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Editorial comment.
Valve development of recent a years has taken two distinct trends. On the one hand we have the tendency towards the production of multi-electrode valves as exemplified by the screen-grid, the pentode, and the heptode or pentagrid, while on the other we have the trend towards the multiple valve, of which the Class B valve and the multiple diode types are good examples.
No one can have anything to say against the development of multi-electrode valves, since these represent a definite advance in technique, and function in a manner which cannot be duplicated by any other valve or combination of valves. The same cannot be said, however, of multiple valves, for their equivalent can always be obtained by a combination of existing valves oi simple types.
The multi-electrode valve represents a definite contribution to the advance of radio, but the multiple valve has no technical merit, and it may even be argued that it is a retrograde step, except in so far as it serves as an exercise for the ingenuity of the valve manufacturer.
The characteristics of the individual sections of a multiple valve are usually inferior to those of their equivalents built as separate valves, and the common. cathode to the sections renders such valves less convenient to use. It is the experience of most receiver designers that the grouping of so many connections in a single valve base leads to such a serious congestion of components and wiring around the valve holder that efficiency and stability are impaired.
Perhaps the strongest argument which can be advanced against the multiple valve, however, is the infinite variety of combinations possible. Valves to-day are quite numerous enough, and it is easy to visualise the difficulties which would be brought about by still further increases in the numbers of the multiple types. If we use a duo-diode-triode, it is but a step farther to a combination of two triodes for an LF stage and the output valve and, indeed, such a valve has already appeared in America. A logical development would be to incorporate all the valves of a set in a single glass envelope, and the absurdity of this will be readily apparent. Not only would the whole multiple valve have to be replaced when only a single section had failed, but the manufacturer would have to provide, and the dealer to stock, innumerable types to suit the infinite combinations of sections. required by different receivers.
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