Choosing Valve Holders

Radio Record, Volume III, Issue 25, 3 January 1930, Page 26

Choosing Valve Holders

HE valve holders used in a receiver are generally chosen by their construction and, of course, price. Modern valves are,on the whole, so well constructed that solid valve holders are suitable, although an exception may be made of the detector. This valve usually has a fairly large magnification factor and any noise that may be set up in the detector circuit is magnified by the complete low-frequency amplifier. There are occasions, however, hen it is most important to consider the electrical properties of- valve holders. I am not referring to such matters as the size or fit of the contact sockets or the connections between the socket and the terminals, although these must obviously. be correct. I am referring to the high-frequency losses of the valve holder. The grid, anode, and filament contact sockets, -and the connecting terminals, are held in & shell of insulating material. This is seldom of ebonite. It is usually of bakelite, for the reason that this material is easily and cheaply moulded. Good _bakelite is not always employed. Nevertheless, the resistance between one terminal and another is usually many megohms, which is so high that the leakage is of no importance. What does matter is the high-fre-quency loss resistance. There is capacity between the terminals. The condenser has as its electrodes the metal connections, and as its dielectric the insulating’ material. Now the con.denser formed by the grid and filament connections, and for that matter the _other connections as well, ‘is across the tuned circuit connected to the valve. Tf, therefore, the losses of the valve holder are high, the signal strength

will be reduced and the selectivity impaired. One might imagine the losses could not be so high as to. produce noticeable effects, but when. the tuned ¢ircuit has a very good tuning coil, such as one of the Touchstone type, the amplification may be reduced by as , much as 80 per cent. One shoulk’ re-. member this when about to purcPse the valve holders for a new’ set, xd when the coils are particularly ood ones every endeavour should be made to obtain the valve holders particularly specified by the designer of the set.

STAN

MOORE