Here is a link to WB8ERJ 's linear amp project using a pair of 813 tubes. Here is an Australian ham club's homebrew kilowatt amp using a pair of 813 tubes and a pair of microwave transformers in the power supply. If you recognize this amp or if you have experience with similar homebrew amps and/or use of microwave oven transformer power supplies, let me know. These came with 10K ohm ten watt equalizing resistors. I located a set of ten 500 MF electrolytics at a voltage rating of 310 each.
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Here is a PDF file of an article in QEX that describes such a supply. The use of a pair will allow for a full-wave power supply while keeping those connections on the frame at or near ground potential. The transformers have one side of the secondary connected to the frame. Opinion is divided on whether this idea will work. I inquired about using an electrically matched pair of junk microwave oven transformers (secondary is about 2100 volts AC) and rectifiers for a high voltage power supply. The exciter input circuit is untuned, just a 0.001 mike cap to the cathode. The filament choke is a nice homebrew bi-filar winding on a ferrite rod. The current meter responded to a low DC voltage test, but it proved to have a sticky meter pointer. On powering the unit with filament voltage (shown), I measured 10.0 volts at the tube pins, the correct voltage. Since the unit is homebrew, I modified the opening and mounted a computer-style 3 wire male chassis connector. The AC power chassis connector was a FEMALE connector requiring a plug-to-plug AC cord, a dangerous situation.
The variable cap pre-loading had been loosened to the point that the caps scarcely held their settings. Apparently the heavy copper wires needed a hotter soldering iron than was available. I cleaned the unit and found several bad solder joints on the bandswitch, its coil, and on one of the filament connections. The power input was designed to be about a kilowatt. Editors and Engineers' RADIO HANDBOOK, 17th edition, by Bill Orr comes the closest with a grounded grid circuit that allows for various final tubes including a pair of 813 types. I have not yet found the exact circuit in any of the ham radio magazines. An understanding of the circuit is a must. Also, lethal high voltage (Over 2500 volts DC) is present on top of the chassis as well as under when a high voltage power supply is connected.Due care and caution are warranted. I have limited experience with transmitters and RF amplifiers so tend to err on the side of caution. As can be seen, the 813 tubes glow quite brightly with the filaments powered (10 volts at 10 amps). The piece has a filament transformer mounted under the chassis. I did not have a high voltage power supply for it so had to build one (more info on power supply below). I originally bought the piece at auction because of the rack-mount cabinet in which it was installed but became intrigued by what seems to be a relatively simple circuit, a parallel wired pair of 813 tubes in a grounded-grid configuration. I am assuming it to be about 35 to 40 years old. This home-brew amp with two 813 tubes for the final was made by an unknown amateur radio operator (ham) some years ago.
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