01/27/2016 21:34

Rectified Mains TX


An attempt to feed a valve oscillator by a directly rectified mains supply had to be done. The chosen tube has been a 6159A, the 26.5V filament sister of the better known 6146A. I only needed a transformer to feed the filament, the antenna changeover relay and to provide a negative bias, in the range of -50V, to the tube for the telegraphic keying.

Here's what I came up with:





One thing is to build an ultra-compact device when you can design each component for a specific purpose. A totally different matter is to build an equally ultra-compact device by all what you can grab from your junk-box. The result has been pretty satisfying in the sense its overall size is comparable or perhaps even less than the SSTR-1 (Strategic Service Trasmitter & Receiver 1), developed in the US and used during the Cold War. The only problem is that the -50V bias is not totally sufficient to cut off the oscillator tube. So, under certain conditions this prototype is of unstable operation. But it works. Slightly more than 5 W out are more than enough to go Full Spectrum.

An innovation in comparison to my usual practice, this time has been the use of a PI-Network antenna impedance matching circuit made with a switched set of capacitors on the antenna side, instead of using a variable capacitor. This suggestion came from the study of some other transmitters used by agents & spies in the fifties. I discovered how practical is this approach. On the Internet you can buy a set of ceramic capacitors having a well distributed set of values, from 22 pF up to 1500 pF. In my prototype I've chosen 11 values and I chose them by a rotary switch. I've tested this transmitter from 3.0 up to 10.0 MHz and its tuner matches almost everything anytime. This, then, resulted a very reliable approach.

The only thing a real SPY cannot yet manage comfortably is how to hang up a decent wire to radiate it's signal sure fire the first time from his 'Wireless Set' !



Radioman, Jan 2016

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