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The circuit of the low cost amplifier
With the current trend towards complicated transistor Hi-Fi amplifiers and pre-amplifiers, it is often forgotten that very good quality can be obtained from fairly simple, low cost valve amplifiers. The amplifier described here is a two valve one giving very acceptable quality at a cost of under £6
The Circuit
The input is fed directly to the tone and volume control stage and although this is, a passive network, gives wide control over the tone. The output from this is fed to the first valve, EF86, which is a high gain, low noise type, this acts as a simple voltage amplifier. Negative feedback from the output transformer is fed to the cathode of this stage which makes use of extra gain by improving the overall quality.
The wiring of the tone control panel
The output of the first stage is fed to the triode section of the ECL86 which further amplifies the signal and applies this to the grid of the output section in the same valve which applies the signal to the output transformer in the anode circuit. The only component that is critical with regard to quality is the output transformer, the difference between a good one and a small cheap one is tremendous. Any good transformer having a primary impedance of about 8kΩ will do.
The under chassis wiring
Because of the high gain of the EF86, care should be exercised in the layout of the components. The main sources of noise in an amplifier are the components associated with this valve and if good noise figures are wanted R3, R4, R5 and R6 should be high stability types. The unscreened lead to the grid of the first valve should be kept as, short as possible. All the main components are mounted one chassis about 6 x 6 x 2½ inches, and the controls on a panel 6 x 2½. No details are given for a power supply as any capable of giving between 200 V and 350 V, at 75 mA and 6.3 V. 1.5 A. will do.
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