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

Background
 The A.J.S. type ‘E’ receiver used what became 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 two and four valve sloping panel models, the more expensive table models, the pedestal receivers and the top of the range Console model. The type "E" first appeared in 1924 complete with a built-in H.T. voltmeter. In 1925 the voltmeter was removed and an instruction panel added to cover the hole where the voltmeter used to be.

The sloping panel type 'E' receiver with the top open.

Components
Most of the components were made by the company. A.J.S. produced all of the coils including the tuning coils, reaction coil and the audio coupling coil. The company also made most of the components including fixed capacitors, variable capacitors, multi-pole rotary switches, resistors and valveholders, in fact almost everything. Knobs were almost certainly brought-in and it is uncertain as to whether the laminations for the audio chokes were made in-house.


The inside of an A.J.S. carbon resistor.

The carbon resistor appears to be a piece of carbonised card that has been dipped in wax.
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. These valves were very expensive and it was common for suppliers to offer a service to replace a burnt-out filament.


The bottom view of the receiver.
Another view of the underside showing the reaction coil on the left.
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 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. This receiver would have used Mullard dull emitters from the PM series consisting of the following:
HF - PM1HF
DET - PM1LF
LF - PM2
A.J.S. accumulators were made by Joseph Lucas Ltd., and the high tension batteries were made by Ever Ready, Columbia and Siemens.
A close-up view of an A.J.S. tuning capacitor, patent number:

210030 and 209986
This view of a tuning capacitor shows the flexible brass strip at the top of the shaft to connect the moving vanes to the solder terminal. The brass strip also acts as a light spring and so when the capacitor is fully closed it will tend to open again. To eliminate this a thin spring washer is fixed under the shaft mounting nut to apply a small amount of friction to the shaft so that it can't move too freely. The nut is adjustable to set the pressure.
This view shows both of the tuning capacitors in the receiver, aerial tuning on the right and high frequency tuning on the left. The metal plate in front of them eliminates hand capacitance effects.
An unrestored receiver minus long wave coils, valves, tuning and reaction knobs. The thin spring washers that apply a small amount of friction to the tuning control shafts can be seen under each central nut.
Circuit Diagram


The circuit diagram of an A.J.S. type 'E'.
The type ‘E’ receiver is a three valve T.R.F. design consisting of a tuned high frequency amplifier, a detector and an audio amplifier. It covers three wavebands; short wave (higher end of the medium wave band), broadcast (lower end of the medium wave band) and long wave (with the external coils).
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 hand capacitance effects.

Provision is made for switching-out the last valve, so that the receiver can operate as a two or three 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 3.2Ω plug-in resistor in series with the filament that can be changed to allow a wide range of valves with different filament voltages to be used. A range of suitable plug-in resistors was available from A.J.S. dealers.


The instruction panel.
The high frequency tuning coil showing the solid construction and the internal basket wound reaction coil.
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 leaky grid detector also provides feedback for reaction and is choke coupled to the audio output stage. 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. 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 H.T. voltage was not that high. In practice any improvement would only have been marginal.
An A.J.S. capacitor. It consists of a bakelite cover with nut and bolt terminals. The cover screws directly on top of the capacitor itself.
The capacitor consists of copper foil pressings interleaved with pieces of mica. The assembly is held together by the external brass sleeve and has been dipped in wax.
 

The audio choke that is used for the choke coupling between the detector and audio output. On top are the two A.J.S. carbon resistors that are held in place by the end contacts and their tightening screws.


A close-up view of one of the resistors.

The neutralising capacitor
and wavechange switch,
both made in-house.
The top of the receiver with the cover open showing the top panel.


The top panel.

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 neutralising capacitor 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 top 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.

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 audio amplifier is RC coupled from the detector by C6 and R2. The output is connected to a moving iron horn loudspeaker or headphones via the selector switch.


Audio Output.


A final view of the receiver.

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