by replacing the tubes with a 1k resistor and measuring the impedance looking back into the amplifier output. The 1k is an approximate value based on Vp/(2Ip). The amplifier could be tuned to 50+j0 on each band.

Still, output power was not as expected. I finally put new 572B tubes in, and full power was available (as limited by the circuit breaker to the shack - I am still running the amp on 120VAC).

Resources

The photo below shows the inside of the amplifier before replacing the filament voltage divider with the transformer. The two large power resistors just to the right and above the filter capacitors are the voltage divider.