Re: Questions on 160/80 metre AM transmitters.
For anyone thinking of building a rig similar to the AT5, here's some details of the neat little rig I built back in 1970 that was similar in size to the AT5.
It only had 4 valves, as I used an ECF80 as the VFO/Buffer. I chose a 5763 as the PA (really an RF version of the 6BW6).
The modulator used the same lineup - a 12AX7 into a 6BW6. However, I didn't use a centre-tapped choke, just a choke for the modulation. (you just connect the 6BW6 anode to the DC side of the PA RF choke, and the HT to the other side).
I just used a small mains transformer primary as the Choke, and the audio quality was actually better than from an AT5 ! (the transformer had a 6V secondary, so I connected that to a front-panel bulb, to indicate the modulation).
For 80m there was no switching - I just doubled in the PA - didn't even bother to tap the PA coil !
Oh and I managed to make the VFO very drify-free, just by selecting temperature compensating capacitors by trial and error.
Hope this may be useful to people.
Roger G3YRO
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