Posted on Feb 4, 2014

This project was inspired by a sense of wanting to build something a little unusual rather than with the expectation that it would become a useful transmitter. At the time of writing I`m at the breadboarding stage and have yet to decide if I will keep the unit in its current form or dismantle it to re-use some of the parts in other projects. One of my friends egged me on to build this device and although the rather large size of the valve

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is more his enthusiasm, I did get to wondering if I could actually use this on air. The valve is a Russian made "GM100" directly heated triode. This valve will withstand anode voltages of up to 5 kV, pass currents of up to 1. 6 amps (!) and has an anode dissipation of 1 kW. The filament takes a massive 18 amps at 17 volts although the only slightly suitable transformer I have drives the heater to about 15. 5 volts which is still pretty bright. To give some idea of the scale, the tiny tube on the left (by the multimeters) is a 12AT7 double triode, the larger tube on the right hand is a 3-500Z, a modern 500 watt triode. The variable capacitor and inductor shown in the photograph are moderately big components, at least by UK ham standards, but are dwarfed by the huge tube. Having "short" wiring is obviously quite difficult on something so large but ferrite beads on the grid and anode leads seem to have prevented parasitic oscillations. To run this tube at full power much larger components would be required and the wooden frame would probably be a fire risk! The whole rationale behind this transmitter is to under-run the tube in the interests of frequency stability and longevity although the heater power consumption is a little excessive. The circuit I`ve used for this is identical to that of the 812 Hartley shown further down the page. At the time of writing, 20 watts of RF has been obtained at 1. 9 MHz with a 730 volt supply...

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