How are tubes named?
In the United States vacuum tubes are identified by a naming system that is fairly standard. • The first part of a tube designation is a number which tells us the approximate voltage required for the heater of the tube. For example, the heater of a 6J5 requires 6.3 volts, the heater of a 5U4 requires 5 volts, and the heater of a 50L6 requires 50 volts. 12AX7s, which are dual triodes, require 12.6 volts when wired in series, but when wired in parallel, as in almost all musical instrument amplifiers, they require 6.3 volts. It’s no coincidence that the 6L6-GC requires 6.3 volts, this allows us to hook them to the same voltage source. • The second part of a tube designation is a code letter (or group of letters) for identifying the tube types and functions. • The third part of a tube designation is a number which sometimes indicates the number of useful elements brought out to the base terminals. For example, a 12AX7 has seven elements if we consider the filaments (pins 4, 5, and 9) as on
