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.


The photo below shows the inside of the amplifier. Note the silver plated copper strap from the top of the plate choke to near the top of the tubes, then to the DC blocking capacitor mounted on the plate tuning capacitor. This strap should reduce lead inductance compared with the previous thin long wires. The RL parasitic suppressors have been replaced with ferrite beads as described here. Later the stranded wires through the ferrite beads were replaced with solder-wick braid. The braid has a plate cap soldered on each end. The braid is then wrapped around the copper strap and soldered to it. Note also the small transformer bolted to the side of the main transformer. This implements the W2XC modification to center the filament about ground, reducing hum.

On 1/28/23, the amplifier stopped working. When RF was applied, the plate current would go up, but there was no RF output. Further, adjusting the plate tuning resulted in no change in the plate current. It as suspected that the DC blocking "doorknob" capacitor shown above had failed to an open. It is rated at 1,000 pF, 5 kV. The capacitor was removed and measured on the SARK antenna analyzer. It was found to be 14 pF. The capacitor was replaced.

The photo below shows the RF section of the amplifier with the new DC blocking capacitor. In addition, the insulated wire from the plate copper strap to the plate caps was replaced with "Solder Wick" braid for flexibility without the danger of the wire insulation melting.

Tube Replacement 22 April 2022

As tubes go flat, the loading has to be reduced (increased capacitance) to get maximum power. At some point, maximum power is with loading at the absolute minimum. If I can't get reasonable power at that point, I replace the tubes. Four new 572B tubes were installed 4/24/2022. The table below shows tests on each band (except 160 meters) in both the CW and SSB modes. The amplifier was driving a 50 ohm load (Heath Cantenna). The amplifier was driven with the SEA245 transmitter in the low power CW modes (50 to 75 watts). The mode column indicates whether the amplifier was in the CW or SSB mode. The SSB mode increases the plate voltage. The loading value is the front panel loading knob setting for maximum output power.
ModeEp (kV)Ip (mA)Output Power

Tube Replacement 2 June 2023

As discussed above, a clue that tubes are going flat is when maximum output power is achieved with minimum loading. We were once again in this situation, so the tubes were replaced. Four PSVANE 572B tubes were ordered from Ali Express. They work great! The table below shows tesults with the new tubes installed.

The amplifier has CW and SSB modes. The difference is that the SSB has a higher plate voltage. The transceiver has three power levels (about 50, 100, and 150 watts). The table below shows various amplifier parameters in the CW and SSB modes at each input power level on several bands. All measurements were done with the amplifier driving a Heath Cantenna and the transceiver in CW mode (continuous carrier). The Loading value is the position of the amplifier loading control for maximum output power. The amplfier input SWR is included since the amplifier is grounded grid with an untuned input.

BandAmp ModeAmplifier
Drive (W)
Input SWR
Plate Voltage
Plate Current
LoadingAmplifier Output
Power (W)

Running the system on 20 meter RTTY with the TU-170, the transceiver set to high power (150 watts), and the amplifier set to CW, the following paramters were measured. Since RTTY is run with an AFSK input to the transceiver in LSB mode, and the transceiver transmit audio compressor appears to use RMS level, the average power driving the amplifier was 40 watts even though the transmitter was in high power (150 watts PEP).

BandAmp ModeAmplifier
Drive (W)
Input SWR
Plate Voltage
Plate Current
LoadingAmplifier Output
Power (W)