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 arced. 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. Further, maximum output power was always achieved with the loading control set at minimum loading (maximum capacitance) suggesting that the plate resistance of the four tubes in parallel was greater than the predicted 1k (maximum power when the PI network presented the tubes with something higher than 1k). 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). The circuit breaker panel is outside, so I can run more power when it's cold outside since the circuit breakers are thermal. Further, the loading control is now around 4 to achieve maximum output power.

The coax connection at the output loading capacitor started to arc. The shield was grounded at the capacitor and the center conductor continued on to the high side of the loading capacitor. Apparently the coax insulation had melted some when the shield was soldered down. With an antenna analyzer on the output and a 1k resistor to ground on the tube plate caps, the impedance would change as the coax was flexed. This coax from the loading capacitor to the RF bpypass relay was replaced.

Next, the common contact on the band switch wafter section that shorts turns in the output inductor arced. This apparently happened before, as this contact was held in with a screw, while the other contacts were rivetted in. As a temporary measure, a jumper was run from the common to the 80 meter contact. This prevents operation on 160 meters, but I do not run the amplifier on 160 meters.

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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.