Combination Tubes

Author: J.B. Hoag

It has been found advantageous to mount two or more tubes within the same glass or metal envelope. Sometimes the separate tubes which are so contained in a single envelope are of the same type as, for example, the double-diode tubes. In this case, the two diodes use a common filament, have separate plates, and are assembled together in a common bulb. Then there are triode-tetrode tubes and, indeed, many other useful combinations. These various tubes operate, part for pari, as though the diodes, triodes, and pentodes had been constructed in separate vacuum chambers. Figure 15 F shows some of the possible combinations for tubes used in receivers.

Fig. 15 F. The names and symbols of some multi-electrode and combination tubes

In the code numbering of tubes, the first number stands for the approximate voltage to be applied to the filament, G stands for a glass rather than a metal envelope, and T at the end stands either for a certain kind of base, known as a miniature " octal " (octa means " eight "), or for a tubular rather than a pear-shaped glass envelope. Tubes with a T only are smaller than those not so marked (T for " tiny "). The central number stands for the useful number of lead wires brought out of the tube. The letter S means that the tube is single-ended, all connections coming through the base. For example: the triple grid 6SK7GT amplifier tube uses 6.3 volts to heat its cathode; has 7 useful leads (1 and 2, filament leads; 3, cathode; 4, grid; 5, grid; 6, grid; 7, plate); has a tubular and small glass envelope, uses a small wafer octal 8-pin socket and is single-ended.