A History of Mullard Valves 1920-40

During the First World War Captain Stanley R. Mullard worked for the Admiralty on high vacuum development and supervision of the production of transmitting and receiving valves at government establishments. He held several patents with the Admiralty. After the end of the war, in 1919, he joined the Z Electric Lamp Co. at Southfields, where he soon became managing director, and manufactured high power transmitting valves for the Admiralty, the Post Office and other government departments. The valves were sold under the S.R. Mullard brand and exploited his patents.

He left the Z company, which collapsed, and established the Mullard Radio Valve Co. Ltd. in 1920, first in the Z factory and later at Hammersmith. The company moved to larger premises in Nightingale Lane, Balham, SW12 in 1922. Production was mainly concentrated on transmitting valves with a small production of hand built 'R', Weco dull-emitter (thoriated tungsten) and other receiving valves.

In the same year the Mullard company was one of the founders of the British Broadcasting Company Ltd. and public broadcasting began in the U.K. There was a much increased demand for valves for domestic receivers and by 1924 Capt. Mullard needed to expand production. He financed this by selling half the shares in the Mullard Radio Valve Co. Ltd. to N.V. Philips Gloeilampenfabriken of Eindhoven, Holland.

By 1923 United Kingdom domestic receiving valves were more-or-less standardised on a 4-pin base and a 4-volt heater supply derived from a two cell, lead acid accumulator. Later both single cell (2-volt) and triple cell (6-volt) series were introduced. By 1930 the single cell, 2-volt series were almost universally adopted for battery sets.

Philips had invented the 'Azide' process which increased the emission of dull emitter cathodes beyond that obtainable from thoriated tungsten. This process produced an oxide covered cathode which was more robust and long lasting than other competing methods, for example the A.C. Cossor 'Wuncell' type. Before assembly the tungsten filament was directly oxidised or plated with copper and oxidised. The anode was coated with barium nitride (Azide). After evacuation the anode was heated (probably by induction) and the evaporated barium nitride reduced the oxide on the filament forming barium oxide, metallic tungsten or copper and nitrogen which was adsorbed by the getter.

1925-6 PM series valves

Mullard commenced production of these P.M. (Philips Mullard) valves in 1925 with the PM3 and PM4.

Very early PM valves had solid pins with a single slot cut into them. Connections to the electrodes were by wires that emerged from the black ebonite bases via small holes just clockwise of each pin and were then wrapped around its and soldered in place. The bases were plain cylinders without an anode indication moulding. They all originally bore a paper label giving the warning "THERE IS NO GLOW FROM THIS FILAMENT" and details of filament voltage and current as well as maximum anode voltage. The paper labels are often missing today and may have been re-applied when present. The identity of valves should be checked by examining the engraved markings. Mullard boxes had an unusual feature. The valves were packed with the pins up and a hole was provided in the top so that a contact probe could be inserted. Testing was carried out at the dealers at the time of sale without unpacking the valve. The hole in the top flap is visible in the example at left.

The old style Mullard logo, the aerial-coil-earth symbol with the valve type and a BBC type 3 symbol with an abbreviated valve type were all engraved on the side of the envelope. The engraving is often very feint and hard to read. A good hand lens, high contrast lighting with a dark background and breathing on the valve to mist it may all help to discern the markings. They are engraved in the glass and immune to normal cleaning methods such as a soft cloth moistened with a dilute solution of washing-up liquid.

There are some suggestions that these very early types were imported from Holland and not made in the U.K. Valves of this type were probably marketed during 1925 and early 1926. Small quantities of the less popular variants were probably supplied by Philips throughout later production.

1926-27 PM series valves

These 'split-pin' types were very quickly replaced with valves with similar envelopes but with different pins and bases. The pins were tubular with two slots at right angles. The electrode wires were passed up the centre of the pins and soldered at the tips. The bases had two adjacent ridges up the vertical curved surface near the anode pin. The groove formed was infilled with white paint as was a letter 'A' between it and the anode pin on the bottom surface. The envelope engraving was as before. The wording on the only example I have with a paper label has "THERE IS PRACTICALLY NO GLOW FROM THIS FILAMENT"- 'practically' added. The wording seems to change from one to the other with no particular order or timing. These valves were made in 1926 or 1927.

1927-30 PM series valves

From about 1927 the BBC engraved label was replaced by the BVA logo. Mullard was a founder member of the British Radio Valve Association (BVA) cartel in 1926. The white infill was omitted although the moulding remained until well into the 1930s. Valves with the BVA logo on the side  and the old style Mullard logo cannot date from before late 1926 nor from later than 1930.

Post 1930 PM series valves

Around 1930 Mullard moved all the envelope engraving to the top of the valve and adopted a new logotype. The envelope shape also changed, still being a balloon but having a much steeper neck at the junction with the base. I have one post-1930 PM2, which is still of the balloon shape but lacks the anode indication moulding. The box changed considerably, on at least two occasions, as can be seen from the examples shown. Very late PM series valves, probably made only for revalving old sets,  were made in the  'shouldered' ST envelope, similar to that of this IW4/500.

All of this is based on the valves in my collection.