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Audio for young ears ’60s style

MORE MUSINGS ON MULLARD, CIRCUITS IN PRINT AND MEMORIES OF A LEAK LEGEND

The letter from reader Nick Willans in
the September issue on Mullard valves
brought back many memories for me.
As a schoolboy radio amateur in the
1960s with an interest in audio, fi nancial
constraints necessitated that most of
my equipment was either home-built or
sourced secondhand, the latter often
from the plentiful stocks of government
surplus equipment to be found in the
shops of London’s Lisle Street.
Almost all of this equipment used
valves; indeed for many years the
only solid-state device in my set-up
was a UHF converter employed as the
‘front end’ for an ex-US forces HRO ABOVE: Leak’s ‘Plus’ version of its Stereo 30 amp used silicon rather than germanium transistors
receiver. Home-build projects relied

on designs from the RSGB Amateur ‘hi-fi’ AM modulation! I used KT66 output captain, Stanley Mullard, it went into
Radio Handbook and the excellent valves instead of EL34s as I found their partnership with Philips of Eindhoven
Mullard Circuits For Audio Amplifi ers shape more aesthetically pleasing... in 1924 and was wholly taken over by
publication (eight shillings and six For audio purposes I used a Mullard Philips in 1927. The letters and numbers
pence) which, of course, I still have. 5-10 design. This was fed from a Decca nomenclature was invented by Philips
Regarding the latter, Mr Willans refers Deram ceramic pick-up mounted on a and adopted by all its subsidiaries,
to a ‘5-30’ design using EL34 valves in Garrard SP25 turntable. The speaker including Mullard. Most other British
the output stage. I have not heard of this (only one of course, these were all mono companies, like M-O Valve and Brimar,
one. The design detailed in the Mullard designs) was a large Goodmans housed carried on using their own schemes.
publication is for a 20W amp (ie, a 5-20) in a plywood box of dimensions sized to The EL34, introduced by Philips in
using EL34s. I built two of these amps, fit in my shed – no science involved, but 1949 (gm = 11mA/V) did indeed have a

but not for use in an audio set-up. Rather, it sounded great to my young ears! higher transconductance than the M-O
I used them as AM modulators for my This set-up saw me through student Valves KT66, introduced in 1939 (gm
transmitters – output transformers for years, after which, although still an = 6.3mA/V) but not more than the M-O
that application were readily available impoverished postgraduate, I was able Valves KT88, introduced in 1957 (gm
in those days, and there was no way I to buy a Thorens deck with a Shure = 12mA/V) or the KT77, M-O Valves’
could afford an audio output transformer M75ED cartridge. I also built a Linsley- beam tetrode equivalent of the EL34.

of suitable quality. Hood 75W amplifier to the HFN design. Another valve in the KT series, the KT55,
In the VHF/UHF bands one could I was also fascinated to see the had a transconductance of 19mA/V.
(just about) get away with the ‘antisocial’ Vintage Review of the Leak Stereo 30 Incidentally the Mullard design for push-

high bandwidth signal resulting from Plus amplifier in the same issue. Another pull EL34 amps was the 5-20, not 5-30.
friend in North London had, I believe, David Mansell, via email
worked for Leak and he had a complete

system – amplifier, Troughline tuner and Steve Harris replies: Thanks to David
Leak Sandwich speakers, with a Garrard Mansell for the comparison of the EL34 and
301 turntable. I was in awe and could the various competing ‘KT’ tubes. Invented
only dream that one day I might be able in 1926 and patented by Philips, the pentode

to afford such components! added a fifth electrode, the suppressor grid,
I have been reading HFN for the to overcome the kink in the characteristic
past 55 years and have found the performance curve of a tetrode. In the early
contributions of Chris Breunig, Jim 1930s engineers at the recently-formed EMI
Lesurf and Ken Kessler of particular proposed another solution, using beaming
value. May they continue! plates instead to create a ‘kinkless tetrode’.
Dr Alan Craggs, via email This was then developed by RCA, which
had a big stake in EMI, rather than the
I’d like to add some clarifi cations to associated M-OV company in the UK, as the
the letter from Nick Willans published American giant had far greater resources.

in the September issue. Firstly Mullard So it first came to fruition as the 6L6 in 1936.
did not ‘devise its own nomenclature’ Incidentally, Stanley R Mullard served with
ABOVE: Mullard Circuits For Audio Amplifi ers as Mr Willans says. Although Mullard the RNVR during World War I, working on
was first published by Mullard Ltd in 1959 was founded in 1920 by a British ex-army valve design for the Admiralty.

118 | www.hifi news.co.uk | DECEMBER 2020