12/05/2014 22:30

Two Valves AM-CW TX


In my early days of radios homebuilding , when I had little money and an enormous will to stay in touch with my radio-fellows, I was attracted by the simplest schematics that would have allowed me to surf the aether.

One day I stumbled on the Web on the 'Carter Mobileer': a two tubes AM transmitter, using a double triode and a sweep tube pentode as PA. A triode is a Pierce oscillator (crystal connected to control grid and plate) and the other triode is a cathode follower amplifier, driven by a carbon mike and feeding the PA screen grid.

In the prototype I used the marvellous EL34/6CA7 as PA: very high amplification and respectable dissipated power. While, the double triode was an unknown industrial tube type that I don't recall anymore.

The microphone was taken from a handset. I fed that mike by rectification and filtration of the filaments supply. The mike circuit was driving the low frequency amplifier triode through an audio line transformer, connected as voltage elevator. The PA tank coil had been recovered from a WW II Command Set.

At that time, the antenna was a piece of wire 50 feet long, hang close to the outer wall of my house and running a bit over the roof, ending then up to the TV antenna pole. Not an ideal radiation system !

10W carrier, 25W peak power in antenna: the modulation was 'telephonic', obviously, and not very deep, but that transmitter made me staying in touch with my group of friends for many weekends, on some unused portions of the Shortwaves between 5.0 and 7.0 MHz. 

Those glowing tubes had been a warm presence aside to my workbench: between one soldering and the next, I was used to exchange comments and make technical questions to my friends about some other radio-stuff I was building. Like today one might do by the Internet or by a mobile phone. 

There are today plenty of means to stay in touch with someone. At that time I had the Sortwaves, effective and cheap. I still like that way today, the same way, somewhere on the Shortwaves, where there's only the background noise ...

Radioman, Nov. 2014

Posted by Radioman | Permanent Link