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AUDIO and RF MODULATOR
TETRODE, PENTODE AND BEAM POWER TUBES
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AWA TYPE A-515
This page presents the "AWA - Type A515 Amplifier" - a modified and improved version of the original Williamson.
Announced in Issue No. 128 (Nov/Dec 1947) of "Radiotronics" Magazine, published by the Amalgamated Wireless Valve Co. Ltd, Sydney, Australia.
Notice how well the amplifier performs extremely well on a speaker load with no feedback.
Notice how - even with no feedback - the amplifier does not respond to the bass resonant frequency of the loudspeaker.
This is one of the reasons why it sounds so good to the ear.
These attributes may be attributed to the triode connected output stage and is typical of any power triode output valve system.
One important variant from the original Williamson is the power supply filter cap configuration. Photographs of the 1947 Williamson show oil-filled paper caps for the B+ supply. The A515 circuit shows normal electrolytic caps and it is assumed the test results were obtained with those installed.
However for best tonal results the modern constructor would be wise to use either oil-filled paper or 440VAC motor start capacitors having as high a capacitance value as is practicable.
A second important variant is the use of a choke input to filter rectifier with two equal value filter chokes in the circuit. This design was based on the original Williamson and established before the second version having an indirectly heated rectifier, however if a valve rectifier is to be used the indirectly heated version is gentler on the filter caps. Note that with choke input to filter the capacitors MUST have a peak voltage rating equal to the rms input x 1.414 - in this case as above that is not less than 800 VDC (hence the series connected capacitors)
A third important variant is the addition of a third filter choke between the output valves and the driver stage. This is a vital modification that improves linearity and dynamic response considerably because there is negligible voltage drop under load to the phase-splitter and voltage amplifier stages.
As with the Williamson, modified to accomodate fixed-bias, a modern solid-state power supply with full-wave bridge rectifier, adequate filter capacitors (250 uF or more) and superior M2 or M6 output transformer core steel, this amplifier in either triode or ultra-linear mode will still compete as one of the finest sounding amplifiers ever produced.
Larger image of the A-515 circuit diagramme.
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This page is located at http://www.oestex.com/tubes/A515.html
This page last modified 22 June 2010