How the Ultradyne L-2 circuit works

OCR'd by Charles Days,  How an L2 works as quoted from Loomis

M. T. Loomis 3rd. edition 2/27

380. A different application of beat production to reception of signals is shown in figure 453. This apparatus is named the ultradyne, by Mr. Robert E. Lacault, engineer of the French Signal Corps, who developed it. The ultradyne receiver uses intermediate transformers, and a detector for reducing modulated signals to audibility, but the part of the circuit corresponding to the first detector and oscillator of the superheterodyne is quite different. The first tube, which in the superheterodyne is called the first detector, and which functions in producing the superaudible beats, is called a modulator. In describing this circuit Mr. Lacault says: "No pick-up coil and grid condenser are employed in the ultradyne circuit and no B battery is connected to the modulator tube. The plate filament space of the modulator tube acts as a resistance in the circuit. In this arrangement the plate of the modulator tube is supplied with high-frequency current by the oscillator, the former being active only during half of each cycle when the plate is positive. This produces a change of plate filament resistance which varies from practically infinity to about 20,000 ohms, during each half cycle of the oscillator current when no signal is being received. When the grid potential of the modulator tube is varied by incoming signals tuned in by the circuit L' [10 turn antenna input coil] C' [20 turn coil in the regenerative variometer] the lower resistance value is varied above and below the amount mentioned with various degrees of amplitude according to the phase relation between the incoming signal and local oscillations. This produces a beat note which is amplified and detected. An advantage of this system is that, no matter how small the amount of received energy, a response in the circuit is produced. In order to obtain greater sensitiveness and amplification, one may use regeneration in the circuit of the modulator tube by merely connecting a feedback coil in the plate circuit and coupling it to the grid circuit of the same tube as shown in the illustration. [Refer to Radio News schematic] This produces great amplification for the reception of weak signals and is quite easy to adjust after a station is tuned in."

The radio-frequency transformers used in the ultradyne receiver are of a different design from those employed in the regular superheterodyne. They are so constructed that they amplify at one wave length only. The band of frequencies amplified by the ultraformers is just wide enough to avoid distortion of radiotelephone signals, but they are, nevertheless, sharp enough in tuning to provide the necessary selectivity. The first ultraformer connected between the plate of the modulator tube and the grid of the oscillator tube is of a slightly different construction, the primary being shunted by a small fixed condenser to tune it exactly to the proper frequency. It is important that the capacity of this fixed condenser be exactly .001 mfd. as otherwise the frequency of the input circuit would be different and the amplifier would not operate as efficiently as it should.
In the diagram "Amperites" are shown in series in the filament circuits instead of rheostats. The latter could easily be substituted if desired.

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Here's a great article by the inventor himself, Mr. R. E. Lacault, sent to me by Dale Davenport:

This is a schematic of  the original 6-tube version of the Ultradyne.  Note the different arrangement of the oscillator tuning and lack of regeneration.  A two-step audio amplifier was presented in Radio News as a separate article..

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This was the last version of the Ultradyne as produced by Mr. Lacault before his death ca 1929.  It was called the RE-29 and used screen grid tubes.  Note the regeneration scheme has gone by the wayside.  It was intended to drive an audio amplifier with a pair of 45 tubes.

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