Neutrodyne

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The Neutrodyne was a particular type of Tuned Radio Frequency (TRF) radio receiver, in which the instability-causing inter-electrode capacitance of the triode RF tubes is cancelled out or "neutralized". In most designs, a small extra winding on each of the RF amplifiers' tuned anode coils was used to generate a small antiphase signal, which could be adjusted by special variable trim capacitors to cancel out the stray signal coupled to the grid via plate-to-grid capacitance.

The circuit was developed in the early 1920's by Alan Hazeltine and made by a group of more than 20 firms that were licensed to produce Hazeltine-Neutrodyne receivers known as the Independent Radio Manufacturers. Hazeltine's invention effectively neutralized the high-pitched squeals that had plagued early radio sets. The design also neutralized the stranglehold that RCA then held on the commercial radio industry. Compared to the technically superior Superheterodyne the Neutrodyne was cheaper to build and operate, and much easier for non-technical owners to use.

To properly set up a Neutrodyne receiver, not only did the circuitry need to be aligned for peak performance, (that is, getting all its tuned circuits operating "in step"), it also had to be neutralized. However, this procedure only needed to be done once (albeit by a serviceman) and thereafter the radio could be tuned by anyone without special skill, a unique feature at the time. The neutrodyne was the first commercial receiver suited to use by the general public. By 1927 some ten million of these receivers had been sold to consumers in North America.

By the 1930s, advances in vacuum tube manufacturing finally made Edwin Armstrong's Superheterodyne design practical for domestic receivers. Ironically, these same advances made it also possible to build superior TRF receivers that did not need neutralization, but since an even more superior superheterodyne could be made for about the same cost, the TRF technique for AM radios fell into disuse.

However, with the proliferation of TV receivers in the 1950s, neutralized triode amplifiers once again became popular, this time as the RF amplifier stage of tube VHF TV tuners, due to their superior noise performance.

[edit] Neutralization Technique

To neutralize a Neutrodyne receiver the procedure went something like this:

Turn the receiver on. Tune the receiver to a strong station near the high end of the dial (e.g., 1500 kHz) or use a modulated signal generator set to that frequency and tune the receiver to it.
Turn the receiver off.
Start with the final RF stage and work backwards towards the antenna.
1. Disable the tube filament (so the tube is not conducting) either by unsoldering one lead and taping the wire to prevent it from shorting to anything or (preferred) using a neutralizing adapter between the tube and its socket.
2. Turn the receiver on. If you can hear the signal that the receiver is tuned to, that tube needs neutralizing.
3. Adjust the neutralizing capacitor for that tube for minimum volume.
4. Retune the receiver for maximum volume.
5. Adjust the neutralizing capacitor for that tube for minimum volume.
6. Turn the receiver off.
7. If you disabled the filament by unsoldering it, resolder the lead.
8. Repeat with each tube until all RF stages have been neutralized.
The receiver should be realigned after being neutralized and if it still squeals the neutralization procedure repeated.

It is important to neutralize using the actual tube that will be in that socket as grid-plate capacitance varies some from tube to tube. Also once neutralized the tubes should not be exchanged between sockets. Often replacing a defective tube with a new one required neutralizing the receiver again (therefore tube replacement usually required a serviceman).