A Push-Pull Hartley VFO transmitter . This is a variation of the the Electron Coupled Oscillator circuit, pioneered by Dow in 1931. Dow's ECO has the screen "grounded" for RF. In this circuit, because I am using filament-type tubes instead of heater-cathode types, and because I didn't want to build or use a pair of filament chokes with this first attempt, I chose to use the "cathodes" as the "grounded" or "common" electrode. I am hoping that it otherwise works much the same way as the standard ECO: i.e., the cathode-grid-screen are the oscillator section, with the screen acting as the oscillator's plate, and the plate of the tube and its associated tuned circuit making up the amplifier section of the transmitter. So, essentially, we have two tubes performing the work of four. Should be interesting. This circuit uses grid-block keying, and all components, with the exception of the values of tuned circuit components, are shown on the schematic. Plate voltage should be about 1500 VDC, screen voltage should be 300 - 500 VDC, suppressor voltage should be about 40 VDC, and bias (actually blocking bias) should be about -150 VDC. Power input should not exceed about 150 watts. Adjust voltages and loading for best keying. It might be better to use a push-pull grid/parallel plate (push-push) circuit, as suggested by one of the members of the GB list. The plate circuit would then be tuned to twice the grid circuit frequency. This should provide much better stability, and eliminate any parasitics or need for neutralization. Experimentation is in order.
For any of these oscillator circuits which use tetrodes or pentodes, be sure to never apply screen voltage before applying plate voltage. Otherwise the screens may disappear in a blinding flash of light. This circuit should feed a good antenna coupler.
I will post photos of a finished version of this rig asap.
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