USA Tube Numbers

The following is a faithful reproduction of text from the 4th edition of the Radiotron Designer's Handbook (commonly known as ``Radiotron''). Grammar, spelling and punctuation are, to the best of my ability to proofread, identical to the original. -Miles

``Receiving tubes having the American numbering follow two main systems. The first of these is the numerical system, which is the older, and the second the R.M.A. system. Originally various manufacturers produced the same valve under different type numbers such as 135, 235, 335, 435 etc. This was improved upon by dropping the first figure and using only the latter two figures, e.g. 35.

``All the more recent American releases follow the R.M.A. system (Ref. 8) of which a typical example is 6A8-GT. In this system the first figure indicates the approximate filament or heater voltage-- 6 indicates a voltage between 5.6 and 6.6 volts, while 5 indicates a voltage between 4.6 and 5.6 volts; 1 indicates a voltage in excess of 0 and including 1.6 volts, while 0 indicates a cold cathode. Lock-in types in the 6.3 volt range are given the first figure 7 (this being the "nominal" voltage), but the normal operating voltages remain at 6.3 volts. In the case of tapped filaments or heaters the first figure indicates the total voltage with both sections in series.

``The second symbol is a letter which is allotted in sequence commencing with A, except that I and O are not used; rectifiers follow the sequence backwards commencing at Z. When all the single letters of a group are exhausted, the system then proceeds with two letters commencing with AB; combinations of identical letters are not normally used. The single-ended a.c. range has a first letter S while the second letter may be that of the nearest equivalent in the double-ended range== e.g. type 6SK7 is the nearest single-ended equivalent to type 6K7. Another special case is the first letter L which is used for lock-in types in the battery range. (Note - this is for two-letter combinations only. A 6L6 is not a lock-in type! -ed)

``The final figure denotes the number of "useful elements" brought out to an external connection.

``The envelope of a metal valve, the metal base of a lock-in valve, and internal shielding having its separate and exclusive terminal(s) are counted as useful elements. A filament or heater counts as one useful element, except that a tapped filament or heater of two or more sections of unequal rated voltages or currents counts as two useful elements. (emphasis mine -ed) An octal-based glass valve having N useful elements exclusive of those connected to Pin No. 1 is counted as having N + 1 useful elements. Elements connected to terminals identified as "internal connection, do not use" do not count as useful elements. Combinations of one or more elements connected to the same terminal or terminals are counted as one useful element. For example a directly heated triode with a non-octal base is denoted by 3; an indirectly heated triode, with a non-octal base is designated by 4; a directly heated tetrode with a non-octal base is designated by 4. A pentode with the suppressor internally connected to filament or cathode is numbered as a tetrode. A metal envelope or octal-based glass triode with an indirectly-heated cathode is designated by 5; a tetrode (or pentode with the suppressor internal connected) by 6, and a tetrode-hexode converter usually by 8.

``The suffix after the hyphen denotes the type of construction used. In general, metal valves, lock-in types and miniature types have no suffixes, but octal-based glass valve types are given the suffix G for the larger glass bulb or GT for the smaller parallel-sided T9 bulb. The letter M indicates a metal-coated glass envelope and octal base. X indicates a "low loss" base composed of material having a loss factor of 0.035 maximum (determination of loss factor to be in accordance with ASTM Designation D-150-41T). The letter Y indicates an intermediate-loss base composed of material having a loss factor of 0.1 maximum. The letter W indicates a military type. The letters, A,B,C,D,E and F assigned in that order indicate a later and modified version which can be substituted for any previous version but not vice versa.''

Last updated: 23 March 2004

Copyright 1952 Amalgamated Wireless Valve Company Pty., Ltd., Glebe, Australia. All rights reserved.

Miles O'Neal <> [remove the "XYZZY." to make things work!] c/o RNN / 1705 Oak Forest Dr / Round Rock, TX / 78681-1514