Another idea for a 1929 AWA QSO Party rig.

I have seen many descriptions of early transmitters that used raw a.c. on the plate(s) of the tube(s), usually a self-excited oscillator. Those things usually put out a signal that was not much better than what you would get from a spark transmitter, and the new regulations that went into effect in 1929 required "adequate filtering" for the transmitter power supply.

I had thought of a very simply way to make a MOPA put out a clean signal without rectifier tubes or filter capacitors for the high power stage, although rectification and minimal filtering would be required for the low power master oscillator stage.

Build the final stage with two triodes so that the grids are in parallel and the plates in push-pull. Split the plate tank coil and rf-couple the two halves together with a mica capacitor. Then feed the plate of each final tube directly off the HV transformer, with the midtap grounded. This would form a high level balanced modulator, a DSB suppressed carrier rig modulated by a 60~ tone. The result would be two clean carriers spaced 120~ apart, to give a clean 120 Hz tone in the receiver.

The circuit would have been feasible in 1929, since the high level balanced modulator circuit was already well known before the mid-20's. I have a description of a circuit in a 1922 Army Signal Corps manual. This was one of the very earliest forms of SSB, used for VLF trans-Atlantic communication. The high Q of the tank circuit and short (for the wavelength) loaded Marconi antenna provided the selectivity that suppressed one sideband. AM would have been impossible because of the difficulty of designing an antenna at those frequencies, broad enough to radiate both sidebands.

I seem to have read somewhere that my idea for a CW rig was actually used commercially, since it saved the costs of the high power filter/rectifier components. This was during the days when "slop jar" rectifiers were popular with hams, I believe by mixing up a borax solution and dipping two aluminium strips into the liquid, so the economic saving would have been worthwhile. Larger rigs used mercury arc rectifiers, a large glass bulb where the hot cathode was an arc between an electrode and a pool of mercury. You started the arc by tilting the rectifier. Those must have been a real PITA to keep running. It would have been economical enough to build a simple half wave low power rectifier/filter to run the master oscillator to provide clean drive to the amplifier. I'm surprised no-one thought of using this circuit for amateur transmitters when the 1929 regs went into effect. MCW was still legal then; you just couldn't run raw a.c. on a tube, particularly a self-excited oscillator.