Vacuum tubes

This document contains some practical information about what kind of vacuum tubes there are and what tubes to expect in a radio. It is very much biased toward the European situation and inspired by the following question.

I picked up a beautiful Birkenstone V602 radio, but the type number of the second tube from the left (seen from the back) is unreadable. What tube is this?

Contrary to what some may believe, this question can usually be answered without specific knowledge of the set involved. The tubes in a radio form a well-organized collection, not unlike the musicians forming an orchestra. Given the readable tube types, an educated guess for the unreadable one can usually be made.

Important note: In the extreme case where the unreadability of the type number is due to the very absense of the tube, you will become tempted to plug in a tube of the type you find. However, we can only describe what is logical to do with tubes, and sometimes manufactures do illogical things with them (examples are ahead). In some cases the insertion of a wrong tube causes damage to the tube and/or the radio! Experimentation is at your own risk! If in doubt, post the model number and the visible tubes in and somebody may able to look up the precise data for you.

Another note: This document is based on the Mullard codes for tubes. The other codes are less informative about what is inside the bottle; they were described in a note by Tony Duell.

Sometimes the type number can be read with some effort, for example under very strong light. Some people rub the tube over their hair and find the number clearly visible in the layer of grease picked up by the tube. Or put the tube in the freezer for a while, and after taking it out, read the number in the condensation when it comes or vanishes. Oh, and don't forget to look inside the back cover first. A 'position of valves' chart was often stuck there. I know it's obvious, but some people have missed it.

To determine what you cannot read, and also to understand what you can read, here is an explanation of the Mullard tube type numbering system introduced in the early nineteen-thirties, and used widely in Europe. A tube type consists of two or more letters and two digits, as in: ECH21, PL84, AZ41, UBC42, EABC80, and EM4. In EM4 there is only one digit, but just pretend there is a 0 in front of the 4.

The first letter encodes heater type.
The following letter[s] encodes what systems are in the bottle.
The first digit encodes the base.
The last digit distinguishes tubes with otherwise equal type numbers.

The first letter: filament

In theory you can use any tube for either series or parallel supply to the filament of the tube. In practice, if you want a radio with parallel supply you need a series of tubes with the same heater VOLTAGE, the tube drawing more CURRENT (hence have LOWER resistance) if it needs more heater power. If you want series supply, you need tubes with the same heater CURRENT, the tube dropping more VOLTAGE (hence have HIGHER resistance) if it needs more heater power.

The two series found in postwar European radios are the E-tubes for parallel supply (6.3V, radio has a power transformer) and the U-tubes for series strings (100mA< radio is transformerless). Battary sets usually have D-tubes; both parallel and series supply of these tubes is found. The following heater types are found:

A 4V parallel.
B 180mA series connection (almost unused).
C 200mA series connection (almost unused).
D 1.5V parallel heating, found in battery sets.
Sometimes used in series chains!!
E 6.3V parallel indirect heating.
F 12.6V parallel indirect heating.
G 5V (used for rectifiers GZ32 and GZ34).
H 150mA series heated (almost unused in Europe).
K 2V (often directly heated).
L 450mA series connection.
O Cold device, not heated.
Eg, diodes, transistors, mercury vapour rectifiers.
P 300mA series indirect heating (televisions mainly).
U 100mA series indirect heating.
V 50mA series connection.
X 600mA series connection.
Y 450mA series connection.
Usually the heaters in a radio are all of the same type, so you can read the first letter of your missing tube from the other tubes! To make sure, here is some additional information.

1. If all tubes remain cold when one is missing, you have series heaters (or an additional defect such as blown fuse). For radios this usually means a U-tube, for televisions a P-tube. If the other tubes light up despite the missing tubes, you have parallel heating in your set. E-tubes are very likely (D-tubes for battery operated sets), but in the thirties sets were built with A-tubes.

2. Another rule of thumb: with a power transformer you usually have parallel, and without a transformer, you have series heating. Exceptions are series-heated filaments fed from a separate HT winding from the power transformer. (Don't mistake the output transformer for a power transformer.) An important exception: in some sets with E-tubes, the rectifier is an A-tube, or, in later sets, a G-tube!!

Plugging a tube of the wrong heater type usually causes damage; here are two examples provided by A.R Duell:

Example 1: Philips record player with visible valves: a couple of ECC83's, an EL84, and an emptry socket. What is the missing one? You'd reasonably guess a second output valve (push-pull or stereo operation) and try an EL84, right? Well, you've just ruined a new valve. The missing one is a UL84 - it's the wierd transformerless Philips output stage. The heater rating of that one is 45V@0.1A, not 6.3V@0.76A....

Example 2: There were portable sets with 4 1.5V valves (DK91, DF91, DAF91, DL91) used when it ran off batteries, and a separate EL84, UL84, or something like that to give more audio power when it was run off the mains. Some of those had an EZ40 rectifier to add complication.

To make things more confusing, also plugging the right tube can cause damage; here is another horror story from Duell's collection, and again it involves D-tubes. Watch out for battery sets with D-valves in series! Battery/mains sets often had a string of D valves in series, operated off the 90V HT line via a resistor (to drop the voltage) and a smoothing cap. Watch out for those especially - if a filament is open, the cap charges to 90V, and then discharges through the string of valves when you plug a new one in. This normally burns out at least one filament! If you have such a set, and an open filament is found, make sure you discharge the smoothing cap before inserting a new valve.

The D-tubes can also be used with series heating. There were 2 common sets of tubes used in Battery sets:
1. DK91, DF91, DAF91 (1.4V/50mA) and DL91 (3V 50mA Centre tapped);
2. DK96, DF96, DAF96 (1.4V 25mA) and DL96 (3V 25mA Centre tapped).
Now, those valves can either be wired in parallel (1.5V battery), or in series (7.5V battery). You wire the output valve filament halves in series or parallel as appropriate. The bottom line is to be on the alert when you see D-tubes; their filaments are very easy to damage.

Second and following letter: Content

Tube systems come in varieties; here is what the manufactures have made for us:
A, Diode, contains cathode and plate.
Usage: audio detection, avc detection.
B, Duodiode, contains one or two cathodes and two plates.
Usage: separate audio/avc detection, ratio detection (FM).
C, Triode, contains cathode, control grid, plate.
Usage: amplification, oscillator.
D, Power Triode (not often seen, but comes up in colour TVs).
E, Tetrode (again, not often seen).
F, Penthode, contains cathode, control grid, screen grid, suppressor grid, plate.
Usage: amplification (can do oscillation as well).
H, Hexode or Heptode, contains additional cutoff grid and sometimes additional screen grid.
Usage: amplifying mixer (can do plain amplification also).
K, Octode, contains `a lot of grids', or heptodes designed as self-oscillating mixers.
Usage: Self-oscillating mixer (i.e. frequency conversion).
L, Power penthode, contains cathode, control grid, screen grid, suppressor grid, plate.
Usage: Output stage (in TVs: also for flyback).
M, Magic eye, contains a triode for signal amplification and a triode with light emitting anode.
Usage: signal level indication.
N, Thyratron (never used in radio?).
P, Secondary emission tube.
Q, Nonode. The EQ80 tube (with 5 MOhm plate resistance) was used as FM detector in television sets.
X, Gas-filled full-wave rectifier.
Y, Power diode, contains cathode and plate.
Usage: `single wave' rectifier in power supply.
Z, Power duodiode, contains one or two cathodes and two plates.
Usage: `full wave' rectifier in power supply.
Some tubes contain more than one system; in this case the corresponding letters are listed in alphabetic order. Some combinations are:
CH as in ECH3, UCH41, ECH21, ECH81:
Triode plus hexode, this combination is used as frequency changer.
AF,BF, AC, or BC as in EAF41, EBC41, DAF91:
Diode or duodiode with triode or penthode, combines detection with one amplification stage.
ABC as in EABC80:
A diode, duodiode, and triode combine AM-detection, FM detection, and audio amplification. Virtually all nineteen-fifties AM/FM sets contain this tube. Was it used in stereo decoders?
CL as in ECL82, ECL86, or UCL82:
Triode plus power penthode, this combination suffices for a complete audio section.
CF as in ECF80 or PCF80:
Triode and pentode, used as oscillator plus mixer in TVs.
LL as in ELL80:
Twin-pentode for stereo or push-pull output. Tube is reported to be very rare!
CLL in ECLL800:
Triode/twin pentode implements almost a complete push-pull output stage, with the triode used as phase inverter.
What do we need in a normal radio? The signal chain of a standard AM radio (such as the All American Five or its European collegue) processes the signal in the following steps:
1: Frequency conversion (sometimes called: first detection);
2: Intermediate Frequency amplification;
3: Detection (sometimes called: second detection);
4: Audio pre-amplification;
5: Audio output amplification.
The following are sometimes found in addition; check the listed condition to see if your set has these:
0.5: Radio-frequent pre-amplification (BEFORE conversion).
Can be recognized from having a triple-gang tuning condenser; RF amplification is rarely seen in European radios.
0.7: FM unit.
Your set has it if it tunes the FM dial.
2.5: A second IF amplification stage.
Can be recognized from seeing a third `IF can' (that is an aluminium box with two adjustable "DON'T TOUCH" skews).
3.1: FM detector.
Again, if the set has the FM dial.
3.5: A tuning indicator (Magic eye).
Should be visible from the outside, if the set has it.
5.5: A second output penthode.
Indications for this are: possibility to play Stereo records or having a push-pull output stage (don't know how you can see this easily though). An EL86 pair can be found in series balanced output stages, which have low output impedance and allow a transformerless connection to the speaker.
6: Rectification in power supply.
Battery or DC sets don't have it, later AC sets have selenium rectifiers.
The numbering I put here seems a little strange, but they are indicative for the position of the tube in MOST SETS. The usual build-up is to have low numbers on the left of the set (seen from the back) and high numbers on the right.

After finding out which of the listed tasks are performed in your set you can determine the necessary glassware.

0.5: C or F.
Examples: EF39, EF5 (vari-mu).
0.7: CC (or C or FF).
Examples: ECC85/UCC85 is the standard here. Before 1955, EC92 or two of EF40/EF80 was observed.
1 : CH or K.
Examples: UCH81, ECH3, AK1, DK92
2 : F.
Examples: EF5, EAF41, EF85, EF89 (must be vari-mu!) OC45 transistor, coded as `cold cathode' triode!
2.5: F.
Examples: EF5, EAF41, EF85, EF89 (must be vari-mu!)
3 : A or B.
Examples: EB4, EAF41, EABC80, EB91. OA75 germanium diode with `cold cathode'.
3.1: A/B.
Examples: EABC80, EAA80.
3.5: M.
Examples: EM4, EM34, EM80, EM81, EM84.
4 : C or F.
Examples: EBC41, EF6, EABC80.
5 : L.
Examples: EL2, EL3, EBL21, EL34, EL41, EL84, DL92. OC72 transistor, coded as `cold cathode' triode!
5.5: L.
Examples: Second EL84/UL84, ELL80.
6 : Y or Z.
Examples: AZ1, EZ3, AZ41, EZ80, UY41, UY80.
A few more hints:
A valve mounted on a separate screened box in an FM-capable set is likely to be an ECC85 (or UCC85) FM frequency changer.
A valve mounted on top of the mains transformer is likely to be the rectifier.
If you have the tube, a few clues can be gained by inspecting it. If it's clear glass, then you should be able to see if it looks like a screened HF valve (mesh screening on the outside), a power-handling valve (output or rectifier) (black, possibly finned anode on the outside), a multi-section device, or what. Similarly, look for _any markings_ on the glass - the names 'Binode' and 'Pentone' were used by Philips on double diodes and pentodes (both signal and output) respectively.

Some quite unusual line-ups have been reported.

  • A.R. Duell mentions that of a four tube radio using mostly Loctal valves; we observe ECH21, an unreadable locktal, EBL21, AZ31. In this case, the unknown loctal is a second ECH21! Its heptode section is used as IF amplifier, and its triode section is the audio driver. Detection finds place in the duodiode section of the EBL21. The ECH21 was specifically designed with NO internal connection between the triode grid and the sharp cutoff grid of the hexode, so as to make other uses than as a converter possible. The ECH21/ECH21/EBL21/AZ1 and UCH21/UCH21/UBL21/UY1 lineups were quite frequently used in the late forties by Philips and Tesla. Similar lineups (can you figure out ECH81, ECH81, EL84) were reported by Martin Ackroyd. You cannot play this trick with the Rimlock converters ECH41/2/3 because they have only eight pins and connect the triode grid to the heptode internally.
  • With high-gain tubes it is possible to eliminate one amplification stage, and with a reflex design the IF and first audio amplification can be combined into a single penthode. Example: the Amroh Junior reflex, a Do-It-Yourself kit project from the thirties.
  • A. R. Duell reports a hybrid receiver with 3 valves (DK96, DF96, DAF96) with a 1.5V filament supply and a 67.5V B+ supply, and a separate 9V (?) supply for a 2-transistor audio output stage. If you cannot understand the complete tube line-up with the standard rules given here, this should be an indication that something not covered by them is going on.

    The first digit: Base

    The first radios used tubes with various bases, including four-pin directly heated triodes. Later a small collection of bases was standardized, including the numbering of the pins. The layout of the socket guarantees that the tube can be inserted only in one position (the right one :-). In the ascii drawings below, the view is from the BOTTOM (where you approach the socket with your soldering iron if this is necessary).
    0 and 1: Various bases.
    Quite common (used until about 1950) is the European P-Side contact base. Four contacts are close to each other, four are further spaced, and counting starts in the middle of the widely spaced ones:
                       8    1 
                    7          2
                      6      3
                        5  4
    2: Lock-in-Octals or Locktal tubes aka B8G.
    The eight pins are evenly spaced and correct insertion is enforced by a center extension with a little `nose'. The center is sometimes used as an additional contact. The Locktal base is slightly larger than the Octal base, the pins are much thiner, the central spigot is metal, and the pins come out through glass seals in holes in a metal base shell
                       8   1
                     7   ^   2
                     6       3
                       5  4 
    There are not so many types of Loctals.
    3: Octals.
    Very similar to locktals. The Octal valveholder is about 1.125" or 27mm in diameter, and valves have a plastic base bonded to them. The pins are fixed to this base.
    4: Eight-pin miniature, aka Rimlock tubes aka B8A.
    These tubes do not usually have a base, but rather their eight pins come out of the bottom of the glass. They are evenly spaced and the correct insertion is enforced by a little `nose' on the outside of the tube itself:
                     7     2
                     6     3
                       5 4
    The Rimlock series was introduced around 1947 but became obsolete about five years later, due to the successful introduction of the Noval tubes. If your set contains Rimlocks, it most likely dates from 1950 plus or minus two years. A Rimlock tube is about 0.75" or 18mm in diameter. The base has a metal tube surounding it with a groove for the locating 'pip'. The valve is usually all-glass, or sometimes has a metal base shell.
    5: Various bases including:
    (1) Large 9-pin base, aka B9G. This base has an octal-style locating spigot in the middle.
    (2) Wire-ended miniatures. These tubes are mostly soldered in place like transistors, but there was a three pin base (B3G) for the EA50 diode and EC53 triode. These tubes, 30mm heigh and 10mm in diameter, have one, or two, respectively, top connections.
    (3) B9D, a 9-pin base, like an oversized B9A.
    6 and 7: Subminiatures.
    Used in hearing aids and pocket radios, e.g. the B5A based DF64 and DL64.
    8: Nine-pin miniature aka Noval tubes aka B9A.
    Like the Rimlocks, the Novals have their pins coming directly from the glass bottom. There are nine, arranged in a regular decagon with one missing position which enforces the correct insertion. This series was introduced around 1952 and was the universal tube base in European home entertainment sets built after about 1955; octals remained in use for power applications. The tubes are about 0.75" or 18mm in diameter and here is the pin layout.
                     9 . 1
                    8     2
                    7     3
                     6 5 4
    9: Seven-pin miniature tubes aka B7G.
    The pins are placed in an octagon with one pin missing to enforce correct insertion, and the diameter is about 0.625" or 16mm.
    To determine the first digit of the tube you must inspect the socket. If you don't recognize it, you can try to insert other tubes (with radio OFF; equal digit implies that a tube fits in mechanically, not that it fits in without damage!!) and copy the first digit from a tube that can be inserted.

    Many radios have a complete set of tubes with the same base, but mixed series are also seen. Mixtures of Rimlock and Noval tubes are found around 1953, when the Noval series was introduced but no complete set was available yet.

    If you make the wrong guess regarding the base and try to insert a tube, no harm is usually done (unless you try to fit the tube with a sledge hammer ;-) because it simply won't fit in the socket.

    Last digit: sequence number

    Different numbers distinguish tubes with otherwise identical numbers. For example, EF85 and EF86 are both 6.3V-parallel filament, noval penthodes, but EF85 is vari-mu (used in the IF stage) and EF86 is fixed-mu (used in audio section). In general, for pentodes (F as second letter), if the last digit is odd, the valve is variable mu, but if it's even, it is sharp cut-off (straight-mu). ECH41, ECH42, and ECH43 are all Rimlock frequency changers, each improving over the previous one, and they are interchangeable. ECC81, ECC83, and ECC85 are twin-triodes, where ECC83 differs not too much from ECC81 and can replace it, but ECC85 is variable-mu and thus has completely different properties (characteristic). The ECH83 is pin-compatible with the ECH81 (and can be used instead of it!) but was designed specifically for hybrid car-radios; it works with only 6 or 12V on the plate.

    In general, there is no reason to believe that two tubes, differing only in the final digit, are equivalent and can be interchanged.

    Gerard Tel,