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Replies to Queries Raised by Constructors
January 1950
The series of articles recently published on the High-Quality Amplifier has aroused considerable interest and given rise to correspondence. It is hoped that these notes, which deal with matters of general interest arising from the correspondence, may be of assistance to readers who have similar difficulties.
Valves
There is no exact equivalent for the Osram type KT66, and its use is recommended where possible. When the equipment is to be used overseas, the KT66 may be difficult to obtain, and 6L6G glass and 6L6 metal types may be regarded as direct replacements, with the proviso that the total anode and screen dissipation should be reduced from 25 W to 21.5 W by reducing the total current from 125 mA to 110 mA by adjustment of R21. The use of these valves with reduced rating entails a slight reduction of the maximum output. The 807 may be used at the full rating of 25 W, with modifications to the valve connections. Since the articles were written, a modification of the EF37 has appeared under the number EF37A. This has improved heater construction giving greater freedom from hum, and its use may be advantageous for V8 and V13. No other changes in valve types can be recommended, as their use would involve radical redesign.
Output Transformer
When assembling the core of the transformer, care should be taken to ensure that the edges of the T and U laminations butt together. The magnetic properties of the core are dependent upon careful assembly and tight clamping.
Static Balancing
The method of balancing the standing currents in the output valves, which was suggested in the article in the August, 1949, issue, is dependent for its success on close matching of the DC resistances of the halves of the output transformer primary. Nominally the sections are identical, and when carefully machine-wound from the same reel of wire, the resistances should not differ materially. It is possible, however, due to variations in wire diameter and insulation thickness, for the resistances to differ by up to 5% and even, in extreme cases, 10%. Should this occur, a compensating resistor should be added in series with the low-resistance side in order to equalize the resistances, and the meter connected across the equalized sections. Other more direct methods may, of course, be used to adjust the anode currents to equality, but unless the transformer has a split primary winding they are inconvenient, and great care should be taken to ensure that the insertion of instruments does not cause oscillation which could give misleading readings.
Construction
There is little to add to the constructional data on the main amplifier given in the August, 1949, issue, except perhaps to explain that the purpose of the sub-chassis screen, shown in Fig 3, is to prevent feedback from the anode connections of the output valves to the input of the amplifier. It should extend downwards to the full depth of the chassis.
The method of construction of the pre-amplifier and tone-compensation units will usually be adapted to individual circumstances. One suggested method of construction for the pre-amplifier circuit of Fig. 15 is to use a shallow chassis about 9 x 3 x 1 inches. The valves and electrolytic capacitors are mounted in a group along the centre of this chassis, and the other components mounted vertically above the chassis on tag strips arranged on each side of the central group. The connections to the valve-holders are taken through slots cut in the top of the chassis. The input transformer should be mounted on the top of the chassis at one end. With the sizes given, there is ample room for a screened component of dimensions up to 3 x 3 x 2 inches. The whole unit should be fitted with screening covers, and mounted on the underside of the motor-board as close as possible to the pickup.
The tone compensation unit of Fig 19 may be constructed on orthodox lines, the only essential being to provide sufficient frontal area to accommodate seven controls. Grid leads should be kept short to avoid hum pick-up. The blank valve-holder terminals (pin 6) should not be used as anchors for the leads to the top-cap grids. The power supply components can, with advantage, be assembled on a separate chassis.
Conclusion
The circuits published in the series have been evolved over a considerable period of time and are capable of giving a very high standard of performance. Requests have been received for data on modifications, but as it is rarely possible to determine the full effect of these without carrying out tests, in general, no such data can be supplied by the writer. Or for that matter by Wireless World.
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