The A.J.S. Type 'F' Receiver - Inside Out

The A.J.S. type ‘F’ receiver used the company’s standard T.R.F. design that was used in most of the A.J.S. non-superhet receivers. The same components are used in many of the cheaper two and three valve sloping panel models, the more expensive table models, the pedestal receivers and the top of the range Console receiver.

The sloping panel type 'F' receiver with one of the long wave coils removed. Courtesy of Marco Manfredini.

Most of the components were made by the company. Charles Hayward who was in charge of the sidecar section at Lower Walsall Street works was asked to design a coil-winding machine so that A.J.S. could produce its own coils. Not only R.F. coils were produced but also the audio coupling coils that were a feature of many of the receivers. The company made everything except for the valves, control knobs and the meter. It is uncertain as to whether the laminations for the audio chokes were made in-house.

The receiver's back panel. Courtesy of Marco Manfredini.

Part of an advert from 1926

The valves used in A.J.S. receivers were initially made by Mullard and carried the company’s logo. When A.J.S. started receiver production in 1923, valves used plain tungsten filaments which had to be heated to around 2,000 degrees centigrade to obtain adequate emission. These were understandably called ‘Bright Emitters’ and required high filament currents and had a relatively short life. They were very expensive and it was common for suppliers to offer a service to replace a burnt-out filament. Around 1925 this all changed with the development of ‘Dull Emitters’. These use a filament that is coated with Barium or Strontium oxide and gives a much higher electron emission level than plain tungsten. The filaments now only have to be heated to around 800 degrees and only emit a dull glow, hence the name. The filament current is greatly reduced and the valves last a lot longer. Mullard’s dull emitters are the PM series and these were used in the A.J.S. models for 1925. From 1926 onwards the company decided to use ‘Six-Sixty’ valves, which were made by The Electron Company in London.
Six-Sixty valves were available with different filament voltages. The types used by A.J.S. with a 2 volt accumulator were:

H.F. amplifier or detector – S.S.2HF

Detector or L.F. (non-microphonic) – S.S.2a

Audio output – S.S.10

A.J.S. accumulators were made by Joseph Lucas Ltd., and the high tension batteries were made by Ever Ready, Columbia and Siemens.
A receiver front panel showing the A.J.S. made valve holders.
A.J.S. always produced it’s own loudspeakers. Initially these were horn loudspeakers which used a telephone- type earpiece with a metal diaphragm. They were available with a wood or metal horn, but in either case the quality from such a device is very poor when compared with what we are used to today. Later on moving iron loudspeakers with paper cones were produced in the Stewart Street works. Not many of these have survived and it’s hard to know how they sounded.

The receiver's front panel.

The moving iron meter and meter switch.

The underside of a type 'F' receiver.

When A.J.S. radio production ended, Stewart Street works were purchased by the Symphony Gramophone and Radio Company and they used moving iron loudspeakers based on the A.J.S. design. Their loudspeakers were very good indeed and compared favourably with modern types, so it’s likely that the sound quality from the later A.J.S. loudspeakers was equally good.

A.J.S. also sold headphones. These were of the standard type of the day and it is doubtful if they were made in Wolverhampton.

Circuit Diagram

The circuit diagram of an A.J.S. type 'F'.

A close-up view of the long wave coils.
The plug-in long wave coil assembly.
The underside of one of the A.J.S. made capacitors.
The wavechange switch. All the switches were made by A.J.S. and had large and reliable contacts.
One of the A.J.S. patent tuning capacitors.
The type ‘F’ receiver is a four valve T.R.F. design consisting of a tuned high frequency amplifier, a detector and two audio amplifiers. It covers three wavebands; short wave (higher end of the medium wave band), broadcast (lower end of the medium wave band) and long wave. A vario-coupled reaction circuit is used which prevents external radiation on the short wave and broadcast bands and a metal screen is incorporated to eliminate the possibility of hand capacitance effects. Provision is made for switching-out the last two valves, so that the receiver can operate as a two, three or four valve radio to reduce battery consumption. In the 1920s high tension batteries were very expensive and the accumulators usually had to be taken to a local shop for charging, so this kind of facility would appeal to purchasers. Each valve is fitted with a plug-in resistor in series with the filament so that a wide range of valves with different filament voltages could be used. A range of suitable plug-in resistors was available from A.J.S. dealers.
The high frequency amplifier has separate aerial and output tuned circuits and is neutralised. The coils are wound on large diameter formers using heavy gauge copper wire. In the mid 1920s this was considered to be good practice because it offered excellent mechanical stability and a low internal capacitance and resistance.
The plug-in long wave coil holder and coil arm. Courtesy of Marco Manfredini.
The basket wound reaction coil.

An A.J.S. patent audio coupling choke.

The leaky grid detector also provides feedback for reaction and the audio stages are choke coupled. A.J.S. always favoured choke coupling and claimed that it produced a higher quality sound than conventional transformer coupling. Choke coupling was never popular and was developed because a choke was cheaper to produce than a transformer and the low dc resistance of the coil meant that the valve anode was at a higher voltage than when used with an anode load resistor so that a larger audio signal could be produced. This was felt to be important because at the time most receivers were battery powered and so the HT voltage was not that high. In practice any improvement would only have been marginal.

Circuit Description

High Frequency Amplifier

A simplified version of the high frequency amplifier showing just the short wave and broadcast band components.

Broadcast and Short wave bands
The aerial tuned circuit consists of L1 and C1, L1 being tapped for short wave. La is the aerial coupling coil which acts as a coupling capacitor (one end is not connected).

The output of the amplifier is tuned by L4 and C2. L4 is tapped for short wave and is mounted at right angles to L1 to prevent feedback.

The internal capacitance of the valve could produce instability and this is eliminated by neutralising circuit L5 and C3.

The negative feedback from L5 is adjusted by C3 to counteract any positive feedback resulting from the valve’s internal capacitance. 

Two terminals on the front panel, normally linked, allow for the connection of an external tuning coil and two terminals on the back panel (labelled R and R and normally linked) in series with V1 grid, allow an A.J.S. Patent Rejector Circuit to be used with the receiver.

Long wave band
External plug-in aerial and anode tuned circuits are used on long wave. Aerial coil L2 and output coil L3 plug into a switched socket on the front panel. When the coils are removed their connections on the socket are automatically short-circuited.

In use long wave is selected on the rotary wavelength switch and the coil arm is raised. If the arm is fully lowered the coils are switched-out. The arm controls the coupling between the coils.

They are connected so that positive feedback will result if the coils are closely coupled and so the arm acts as the long wave reaction control. L4, L5 and C3 remain in circuit on long wave to provide neutralisation and the aerial is directly connected to the aerial tuned circuit. 

A simplified version of the high frequency amplifier showing just the long wave components.
A range of plug-in coils was available allowing the receiver to operate down to 55KHz.

The detector.


The detector is a conventional leaky grid detector, directly RC coupled from V1 anode. L6 is mounted inside L4 and is rotated by the reaction control knob to alter the amount of coupling between the two coils.

This provides positive feedback for reaction. L6 is a very thin coil so that the coupling between the coils at zero reaction can be reduced to an extremely low value. It also provides an extremely fine and precise reaction adjustment.

This type of coil used to be described as ‘basket wound’ and sometimes called a basket coil. On the short wave and broadcast bands this is the only reaction control, but on long wave the main reaction is adjusted by the coil arm and the reaction knob becomes a fine reaction control. C4 filters out any remaining carrier wave and the audio signal is developed across audio coupling choke L7.

Audio Amplifier
The first audio amplifier is RC coupled from the detector by C6 and R1, the audio output being developed across coupling choke L8.

The two coupling chokes L7 and L8 are mounted at right angles to one another to minimise feedback in the audio amplifier. The output valve is RC coupled to V3 anode by C7 and R2 and provides a direct output for a horn loudspeaker, moving iron loudspeaker or headphones.

The loudspeaker positive terminal is directly connected to the HT(3) + terminal on the back panel so that the audio output can be supplied from a separate, or higher voltage supply.

The two-stage audio amplifier.

The receiver front panel.

We would like to thank Marco Manfredini for providing some excellent photographs. We would also like to add technical details of other A.J.S. receivers and if anyone has any information please email the webmaster.

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