THE GREAT MULLARD MAGIC BLOG — EL84

'Technical advances have effected considerable increases in the quality of broadcast and recorded sound with television sound and FM transmissions including the full range of audio frequencies.  It is thus essential to have an amplifier which does full justice to a high quality audio signal.  Details of such a new amplifier have been released by Mullard in the booklet "Mullard 5 Valve 10 Watt  High Quality Amplifier Circuit (ref. no. MV8104).' The above entry is from a press release by Mullard dated October 1954.  Today, the 5-10 is much revered  and not at all in the shadow of it's larger...

Today's photograph from the annals and archives of Mullard is a press photogaph from early 1954 where Mullard representatives chatted animatedly with Mr A J Walker Hon. Secretary of the Association of Public Address Engineers at the Mullard Stand during the APAE exhibition.  It was recorded that Mr Walker was very excited by the possibilities that the EL84 presented to the world of PA: -

You can read more about this Noval based output pentode 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 much loved device: - 

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...

This is an interesting one as a number of insulation failure modes exist for ageing valves.   Often poor insulation may be caused due to cathode emissive material or metallic deposits which have evaporated from the  electrode cage being deposited on various parts of the valves interior.  Deposits can build up on mica separators or the glass pinch where the connection pins/leads enter the envelope and even sometimes the inner envelope glass - remember those manky EL84 which had been run hard and hot and built up those yukky black deposits on the glass.........? In manufacture, the mica separators are...