Contrary to what some may believe, the missing or unreadable tubes in a radio can often be guessed quite well. 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. Of course, experimentation is at your own risk! If in doubt, post the model number and the visible tubes in some group and somebody may be able to look up the precise data for you.
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 two series found mostly 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).
Prewar sets used the A-series (4V parallel).
Battary sets usually have D-tubes;
both parallel and series supply of these tubes is found,
because they have the same current and the same voltage.
The following heater types are found:
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
(Radiomarelli 9A75).
(Don't mistake the output transformer for a power transformer.)
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....
This output stage sometimes inspired the use of
"series" tubes because of technical reasons.
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.
After finding out which of the listed tasks are performed in your set
you can determine the necessary glassware.
Some quite unusual line-ups have been reported.
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.
In general, there is no reason to believe that two tubes, differing
only in the final digit, are equivalent and can be interchanged.
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.
This sounds complicated, but is an elementary consequence of Ohm's Law.
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.
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 DL92/94 (3V 50mA Center
tapped);
2. DK96, DF96, DAF96 (1.4V 25mA) and DL96 (3V 25mA Center 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.
This book on battery tube radios has a complete chapter covering the filament power solely!
Second and following letter: Content
Tube systems come in varieties; here is what the manufactures have
made for us:
Some tubes contain more than one system; in this case the
corresponding letters are listed in alphabetic order. Some
combinations are:
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:
The following are sometimes found in addition; check the listed
condition to see if your set has these:
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.
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.
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 :-).
The picture shows (from left to right):
a gold-painted Philips P-side AF7,
a red Philips pentode with P-base,
a loctal tube,
an octal tuning eye EM34,
a Rimlock tube ECC40,
a noval tube,
a miniature output pentode EL95.
In the ascii drawings below, the view is from the
BOTTOM (where you approach the socket with your soldering iron if this
is necessary).
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.
8 1
7 2
6 3
5 4
8 1
7 ^ 2
O
6 3
5 4
There are not so many types of Loctals.
8^1
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.
(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.
9 . 1
8 2
7 3
6 5 4
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.