Valve amp nostalgia
November 2005
Valves, Actuators & Pump Control
The accompanying photograph is of a recently constructed audio valve amplifier designated the 'The Mullard 5-20'. The 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 & June 1955. For those of us 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 author of this press release was employed in the workshops of the SABC during the '60s and carried out maintenance work on similar amplifiers, manufactured by the 'H.J. Leak' company in England, which was the standard studio monitor amplifier for the SABC at that time.
The 'Mullard 5-20' was a very popular design during the '60s and enthusiasts believe that for some reason it is able to reproduce the music of that era more faithfully than modern solid-state amplifiers.
For those who are interested, 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 uses 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 and Sovtek and are sourced from Russia and Eastern Europe.
In view of the success of the first amplifier built, Denver Technical Products is considering producing a small batch, if there is sufficient interest.
For more information contact Mervyn Stocks, Denver Technical Products, 011 626 2023, [email protected], www.denvertech.co.za
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