Two Telephone Tips

Radio Record, Volume II, Issue 31, 15 February 1929, Page 32

Two Telephone Tips

QUITE a large proportion of the unpleasant body capacity effects of which so many shortwave enthusiasts complain are brought about by the connection of the head telephones directly, into the plate circuit of the note-mag-nifying valve. There is always a certain leakage of high frequencies through the rectifier, and its effects are particularly marked upon the short waves. These stray high frequencies make their way through the note-mag-nifying valve or valves and so into the telephones. When the head wearing them approaches ‘the tuning controls weird effects are sometimes produced. It may also be found that either the set will howl or an incoming transmission will disappear if the telephone receivers or their cords -are touched. © When the telephones are connected between the plate of the output valve and high tension positive they are, s0 to speak, up in the air. In other words, they are not earthed directly. Now, it is quite easy by means of a filter circuit to alter this state of affairs, and it is a very great advantage to do so, particularly on wavelengths below 20 metres. Here is the way in which it

is done. Connect the plate of the out put valve to high tension through a low-frequegcy choke. Connect the platé also to one contact of a fixed condenser with a capacity of from 1 to 4 microfarads and take a wire from the second contact of this condenser to one of the telephone terminals. The second telephone terminal is connected to earth. Besides materially reducing capacity effects the system has two other important advantages. In the first place the telephones are relieved of the totally unnecessary strain of carrying the direct current that flows in the plate circuit. And this brings us to the second advantage, that, it no longer matters which telephone lead is connected to which terminal. The second point about telephones concerns sets in which the high tension current is derived from the mains. Generally speaking, it is not advisable in such cases to use head telephones less a properly designed filter es or output transformer is incorporated. Without these there is always:a Hiability to a severe shock if. certain parts of the battery eliminating apparatus are touched whilst the telephones are being worn.