September 2000

This little 1mW transmitter is a perfectly straightforward design except that it works down to an h.t. of just 12V. The valve I used in the prototype was a DL96. I also tried the pin-compatible DL94 which also worked well. By choosing the appropriate inductance for the p.a. tank coil, the transmitter can be made to operate on the 160m, 80m, 40m, 30m and (probably) 20m amateur bands. Note: the crystal frequency must be the same as the output frequency. It may operate at even higher frequencies but I've not tried operating a transmitter like this above 40m. Nothing stopping you having a go though! Click here for the circuit diagram (44K gif). The transmitter did surprise me in one respect: instead of the anode current decreasing as the circuit began to oscillate it increased. Since my September column was published I've tried a DL93. This valve behaved as expected in that the anode current did show a marked decrease as the circuit began to oscillate. I think the inverted dip I got with the DL96 and the DL94 involves the space charge in some way. Anyone care to offer an explanation? Seriously, I'd like to contact anyone who has seen this effect at first hand and who knows what causes it.

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