Nostalgic Memories of my 3 x 807 Vacuum Tube TX in 1980s

 807 Valves (Vacuum Tubes) used to be the final RF amplifiers for my hybrid TX in 1980s. A step up transformer with 600 V supplied by VU2PKH used to provide the power to them. Rectified DC voltage would be in the range of 850 Volts. Obviously, I needed huge high voltage capacitors for the rectifier along with high voltage rectifier diodes.

Plate modulation on AM needed a lot of audio power, for which there was another solid state audio amplifier if I remember correctly, using a power IC. There were other valves as well in the RF circuit. I had used the VU2VWN QRP VFO which I had homebrewed earlier, with 2 x BFW 10 MOSFETs.

Homebrew PTT mic using a push button switch and condenser mic in a thermocol enclosure, covered with insulation tape, was the grand control. Relays would switch the antenna input between my Philips Prestige BC RX and the TX. Plate supply of valves in the RX would also be switched off simultaneously by the relays to prevent overloading of the RX with TX output in the near field. Heater coils of the tubes in the RX would remain on so that there will be no startup delay in RX when the PTT was released.

I could work a lot of DX with that TX having 120 W DC input, into a 75 Ohms TV cable and dipole antenna. I had no access to RG 58 or RG 213 in those days. The Pi tank circuit in the final stage with huge insulated copper wire coil and high voltage air dielectric capacitor would have tuned any excess SWR in the antenna circuit! I used to check the TX output with an incandescent lamp, which on thinking back, was also a dummy load for tuning! The Pi tank would be tuned for maximum brightness of the incandescent lamp, which will also give an indication of the power output, depending on the wattage of the bulb used.

My RX in those days was a four band Philips Prestige vacuum tube radio which my father had bought as home radio, in 1970s. I had modified it for switching during TX as mentioned. Tuning indicator in the radio in those days used to be a 'Magic Eye', a vacuum tube with a wonderful green display. The gap between the rectangular display on either side used to close if the received signal was very strong.



73 de Jon, VU2JO

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