807 Vacuum Tube, My Workhorse of Yesteryears!

 I still have the sweet memories of working a lot DX on CW in late 1980s using my homebrew 3 x 807 radio. That was a great upgrade from my 5W VU2VWN QRP which I homebrewed as soon as I got my amateur radio license in 1985. As the power supply, I was using a 600 V AC transformer made by a local ham and I had homebrewed the solid state bridge rectifier circuit with four diodes and large high voltage capacitors. In those days, I had sourced 807 valves from a local radio shop and if I remember correct, they were made by RCA. 807 is a beam tetrode vacuum tube which used to give me 40 W power each so that my TX was 120W on CW. In those days, 807 vacuum tubes were also being used in audio amplifiers, and that is how I could get them from a local radio shop. Even at that time, vacuum tubes were being phased out as solid state audio amplifiers had become commonplace.

For AM, I needed a lot of audio power for modulation and I had homebrewed a solid state audio amplifier for that. But AM was not powerful enough to reach my signals to DX land, though I had a single contact to Indonesia with 4,4 report. CW used to take my signals to the other side of the globe in W land, during the grey line propagations of dawn and dusk. A few of my friends are still using 807 valves in their linear amplifiers which have been homebrewed as recently as a few years back. One of them is using a Radioberry to drive the linear, with probably other stages in between and putting out a very strong signal on HF bands. Some others have secured 807 tubes recently and kept them hoping to start homebrewing soon!

Though 807 tubes were fully rated for up to 60 MHz, as my home Philips Prestige Radio which I had used as RX did not have 20m band, I had operated mostly on 40m. When the Solar Cycle went down and 40m propagation came down, I had made an additional tuning coil for 80m and worked a few VU stations on 80m, mostly on CW. For the same reason, I could not experiment on 21 MHz and 28 MHz in those days. Moreover, the lack of a good communications receiver prevented me from activities on higher bands. Somehow I was not aware of activities on 160m in those days. Otherwise I could have tried that as I had plenty of space in my parents' home for even a 160m dipole to be tied on top of tall coconut trees!

I remember that the anode cap of 807 vacuum tube connected at the top was a potential shock hazard with the full rectified DC voltage from the transformer reaching there. One had to be extremely cautious while making adjustments with the cover of the enclosure removed. Similarly, the high voltage filter capacitors in the power supply had to be discharged using a resistor before handling. I was always wondering why the anode connection was made at the top, instead of at the base as in other valves. Now I came to know that it has an advantage in preventing arcing between the high voltage anode and other pins in the base.