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Jack's One Tube Superheterodyne Radio

One tube superheterodyne receiver

Front view of set with separate power supply

Regenerative sets are a pretty common approach to a one tube radio design. However, the modern superheterodyne (superhet for short) can be built as a one tube radio as well.

The classic All American Five (AA5)set was a superhet. One of the later versions used the following layout:

-Converter tube: a 12BE6 is used as a combination mixer and oscillator The input to the mixer is from a tuned circuit using a variable cap across a ferrite rod or air wound antenna coil. The oscillator tunes 455 kHz above the desired signal, and this signal also goes into the mixer and mixes with the BCB signal. The output of the mixer contains the BCB signal, the oscillator signal, and the sum and difference of the two. The difference (455 kHz) is the desired signal. A 455 kHz transformer is used at the output of the mixer to tune in this desired signal and to provide selectivity.

-455 kHz IF amplifier: 12AU6. An IF transformer at the output of the IF amp provides additional selectivity,

-Combination diode detector/AGC and audio preamp: 12AV6

-Audio output: 50L6

-Rectifier: 35W4

The tube filaments are wired in series and require 121 volts: perfect for our 120 volt AC house current.

The easy way to build a one tube superhet is to get a junked AA5 radio and use the converter tube and first IF transformer. On the secondary side of the IF transformer, treat the set as a fixed tuned crystal receiver, tuned to 455 kHz. Use a germanium diode on the IF secondary, followed by an audio matching transformer and a parallel resistor/capacitor in the bottom lead of the audio transformer (often called a "benny".)

I recommend using a transformer in the power supply to provide isolation from the AC line. A silicon diode rectifier can be used for the DC supply, or you could use a tube rectifier.

For my first attempt, I used a junked Knight Star Roamer as the superhet test bed. First I used the 6BE6 converter tube that came with the set, and it worked great. I wound a few turns of wire around the ferrite rod antenna to couple to an outside antenna. Next, I replaced the seven pin 6BE6 socket with a nine pin one for a 6BA7 tube. The 6BA7 worked even better.

I knew the concept worked, so I moved on to the next set. For this one I used a double tuned front end (a bandpass front end) using two coils wound on FT114A-61 toroids, bottom coupled with a transformer wound on an FT82-61 core. A three turn link on the first coil was used to couple to the antenna.

The 6BA7 was used because of its high transconductance (gain). For the variable cap, I used new old stock three section cap from a BCB ARC-5 navigation receiver. I had a mixer IF coil for that set, so that is what I used. The IF frequency is an odd one: 239 kHz. The IF can contains two air-coupled coils. The IF selectivity can be varied by adjusting the coil spacing via a phenolic rod in the top of the IF can.

A germanium diode was connected to the secondary of the IF transformer. A 200k:1k audio transformer was used to match to sound powered phones, A "benny" was used in the primary of the AF transformer.

A tapped toroid coil was latter added in series with the antenna, so I could tune out the capacitive reactance of the antenna. An RF gain control was added across the three turn antenna link. The power supply was constructed as an outboard unit, so I could use it with other sets.

How does it work? It sounds just like a superhet! Selectivity is consistent at the upper and lower ends of the band. Stability is very good. Weak signals can be copied between the power house stations. The calibrated dial helps in logging stations. For everyday listening, my crystal sets provide greater fidelity, but they cannot match the consistent selectivity of my one tube superhet!

Updated 12/19/06

KE4ID

One tube superheterodyne receiver

Rear view of set

One tube superheterodyne receiver

Bottom view of set

One tube superheterodyne receiver

Schematic


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