The photograph shows a recently-constructed audio valve amplifier designated the 'The Mullard 5-20'. This amplifier was constructed according to the publication 'Mullard circuits for audio amplifiers', published in April 1959. The original design first appeared in 'Wireless World' - May and June 1955.
For those who grew up in the '60s and had the privilege of listening to guitar music by the Shadows, Ventures, Beatles, etc, this type of amplifier certainly brings back memories. The Mullard 5-20 was a very popular '60s design and for some reason seems to reproduce the music of that era more faithfully than modern solid-state amplifiers.
For technical interest, the input stage of the circuit uses an EF86 high gain, low-noise pentode. The second stage is a double triode ECC83 and fulfils the combined function of a phase splitter and driver. The output stage utilises a pair of EL34 pentode output valves in a push-pull configuration with distributed (screen-grid) loading. The amplifier operates on 465 V d.c. supply, which is provided by a full wave vacuum rectifier (type GZ34).
All of the valves were manufactured in the '60s by such companies as; Mullard, Philips, Telefunken, Siemens, etc. These days the valves have such names as Svetlana, Sovtek and are sourced from Russia and Eastern Europe.
In view of the success of this first amplifier built, Denver Technical Products is considering producing a small batch, if there is sufficient interest from the public.
For more information contact Mervyn Stocks, Denver Technical Products, +27 (0)11 626 2023, [email protected]
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