Curbing the Pentode

Radio Record, Volume V, Issue 51, 1 July 1932, Page 30

Curbing the Pentode

MPHASIS has often been laid on the fact that a pentode cannot be regarded as being interchangeable with a triode output valve unless. the appropriate circuit alterations are made. The result of an indiscriminate change is almosts always poor quality manifesting itself by a shrill over-emphasis of the higher notes, and, as often as not, there is little apparent increase in sensitivity unless proper precautions are taken. The problem of matching, which is all important if the full benefits of making a change are to be obtained, has been dealt with from time to time, and the essentials may be repeated for the benefit of those readers who have brought forward specific questions. Briefly, the scheme most usually advocated is to use a tapped output choke, and to join the loudspeaker (in series with a large-feed condenser) between the most suitable tapping of the choke and earth. The tendency toward overemphasis of high notes is curbed satisfactorily by shunting the choke with a ‘correcting device consisting of a condenser of about 0.01 mfd. in series with a variable resistance of some 25,000 ohms, which gives some latitude for adjustment, . Certain highly specialised pentode valves may need different values of correcting components,