Mains-operated sets are rapidly becoming popular, for they hold many advantages for both the ordinary listener and the home-constructor. The following hints on the use of mains valves should, therefore, be of special interest.
If you have a mains-driven set or contemplate building one, you have distinct advantages over your battery-using friends in the question of valves. The advantages are not wideness of choice, because there are very many less mains valves on the market, than battery types, but those available are exceedingly efficient.
It is possible with a. mains-driven set. with valves of the indirectly-heated cathode type, to get a, far higher amplification per stage than it is with the battery type.
In the case of the directly-heated valves with the AC put straight through the filament, very much the same characteristics as the battery types are obtained, but it is with the AC indirectly- heated valves that we want to deal with in this article.
The SG Valves
In the first place if you use a screened-grid stage you will find you have the AC/SG Mazda, which has a magnification of 1,200 with an impedance of only 600,000 Ohms. The AC SG Cossor valve, with mutual conductance of over 2, the Mullard, Osram, Marconi, Tungsram, Vatea, Dario, Six-Sixty, etc., all of which are capable of giving very high magnification, much higher, in most cases, than is possible with the ordinary battery screened-grid valve.
This being the case it behoves any user of mains valves to screen his HF stage very completely indeed to make the most of the magnification which is offered to him. Obviously, if a, valve has tremendously great magnification powers, it is liable to cause instability or feedback if the screening is not properly carried out.
So if you are changing over from battery to mains do not forget that a little added screening may be necessary owing to the fact that you are going to get very much higher magnification.
The AC detector valves, too, are also more efficient than the battery types. Here you have to add positive bias for leaky-grid rectification, for in this indirectly-heated valve you have no positive leg of the filament to which you can connect the return end of your grid leak. Consequently, the grid leak has to go to 1.5 Volts positive on a grid-bias battery, the negative of which is taken to the cathode. The heater, of course, goes direct to the secondary of a transformer, which is supplying AC, usually at 4 Volts, to the valves.
High Magnification LF Types
The LF stages do not hold out so much choice in the types of valves, but the ordinary AC LF valves are all suitable here, whilst for the output stages you have the possibility of such valves as the ACP, which has an impedance of something like 2,650 Ohms and a magnification factor of 10.
Such a valve has not a very big grid swing, but it will give a tremendous output power for the input, and as a loud speaker valve it takes a great deal of beating.
In the operation of AC indirectly-heated valves it must not be forgotten that the cathode must be taken to a neutral point on the LT winding of the transformer.
If the transformer is not centre-tapped, then a 400 Ohm potentiometer should be placed across it and the slider taken to the cathode of the valve. This is essential if hum is to be successfully eliminated from your reception.
There is also a method of putting a resistance in the cathode lead by which automatic grid bias can be obtained, but there are one or two snags in it, especially if you use automatic grid bias in more than one stage, because a certain degree of inter-valve coupling is liable to occur and instability or motor-boating may result.
In the case of one of the valves, automatic bias can quite well be obtained by means of a resistance in series with the cathode, but in the writerbs opinion it is far more satisfactory where several are concerned to use ordinary grid-bias batteries.
The Heater Connections
When wiring up the filaments, or rather the heaters, of indirectly-heated cathode valves, or of any AC valves, it is essential that either lead covered wire be employed, or twisted flex be used, otherwise pick-up may take place between the heater leads and other connections and thus cause an annoying hum in the reception.
For economy of running there is nothing to beat the AC valve, and a well-designed three valver (screened-grid, detector, LF), using AC indirectly heated Valves, is capable of giving you results comparable with those of a really good four-valve battery receiver.