Clegg Thor 6 Meter Transceiver

Clegg Thor 6 Transceiver and Junk-Box PS/ Modulator

The Clegg Thor 6 transceiver was purchased at a hamfest. Designed for 6 meters, I noticed that it could be crystal-controlled but also had a VFO built in. That VFO tunes both the receive and transmit sections making it a truly modern transceiver. I asked the seller if he had a power cord. His answer was no. He apparently had never used the piece since it needed a lot more than just a power cord. Since it was light in weight, I correctly assumed it needed an external power supply.

Thor6.jpg (37k)

That missing second chassis
This was the first time I had seen a Clegg Thor 6. What I did not realize when buying it was that it needed not only an external power supply, but that the factory power supply included the built-in plate modulator. One cannot simply substitute a generic power supply. As shown in the ads, the Thor 6 and its original power supply/ modulator sold for $349.95, serious money in 1963.

Clegg Thor Six Ad 2 (20k)

Testing the receiver portion
I located a 12 pin Jones female connector and wired up the 12 volt filament and the receive B+ connections. Using the bench Heathkit variable power supply, the receiver came to life. I adjusted the VFO trimmer cap so that the tuning dial was accurate. With its Nuvistor tube front-end, the receiver performed quite well. It was very sensitive although it could be overloaded by a strong nearby sigmal. Checking the RF output of the driver tube showed that it was producing RF at the same correct frequency as receive . Since I wanted a VFO-controlled 6 meter rig, I decided to homebrew a companion power supply/ modulator using primarily junk-box parts.

Clegg Thor Six Ad1 (10k)

Power output specs
The RF output tube for the Clegg Thor is a 6883, a 12 volt version of the 6146. With 560 volts on the plate, the RF input power is rated at a maximum of 62 watts with an RF output of about 40 watts.

Homebrewing a "junk box" power supply / modulator

A plate modulator needs to be able produce half the RF input power for 100% modulation.

I had a junk box Magnavox AMP-175FF stereo tube amplifier chassis. Its tubes, output transformers, external preamp, and console cabinet were already missing. Its power transformer had a pair of 6 volt windings that could be wired in series for the required 12 volts of filament. Its 6 pin and 9 pin on-board jacks could be re-used for connecting to a wire harness to the transceiver's 12 pin Jones jack. The power transformer could handle the wattage and be close to the right voltage if converted to the "economy power supply" as described in ARRL Handbooks.

Thor 6 on top of junk box Modulator/ PS
Thor 6 on HomeBrew Power Supply/ Modulator (29k)

Testing the transmitter

I preset the transmitter tuning using a grid-dip meter. After connecting the modulator / power supply, I tested the transmitter using a 60 watt lamp as dummy load. It would not light up. I substituted a small 7 watt lamp as dummy load. It lit up to an output of about 3 watts. I suspected that the 6883 RF output tube, a 12 volt version of the 6146, was worn out and "flat". Sure enough, replacing the 6883 made the dummy load shine to about 30 watts of RF output.

Noise in the microphone brightened the dummy load. The modulator was working. Plate voltage was about 520 under load. I tested it on the air with the local 6 meter net, receiving good reports on both audio and signal strength. An oscilloscope check showed that modulation was limited to about 80%. I suspect that the load resistor substituting for the cathode follower transformer might have been limiting the audio drive.

Follow up
I temporarily lifted the DC to the modulation transformer secondary and connected an audio output transformer of 5000 ohms to speaker voice coil as its load. The purpose was to use a speaker to listen to the quality of the audio as well as the level. I tried substituting a grid to plate transformer for the cathode follower transformer. It improved the maximum level significantly but also introduced some hum. I ended up increasing the value of the cathode load resistor which increased the audio and modulation levels without the hum problem.

I widened the gap between the contacts of the transceive relay because I noticed a split second of sustained arc when releasing the mike key which caused the receiver's AGC to block for a second or two on occasion. I am using the relay in DPST mode so I increased the gap by bending back the normally closed contact pair slightly. (Normally closed describes the contact position when the relay coil is not energized. ) I also added Clegg's circuit of a resistor and cap in series across the relay's normally open contacts. Those changes solved the problem.

A Russian Antenna current meter was the previous item on the bench.

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