The following is an extract from the article.
Dull emitters such as the DER, the ARDE, the LF. Ora, the Cossor 'Wuncell', and the BTH B3, all of which require between 1.6 and 2 Volts and consume from 0.3 to 0.4 ampere. These are all robust and efficient valves, very long lived owing to their comparatively stout filaments and to the low temperature at which they work. There are, again, valves with still smaller requirements such as the Wecovalve, the One Volt ORA, and the Dextraudion, all of which can be used economically from a two-Volt battery. Any of these last three will also function with a single dry cell for filament heating purposes. As both the Wecovalve and the One7Volt ORA draw 0.25 ampere, it is desirable that the single cell, if used, should be of large size; a quarter of an ampere is too great a load for any small dry cell to stand up to for more than a short period on end. The Dextraudion, which consumes only about 0.1 ampere, can be worked satisfactorily off a single bell cell of good make.
Dry Battery Valves
I think, though, that the man who must necessarily use dry cells for providing his low-tension current would do best to confine himself to valves of the '06' type. These are the DE3, the ORA, the AR06, and the B5. All of these require a filament potential of between 2.5 and 3 Volts, and take a current of only 60 milliamperes. (A milliampere, by the way, is 1 thousandth of an ampere.) They, therefore, place a very small strain upon dry cells and can be worked quite well with three of them in series provided that a rheostat with a maximum resistance of about 30 ohms is used.
All of the valves previously mentioned are of the general purpose type, that is they can be used in any part of the set and will give good results there. The general purpose valve is most convenient since only a single spare need be kept. If a valve in any part of the set goes wrong the spare can be used to replace it without detriment to results. The beginner would, I think, be well advised to confine himself to general purpose valves, to start with at any rate. Having decided upon the type of valve he requires to suit his purposes, he will not go wrong if he purchases one made by any reliable and well-known firm of makers.
Buy Good Valves
In valves, as in all other wireless goods, you get a what you pay for. It is therefore a poor kind of economy to save a shilling or two by purchasing cheap or foreign made valves which are usually very greedy in their current requirements, unstable in use, and short lived.
Later on, when the beginner has learned something about the way in which valves work and has had some experience of handling them, he may go in for special valves, such as those designed to function solely as either high-frequency amplifiers or rectifiers. When he is no longer satisfied with head telephones and yearns for a loudspeaker, he may take to the power valve for low-frequency amplifying purposes, and here he will find something to match, in its requirements from the low-tension battery, valves which he already possesses. Such is the range of small-power valves nowadays that whether you use in the rest of the set bright emitters, low-voltage dull emitters or '06' valves, you can obtain a power valve operating at about the same filament voltage, and drawing only a little more current from the battery.