Blog posts of '2013' 'July'


Just when you thought it was all over, the Olympic hype has started up again and deposed the contents of the Royal nappy from prime news so I felt it would be topical to discuss Mullard's part in the UK Olympic games.  

What's that I hear you say "But the Mullard name faded into history in 1982 so how did they get involved in 2012 & 2013?"

Well..... I am talking about the 1947 Olympic games in which Mullard brokered a deal with the Olympic committee to have Philips supply the loudspeakers for the games - these were great games too before Ministry of silly walks for horses and ping-pong were introduced as Olympic sports.    Although a more traditional games, the 1947 bash didn't completely embrace the spirit of the original Olympic games as the athletes were clothed, still, beach volleyball wasn't an Olympic sport at this time either so I suppose this wasn't such a great loss.

Anyhow, enough meandering, here are a couple of photographs to show the Philips/Mullard involvement:-




We did the receiving valve numbering system sometime ago in this blog and I thought it was about time we explained the corresponding transmitter valve system, so here we go:-

FIRST LETTER - this describes the general class of valve

M    LF power amplifier or modulator triode

P    RF power pentode

Q    RF power tetrode

R    Rectifier

T    RF power triode

SECOND LETTER - this describes the type of cathode

G    oxide coated mercury rectifier filament

V    oxide coated indirectly heated cathode

X    tungsten uncoated directly heated filament

Y    thoriated tungsten directly heated filament 

Z    oxide coated directly heated filament

THIRD LETTER - this describes the envelope material

S    silica envelope

FIRST NUMBER - this denotes the anode voltage Va in kV

05   0.5kV (500V)

1     1.0kV (1000V)

5     5.0kV (5000V)

12   12.0kV (12000V)

SECOND NUMBER - this denotes the valve output

For valves of </= 5kV the figure denotes the maximum anode dissipation in watts (W)

For valves of > 5kV the figure denotes the maximum anode dissipation in kilowatts (kW)

For rectifiers, the figure denotes the maximum rectified current per valve in milli-amperes (mA)

SECOND NUMBER SUFFIX - denotes cooling type

A     forced air cooling

W    water cooled

Here are some worked examples using the above convention: -

A QV04-7 is an RF power tetrode with indirectly heated oxide coated cathode with the anode rated at 400V with a 7W dissipation.

A TX12-20W is an RF powee triode, water cooled with tungsten filament with the anode rated at 12kV with a 20kW dissipation.

An RG3-250 is a mercury vapour rectifier having a 3kV anode voltage and a rectified output of 250mA.



The Mullard valve factories had numerous test schedules for valves produced but today, we are going to look at the production test that EVERY valve that went into stock received which was known colloquially throughout the Mullard organisation at ' Test I'.     

It is interesting to note that, with the exception of the knock test, the test schedule is identical to that used by Mullard Magic before any of our valves go on sale.

The Test I schedule was conducted by two girls, however, by mid 1953, for some popular valve types, Test I was automated meaning that approximately 1500 valves per hour could be tested in this way by a single girl and this is what was done: -

KNOCK TEST - in this test,  a valve was supplied with appropriate voltages whilst seated in a standard valve base and then  treated to several sharp raps with a rubber hammer by the operator.  A milli-ammeter in the anode circuit would indicate faults such as low or no emission, open circuits, short circuit or intermittent operation.  An IF component abstracted from the anode current was amplified, rectified and passed to a loudspeaker and neon network where variations in anode current would cause characteristic sounds or indication flashes.

Having survived the 'Ben Nock' test, the valves under test were passed to another test bench where they were seated in banks of valve bases and preheated before being subjected to additional quantitative tests. which included:.................  

INSULATION RESISTANCE  - in this test,  with the cathode maintained at a positive potential, measurements of leakage between the cathode and other electrodes were made under specified voltage conditions.  For indirectly heated valves, the heater to cathode insulation was also checked.

EMISSION - in this test, all electrodes excepting the cathode were strapped to form essentially a diode and an AC supply at a specified voltage was applied and the total rectified current measured.

MUTUAL CONDUCTANCE - in this test, the anode voltage was maintained at a specified value and three different values of grid bias were aplied to the control grid and the corresponding anode current values were noted and checked against the slope of the theoretical V- Ia curve.

VACUUM - in this test, the valve was operated at nominal working paraeters and the vacuum was tested by measuring the reverse grid current or 'gas' flow using a series connected micro-ammeter in the grid circuit.

VISUAL INSPECTION - in this test, each valve was inspected under a 4x bench magnifier in daylight balanced (3200K) light for general cleanliness and internal appearance.

In the following photograph , you can see a valve test station at Mullard Backburn, on the left station is the 'knock test' board, the centre station, the characteristics test board and at the right station, the microphony test board: -



For 1953, a new range of B7G valves for battery receivers was introduced by Mullard.  This range differed from previous battery valves by having a filament current consumption of 25mA - which was 50% less than the previous devices.  The four types were: -

DK96: A heptode designed as a self oscillating frequency changer with the first & second grids forming the oscillator section, the third grid the detector .

DF96: A variable-mu HF pentode designed as an LF amplifier.

DAF96: A short-grid base pentode designed for AF amplification combined with a single diode designed as a detector.

DL96: An output pentode designed with two filament sections which may be series or parallel connected.



In late 1952, a new range of Mullard B9A Noval based valves having 6.3V heaters were introduced aimed at use in audio amplifiers.

PRE-AMPLIFIER - type EF86 now replaced the EF37A or EF40 having similar charcteristics to the EF40 with a gain of up to 140 being achievable.  The heater of the EF86 is rated at 6.3V @ 200mA.

DOUBLE-TRIODES - three new devices were made available  - the ECC81 being a medium impedance valve, the ECC82 a low impedance and the ECC83 a high impedance.  All designs have independant cathodes for each section, centre tapped and capable of operation at 12.6V @ 150mA or 6.3V @ 300mA.

OUTPUT PENTODE - the EL84 meets the demand for an output valve with 30% more output than the EL41. A single EL84 was designed to give an output of 5.4W with 10% THD.   A pair of EL84 in Class AB1 push-pull could deliver 16W.  The heater of the EL84 is rated at 6.3V @ 800mA.

FULL-WAVE RECTIFIER - the EZ80 has identical characteristics to the EZ40 and is capable of a rectified output of 90mA at a maximal voltage of 2x 350V RMS.   The heater of the EZ80 is rated at 6.3V @ 600mA.


Mullard had a number of Valve Service Depots throughout the country which were primarily condiuts for the return of valves and CRT under warranty, however, they had a secondary role in keeping small stocks of commonly ordered items for use in peak supply times  transport not as responsive or easy in the early 1950s and no-one worried too much about inventory and factory turns!!!

The main Mullard Service Depot was at Waddon, Surrey and this was supplemented with strategically placed regional depots at Birmingham, Halifax and after 18th June 1953, another depot at 20, Renfrew Street, Glasgow.   The relatively short life of early CRT and the boom in television sales in Scotland during the early 1950s made a more responsive local Service Depot an essential addition.   And below here we have a photo recording the opening ceremony hosted by (3rd from LHS) Mr V Pringle, Undermanager, who bravely stepped into the breach after the Manager, Mr E B Rogers was unable to perform the ceremony due to illness.

The Valve Service Depot expansion programme continued apace during 1953 with yet another opening, this time in Bristol  at Brunel House, St. George's Road, Bristol on the 10th August 1953. 



You would be forgiven for thinking that here we have a photograph of the piping inside the ballast tanks of a WW2 submarine but no, this shows part of the low tension network at the Mullard Mitcham production facility.  Although this may look arcane and let's face it, the working conditions certainly are ( does it comply with PUWR regulations???),  this was considered state of the art in 1953 with the proud boast that the Mitcham Plant Department was the most up to date in the South of England. 


The EF86 audio frequency pentode had been designed for use in high grade resistance coupled AF voltage amplification circuits.  - yes, this pentode was NOT designed as a TV valve as some ill informed naysayers would have you believe.

In such circuits any hum and microphony introduced by the valve must be kept to a minimum and the EF86 shows a maximal hum level of 5uV referred to the control grid.  This was achieved by using a bi-filar heater winding and careful internal screening, what, you thought that pin mesh screen was just for looks, oh no!  What's that I hear you say "Is he talking about the pin mesh anode?," well you may call it that due to the misnomer that has grown up within audiophile circles but the pin mesh bit is actually the screen.  

The pin routing were also carefully designed to minimise hum with the grid pin being placed equidistantly between the two heater pins such that the heater winding essentially had a centre tapped earth to balance out any residual hum.  The electrode cage had additional internal bracing specified within the design to enforce torsional rigidity to reduce intrinsic microphony.

The main characteristics of the EF86 are: -

Heater Voltage, Vf:.............................. 6.3V

Heater Current,  If:...............................200mA

Anode Voltage,  Va..............................250V

Anode Current,  Ia................................3mA

Hum Level...........................................<5uV

Mutual Conductance, gm.....................1.85mA/V

For more about this interesting landmark pentode, why not have a look at our EF86 product page where I write in a little more detail on uses for the EF86, especially for audio.



I was chatting with my pal Alan Wyatt some time ago - you know him, the nice chap who runs the National Valve Museum and whilst we were chewing the fat on all things thermionic, our conversation meandered to the matter that there are very few photos of the Mullard big cheeses out there - what did they look like, does anyone know?  Alan has put a picture of the Captain on his website so I thought I would follow suit  but additionally also allow you to put names to other Mullard faces: -

OK, for our first line up above, we have, in the centre, 'Stan the man'  Captain S R Mullard, to the left, TE Goldup a Mullard Director and to the right LA Sawtell, another Mullard Director.     

Our second line up below shows, on the left SS Eriks (the Philips nominated head honcho who succeeded SR Mullard as MD soon after his departure in 1930) stood next to DM Hall the Mullard Service Director.



If you felt that a trolley fulfills three important requirements - i) to serve as an attractive self-contained counter; ii) to serve as a compact storage facility; iii) to serve as a mobile platform then you can understand why Mullards commissioned Domain Products of Barnby Street, London to manufacture a purpose built trolley to store your MHSVT on.        This might look like a hospital trolley but it differed in that it could be easily dismantled Ikea style by the removal of eight clamp bolts and it had two hooks on the back legs of the trolley for winding the mains cable on - pretty good huh - and all for £6 15s. 


And that's not all as you could also buy card containers to store over 1000 valve test cards.   Having four compartments and a handy lid to store valves under test these were a snip at 35/- each and fitted beautifully onto the trolley.