Blog posts of '2013' 'September'



Now this really was quite the thing.  In the 1950's the availability of Lay-zee-boy chairs in the UK was but a wistful dream but they had big screen TV you know.  Today's Mullard archive photo shows the demonstration of range of six Mullard forward projection televisions of varying screen sizes up to a  maximum of 4 ft x 3 ft.  The official report states that viewers were wowed by the clarity and quality of the picture which was maintained despite relatively high ambient light conditions, no comment seems to have been recorded about the perceived comfort of the dining chair seating.


Today's Mullard archive photograph shows the evacuation of television picture tubes at the Mitcham works some time in 1953.

The operator is placing the tube on a diffusion pump.    Diffusion pumps use a high speed jet of vapour to direct gas molecules in the pump throat down into the bottom of the pump and out the exhaust, nowadays we use a  polyethylene glycol (PEG),  PEG ethers or more commonly, a silicon oil, however, in 1954, the preferred vapour was mercury as it didn't char, didn't contaminate the tubes with vapor backflow and of course was readily available.  

The tube passed through a heated tunnel where any gaseous emissions were pumped out prior to sealing once the hi-Torr vacuum was pulled.






For Klaus in Germany and Jan in France, your recent orders were shipped earlier this week from this rural Welsh Post Office which amongst other things is also a shop and a petrol station.  The Post Mistress had been in attendance serving the local community as a constant over the last 44 years as the Post Office Long Service Certificate shown proudly in the shop attested.

The reason for this strange choice of Post Office some 100 miles from base is that we have been taking a well earned rest for the past few days but are now back with all hands to the pump and business as usual.  


You can read more about this Noval based twin triode on one of my valve product pages but here, today we have an excerpt from a Mullard 1954 press release extolling the vital statistics of this versatile and today revered device: - 




Today's blog entry is this wonderful photo dating from the late 1950s of the London HMV Oxford Street store, don't you just love the quaint descriptive parlance of "Electric Housekeeping?"      To me the Thames Trader van is special too with it's roof banner proudly proclaiming the delights of stereo.    

Yes, it's the van and not the lorry version, which of course was immortalised as ....the Ten Ton Thames Trader driven by Terry Teddington from Tintwhistel to Tintagel with Ten Tons of  Tin Tin Toilet Tissue bound for Timothy Whites & Taylor's - they're "our sole"  agent.... in my late Father-In-Law's favourite joke.



Valves were from time to time, returned to either the main Mullard service department or one of the regional service centres.   Reasons for return were varied and included,   defective new product and devices that had failed during their guaranteed life. Today's blog entry will describe what happened to these naughty devices when received at the service centre.

On receipt, the suspect device would be unpacked and listed in a return register attached to a numbered report form on which the results of any tests carried out would be recorded.  At this point, the report contained details of the valve type, manufacturers coding, parcel condition on receipt and result of a subjective visual inspection of the valve.

The device and report would then be passed to a service bench where a range of static tests would  have been made using a test board.  These tests were carried out in a definitive sequence and should a test device fail at any point, the sequence was discontinued.  The test schedule comprised of the following tests: -

The first static test was always a filament check where both continuity and filament current draw were checked.  

The second test was for insulation resistance where the cathode was energised to a positive potential and measurements of leakage current to other electrodes was measured whilst gently tapping the envelope with a rubber hammer to reveal possible intermittent faults.  

The third test was a vacuum test where the valve under normal operating conditions was connected to a microammeter in series with the grid to measure any reverse grid current and hence check for a 'soft' valve.  

The fourth test was an estimation of cathode emission where the valve was effectively configured as a diode to form a composite anode with an AC supply being applied to the anode and the total rectifed current hence being measured and compared to specification.  

The fifth and final static test was a check of the anode current-grid volts characteristic curve  at several different grid voltages.

If all static tests passed specification then the test device would be passed to another test area and operated for a period of 15 minutes in a suitable receiver - typically a Mullard or Philips receiver.  This allowed a direct practical check of device behaviour in an actual circuit and additionally allowed a subjective assessment of any noise and microphony.

If necessary, the final test stage would involve valve disssasembly for further visual and microscopic examination, the results of which were recorded on the test report.

At the conclusion of testing, the report would be submitted to a claims department who decided, based on report contents whether or not the valve could be replaced under the terms of the guarantee 

Today's photo presented below shows Doreen Snailpen operating a valve test board at the Mullard Waddon Service Department:-





A number of customers are now showing interest in B7G miniature battery valves so I thought I would do today’s blog on these marvelous little devices.  These first came to the fore in the early 1940s when they were utilized in personal radios particularly Stateside where they did duty in the likes of the Sonora Candid and the RCA BPA 10. As is usual, we didn’t catch up this side of the pond until the immediate postwar period with such designs as the Marconiphone P17B and Ever Ready Personal B. Let’s also not forget how these devices also saw duty throughout WW2 in a variety of man pack radios for the military.

The first B7G devices were produced by Sarnoff’s RCA and commercially distributed during 1939 and the line up comprised of the 1R5 – Heptode Frequency Changer; 1T4 – IF/RF amplifier; 1S5 – Detector Diode and AF amplifier all of these having 50mA 1.4V filaments but there was the; 1S4 Beam tetrode which had a 100mA filament and of course a version of the 1S4 with a 50mA + 50ma Centre tap filament capable of working as either a 3V 50mA series and 1.5V 100mA parallel. Mullard/Philips and Mazda/Adzam licensed copies of these contemporaneously and in 1953 launched their take which was the DK96. DF96, DAF96 and DL96 with improved construction which meant these had 25mA 1.4V filaments except the DL96 which is 50mA, or 25mA + 25ma centre tap.

To help with equivalent identification, I have prepared the following convenient equivalents listing: -

Heptode Frequency Changer 1R5 = DK91 = VT-171 = X17 = 1C1 = 1L6 = CV782
Heptode Frequency Changer 1AB3 = DK96 = VT-171 = X25 = 1C3

Enhanced Frequency Changer 1AC6 = DK92 = 1AC6 = X18 = X20 = 1C2 = CV5172

RF/IF Amplifier IT4 = DF91 = W17 = 1K2 = 1F31
RF/IF Amplifier IAJ4 = DF96 = W25 = 1F1

VHF RF/IF Amplifier IAN5 = DF97

Detector Diode/AF Preamplifier 1S4 = DL91 = 1S4 = CV783
Detector Diode/AF Preamplifier 1AH5 = 1AF33 = ZD25 = 1FD1 = DAF96

Detector Diode/AF Preamplifier (CT) 1S4 = DL912= 3S4 = N17 = 2P2 = 1P10 = CV820 = VT-174

Beam Tetrode DL93 = 3A4 = 2P3 = CV807
Beam Tetrode DL94 = 3V4 = N19 = 1P11 = CV1633
Beam Tetrode DL95 = 3Q4 = N18 = CV818
Beam Tetrode DL96 = 3C4 = N25 = 1P1

With the above information, the B7G affectionado will be able to number cross reference and match devices from various manufacturers and hopefully achieve thermionic nirvana!





As careless consumers were forever popping the HT fuse due to sputtering rectifiers and valve overloads, Mullard hit upon the idea of introducing an HT Overload cutout for all 1954 production models of the MHSVT.   It was based on a polarised relay which would trip at overload condition but could be reset at the press of a button.  The cut out trip current was 200mA.

Another handy wheeze was what Mullard termed "the MHSVT pre-heat device" where a toggle switch was added which allowed the instrument circuitry valves to have heaters lit without a valve setting card inserted and the gate handle in the "Off" position.  To avoid CRT wear, no EHT - just LT was applied.  On inserting a card and valve, the CRT indicating spot appeared virtually instantaneously which was considered a boon for the busy commercial workshop.

These enhancements could be retrofitted to earlier instruments on their return for routine overhaul - Mullard were very specific in that kits of parts could not be sent to users for self installation.

The cost of upgrade was as follows: - 

Pre heat device: £2 10/-

Overload cut-out: £2

Both pre-heat & cutout: £4

The diagram below shows the circuit amendments that were made - does your MHSVT have them?:-