Different Types of Triodes

Author: N.H. Crowhurst

A great variety of tubes can be made by varying the structure of the grid and the position that it occupies relative to the cathode and plate. If the tube manufacturer winds the grid wires closer together and also puts them closer to the cathode, he will have a tube with both higher plate resistance and higher amplification factor.

Variations in structure change the tube parameters

Characteristics of tubes

One-half of a 12AX7 double triode, for example, has a transconductance of 1.6 milliamps per volt, with a plate resistance of 62,500 ohms, which gives an amplification factor of 100. The transconductance is lower than that in the 12AU7 because the structure of the tube reduces the possible plate current. Tubes with a high transconductance usually have a lower plate resistance and vice versa. This fact sets a limit to the amplification that can be obtained from a single triode. What we need, to get still more amplification from a single stage, is a tube with a high transconductance and also a high plate resistance.