Why do some tubes have several number designations?
There are several different numbering schemes for vacuum tubes. The two most common are the European system developed by
Mullard and Philips. The other is the American RETMA (Radio Electronics Television Manufacturers Association) standard.
These systems were employed as a method of standardization in the late 1930s when large numbers of vacuum tubes were being
developed to meet the demands of a growing technology.
In the European system the first letter indicates filament as follows:
A 4 Volts
B 0.18 Amps (series)
C 0.2 Amps (series)
D <=1.4 Volts (series/parallel)
E 6.3 Volts (series/parallel)
F 12.6 Volts
G 5 Volts (parallel)
H 0.15 Amps (series)
K 2 Volts
L 0.45 Amps (series)
P 0.3 Amps (series)
U 0.1 Amps (series)
V 0.05 Amps (series)
X 0.6 Amps (series)
Y 0.45 Amps (series)
The second and subsequent letters indicate the construction of the tube as follows:
A Diode (excluding rectifiers)
B Double diode
C Triode (signal, not power)
D Power output triode
E Tetrode (signal, not power)
F Pentode (signal, not power)
L Power output tetrode or pentode
H Hextode or heptode (of the hextode type)
K Octode or heptode (of the heptode type)
M Tuning indicator
Y Half wave rectifier
Z Full wave rectifier
The first digit indicates the basing arrangement as follows:
2 Miniature 10 pin
3 International octal
4 Miniature 8 pin
8 Noval (9 pin miniature)
9 Miniature 7 pin
The remaining digits are the sequence number. Note that signal pentodes and tetrodes that end in even numbers are sharp cutoff. Those ending in odd numbers are remote cutoff.
The European system has the advantage of providing a lot of information about the tube by deciphering the tube number. Here are some examples:
EF86: 6.3 Volt filament, signal pentode, Noval base, sharp cutoff.
GZ34: 5 Volt filament, full wave rectifier, International octal base.
ECC83: 6.3 Volt filament, two signal triodes, Noval base.
E83CC: Special quality version of ECC83. Swapping the second and third field was commonplace to indicate that the tube was a premium version.
PCL82: 0.3 Amp series string filament, signal triode + power pentode, Noval base.
In the American RETMA standard the first digit indicates the filament voltage as follows:
0 Cold cathode (voltage regulator tube)
1 0.1 - 2.0 Volts
2 2.1 - 2.9 Volts
3 3.0 - 3.9 V0lts
n n.0 - n.9 Volts
The letter or letters that follow indicate the sequence code. When single letter codes were used up, double letter codes were introduced. Note that U to Z generally
(but not always) as the only letter or as the last letter are used for rectifiers.
The final number indicates the element count with the filament counting as element 1. The 6SN7 has two cathodes, two grids, two plates, and a filament.
Additional letters indicate improvements to the tube or special properties as follows:
A Controlled heater warm-up time or improved version of a non-A type.
B Improved ratings.
C Further improvement.
G Glass bulb.
GT Glass tubular.
W Ruggedized version.
X Low loss ceramic base.
Y Low loss phenolic base.
With the RETMA standard, you will need a spec sheet or tube manual to determine the function of the tube and what type of basing is used. Here are some examples
of the RETMA standard:
6SN7GT: Filament rating is between 6.0 - 6.9 Volts, SN=sequence code, 7 elements, glass tubular envelope.
6V6GTY: Filament rating is between 6.0 - 6.9 Volts, V=sequence code, 6 elements, glass tubular envelope, low loss phenolic base.
12AY7: Filament rating is between 12.0 - 12.9 Volts, AY=Sequence code, 7 elements.
Some companies, such as Mazda and Marconi-Osram (GEC), had their own numbering systems. The most famous are the KT-series of Kinkless Tetrodes, which included
the KT66 and KT88 beam tetrodes. The KT66 is an up-rated version of a 6L6 and the KT88 is interchangeable with a 6550.
Many tubes that are intended for special applications, industrial, or military use do not use the RETMA standard. These tubes have a four digit number as the
designation. A prime example is the Tung-Sol 5881 that was developed as a heavy-duty replacement for the 6L6. This tube was unique in the fact that it was a
prime power tube used in amplifiers built during the 1950s hi-fi movement. It also found a home in the Fender Tweed Bassman, as well as being the power tube
used in the servo amplifiers that drove the control surfaces in B-52 bombers. Another tube found in Fender amplifiers is the 7025. This was actually a 12AX7
that was selected for low noise and hum for use in preamplifier circuits. There were versions of the 12AU7, such as the 5814 that had balanced sections and
was primarily used in analog computers along with the 6189 that was optimized for mobile, shipboard, and aircraft communications. These premium special purpose
industrial/military tubes work very well in guitar and hi-fi amplifiers and are well worth looking into.
There is also the CV series of tubes. The format is CV followed by up to 5 digits. This system was used by the British military to codify vacuum tubes, gas tubes,
and some semiconductors. The CV4004 is the British military version of a 12AX7WA. The CV series tubes are of high quality and sought out by audiophiles for use
in high-end audio equipment.
The following is a list of equivalent tubes showing the different numbering systems. Some of the tubes listed are premium special versions.
EL34= 6CA7, KT77, E34L
EL84= 6BQ5, 7189, E84L
6L6GC= 5881, 7581, KT66, EL37, 6L6WGC
12AT7= ECC81, 6201, CV4024, E81CC
12AU7= ECC82, 5814, 6189, CV4003, E82CC
12AX7= ECC83, 7025, CV4004, E83CC
6V6GT= 7408, CV511