Blog posts tagged with '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 brother, the 5-20 - and so it should not be when it boasted an output of 10W showing a THD of < 0.4% with hum and noise being inaudiable being -74dB below maximal output.  All this and a flat frequency response to within +/- 0.5dB within the range 10 - 20000Hz to boot!

Of course, this amplifier was designed to showcase a range of Mullard valves, first, the first stage voltage amplifier used the low hum/low microphony EF86 pentode which was in turn coupled to a cathode coupled phase splitter, courtesy of an ECC83 dual triode.  An elegant design point was the g'' of the ECC83 being capacitively earthed with the bias for the second section being applied due to current flowing through a common cathode resistor - a nice touch that prevented motorboating or any form of instability at normal driving conditions.  

Interestingly, the output stage was configurable with an 8K or 6K secondary to adjust loading such that high power or lower power with a consequently better transient response could be enjoyed.

The powerhouse for all this was supplied by a GZ30 rectifier or if you wished to stay with an all B9A arrangement then alternatively, the EZ80.   THe PSU was of a resistance-capacitance design utilising a comparatively large reservoir capacitance of 50uF to reduce ripple current.

And here is Mullard Laboratories photograph of "one they built earlier." : -




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 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.


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 coated with a solution of Magnesium Oxide to iimit potential leakage path length  and provide a repellant mono layer to prevent surface deposition, however, the user can take measures to prevent deposition by always running their valves within recommended maxmal limits such that they do not get overrun and overheated - remember those manky EL84!   Rectifier and output valves by nature of use have high anode current loadings and consequently can run very hot, however, manufacturers had designed in a number of mechanisms to combat excess heat - did you admire those sexy black anode coatings in your rectifiers ... boutique or not, alas NOT because although it looks pretty, it exists to dissipate heat more effectively - what about that PX4 with a fin mounted orthogonally against the anode ... not just there to look nice and provide a certain sound but yes, you've guessed it, it's there to help dissipate heat more effectively