Wireless World
1938 Valve Data Supplement
New valves are still being produced in great numbers, and The Wireless
World Valve Data Supplement consequently grows in size every year.
Current types of British and American valves are listed, together with
their electrical characteristics, operating conditions, and base
connections. Valves which are definitely obsolete are not included, but
the omission if a specimen should not be taken to mean that it is now
unobtainable. Valve makers can usually supply obsolete types as
replacements.
Quite a number of the valves included are approaching obsolescence in the
sense that one would not choose them in preference to newer types for a
new receiver. They are, however, still used in large numbers in existing
receivers, and the inclusion of their characteristics is justified on the
score of the help it gives to those engaged in servicing.
In general valves are listed in order of filament or heater voltage.
With a few exceptions, 2-volt valves are for battery operation, and
4-volt types for AC mains operation. The 6.3-volt types are intended for
AC operation with the parallel connection of heaters or for AC/DC
operation with the series connection of heaters. They are also suitable
for car radio receivers when a 6-volt car battery is used. The 13-volt
range is intended chiefly for AC/DC sets and for car radio with a 12-volt
battery. Valves of higher heater voltage rating are designed for AC/DC
receivers.
Valves run with heaters in parallel from a single transformer secondary
must all have the same heater voltage rating, but the currents can
differ. When series operation is adopted, as in an AC/DC set, the valves
must take the same current, but the voltages can vary.
The valves listed are classified roughly according to their major uses,
but because a valve is included in one section it should not be supposed
that its functions are necessarily confined to that one application. An
RF pentode, for instance, finds its major application as an RF or IF
amplifier as its name would suggest, but it can also be used as a grid
detector or an anode bend detector or a resistance-coupled AF amplifier.
The operating conditions for these less common applications are often
widely different from those when it is to act as an RF amplifier. To
list conditions for all the possible applications of a valve is obviously
impossible in view of the enormous number of types now marketed. The
conditions given, therefore, are the optimum ones for the purpose for
which the valve is primarily intended by its makers.
Frequency-changers
The first section is headed frequency-changers and includes valves
primarily intended for this essential operation in a superheterodyne.
The chief types are the heptode, octode, triode-pentode and
triode-hexode, and these are essentially two valves in one-comprising
mixing and oscillating sections. There are also a few hexodes. These
valves require a separate oscillator if used for frequency-changing;
where this is their main function they are listed in this section, but
where they are primarily intended for use as amplifiers they are
included with RF pentodes.
The heptode and octode are essentially alike, but the latter has an extra
grid, rather on the lines of an RF pentode. The electrodes are
concentric with the cathode, and the two inner grids form the grid and
anode of a triode oscillator. The third and fifth grids are screen
grids, and the fourth is the signal or control grid.
The triode-pentode consists of a separate triode and pentode in the same
glass envelope. The triode is used as an oscillator and the pentode as a
mixer with cathode injection. The valve is not very suitable for
short-wave operation, and is consequently less commonly used now that it
was a few years ago.
The triode-hexode is probably the most widely used frequency-changer
today. The hexode section has the first grid for the signal input and
the second and fourth for screens, while the third grid is for injecting
the oscillator voltage. It is normally connected internally to the grid
of the separate triode assembly which acts as an oscillator.
Its popularity is due largely to the relative absence of interaction
between the signal and oscillator circuits on short waves and to the ease
with which the triode circuit can be made to oscillate. In this
connection, however, it should be mentioned that some of the latest
octodes have neutralising condensers built in to remove the interaction
effects.
Screen-grid Valves
Ordinary screened tetrodes and pentodes are not greatly used in broadcast
sets now, having been largely superseded by the variable-mu type. They
are, however, used for IF stages which are not controlled by the AVC
system, and they are also used on occasion for frequency-changing,
electron-coupled oscillators, detectors, AF amplifiers, television sync
separators, etc. The early types of tetrode can also be used as a
dynatron oscillators, but the latest kind have pentode type
characteristics and are unsuitable for this purpose.
Quite a number of valves in this category have very high values of mutual
conductance of the order of 6-12mA/v. These are special television
types, and the grid-anode capacity is usually too high for advantage to
be taken of the high mutual conductance in other applications.
When a valve is used as a voltage amplifier its gain can be calculated by
multiplying its mutual conductance (mA/v) by the parallel value of the
dynamic resistance of the tuned circuit and the valve's own AC resistance
and dividing by 1,000. When the valve resistance is very high compared
with the dynamic resistance the gain is nearly equal to the product of
mutual conductance and dynamic resistance divided by 1,000. That is
gRa Rd g Rd
Gain = ----------------- ~ ------- when Ra >> Rd
(Ra + Rb) 1,000 1,000
This equation holds also for other valves. With frequency-changers
conversion conductance must be substituted for mutual conductance, and
with resistance-coupled amplifiers Rd becomes the coupling resistance in
parallel with the following grid leak. The equation does not hold with
reactive couplings.
Throughout RF and IF amplifiers variable-mu tetrodes or pentodes are now
generally employed. With non-variable-mu valves the mutual conductance
tends to remain constant as the grid bias is increased and then rapidly
falls to zero, whereas with the variable-mu valve the mutual conductance
falls continuously and gradually as the bias is applied. In practice, of
course, the mutual conductance with both types falls on increasing the
bias, but the change is much more gradual and a much higher bias voltage
is needed to obtain the same minimum value with the variable-mu valve.
The practical result is that it is possible to control the amplification
within wide limits by varying the grid bias of variable-mu valves without
distortion or cross modulation being introduced. This statement must be
taken conservatively, of course, for there is naturally a limit to the
conditions under which freedom from distortion occurs, and even with
variable-mu types trouble will be experienced if the input exceeds a
certain figure.
Inter-electrode Capacities
With all valves the inter-electrode capacities are important, but they
are especially so in the case of types which are used for RF and IF
amplifiers. The input and output capacities usually occur in parallel
with the tuned circuits and are only important in that they increase the
minimum capacity of these circuits and so restrict the tuning range. It
must be remembered that a receiver will require retrimming if a valve is
replaced by one having different capacities. In television receivers, of
course, the capacities are of the first importance and most definitely
limit the gain per stage.
The grid-anode capacity, however, is very important in all types of
receiver because it very greatly affects the stability: it does, in
fact, place a definite limit to the possible stable gain even will all
other couplings eliminated. With a single stage of amplification having
identical grid and anode circuits, the limit of amplification is
A = 2/w Cgn Rd
when the valve resistance is high compared with the dynamic
resistance. With two stages the number 2 in the above equation should be
replaced by 1, with three stages by 0.76, and with four stages by 0.67.
With Rd in ohms and Cga in microfarads, w = 6.28 times frequency in Mc/s.
Diode valves are used chiefly as detectors and for AVC purposes. The
majority contain two anodes and a common cathode and can provide
detection and delayed AVC. In general they can be safely operated at a
much larger signal input than the diodes fitted to the multiple diode
class of valve and they often have a lower resistance. Westectors are
included in this section since they fulfil the same functions as diodes
of the thermionic type.
Valves which include one or more diodes in addition to another type of
electrode assembly are listed in the section appropriate to the major
elements. Thus, the duo-diode-triode appears under the heading of
"Triodes" while a duo-diode-RF pentode is listed among "Screen-grid" or
"Variable-Mu" valves according to the characteristics of the pentode
section.
Triodes
Triodes are divided into two categories: those with a resistance greater
than 7,000 ohms and those with a resistance less than 7,000 ohms. The
former are now used chiefly as AF amplifiers, grid detectors and
oscillators, while the latter are output or driver valves.
For a grid detector a valve with a resistance of 7,000-10,000 ohms is
usually the best from the point of view of quality, but where high
amplification is important a higher resistance can be selected. The
deterioration in quality will usually be small and in some cases non-
existent, depending very largely upon the circuit conditions. For
amplification purposes the valve should have a resistance of less than
15,000 ohms in most cases when transformer coupling is used. With
resistance coupling a valve with a resistance of 10,000-100,000 ohms is
suitable.
In calculating the amplification from the formula already given, it is
important to remember that the mutual conductance and AC resistance both
depend on the operating voltages. The figures given in the tables are
for zero grid bias and 100 volts HT, following the standard practice.
The actual values at maximum HT and the optimum negative grid bias are
not widely different, but when using resistance coupling it is a wise
plan to take the resistance as being about 25 per cent greater than the
figure given ann the mutual conductance as about 25 per cent less. In
general, the coupling resistance should be about three to five times the
valve resistance and the grid leak of the following valve not less than
twice the coupling resistance and preferably higher. It is, however,
unwise to exceed 2 megohms for a grid leak and in the case of an output
valve 0.5 megohm. Output valves operating with fixed bias instead of the
usual automatic bias may have to be worked with a grid circuit resistance
of 50,000 ohms or less, and the use of resistance coupling is then hardly
practicable.
Values for cathode bias resistance's are not given for small triodes
since they depend on the operating conditions adopted. When resistance
coupling is used the required value of bias resistance can be estimated
by dividing the coupling resistance by the amplification factor of the
valve. This will not always give the optimum value, but it will give a
value which is usually surprisingly close to it.
Output Valves
Valves with resistance's below 7,000 ohms are chiefly of the output type.
In most cases the figures given are for a single valve in Class A
operation. When two valves are used in Class A push-pull, the anode and
grid voltages remain the same and each valve takes the same current. The
value of the bias resistance must be halved and the anode-to-anode load
doubled. In practice the load can often be about 1.5 times that for one
valve with some advantage in output. The output obtainable from two
valves in Class A push-pull is about 2-2.5 times than given by one valve
alone.
In some cases greatly increased output can be obtained by operating
valves in Class AB1, AB2 or B. All valves are not suitable for these
modes of operation. Some are designed especially for one particular
system and a pair will give a surprisingly large output.
Cases of this nature are met with among tetrodes and pentodes in
particular, and valve makers ratings for a valve vary greatly according
to the method of operation. It is impracticable to include all operating
conditions in the tables, and except where otherwise indicated operation
in Class A is to be understood with a single valve and automatic grid
bias.
The classification of output stages is made on the following basis. A
Class A stage is one in which the anode current flows throughout the
whole cycle of input grid voltage and grid current is not permitted. The
mean anode current is substantially constant irrespective of the input
voltage, up to the overload point. It can be either a single valve or a
push-pull stage.
In Class AB1 grid current is not permitted, and the anode current flows
over the major portion of the cycle of input voltage but not necessarily
over the whole cycle. Two valves in push-pull are used and the mean
anode current fluctuates with the signal. The condition of operation is
about half-way between Class A and what is often termed quiescent push-
pull. A Class AB2 stage is the same as Class AB1, but a larger input is
applied and the valves run into grid current.
A Class B1 stage is one in which no grid current is permitted and a pair
of valves in push-pull is used. The valves are so biased that anode
current flows in each for only about one-half of the cycle of input
voltage. That is, each valve is biased to about the point of anode
current cut-off. The anode current fluctuates considerably with a
signal. A Class B2 stage is the same but a larger signal is applied and
grid current flows.
On this basis what is familiarly termed a QPP stage is Class B1 and the
so called Class B is Class B2. In the tables operating conditions for
these systems are quoted chiefly in the case of the special valves, of
which the majority are battery types. The "QPP" valves are double-
pentodes and the "Class B" types double-triodes : for clarity, the
distinguishing symbols Q and B are employed instead of the more strictly
correct B1 and B2.
Whatever type of output stage is used it must be operated into the
correct load impedance. Figures for this are given in the tables. In
practice the speech coil of the loud speaker rarely has the correct
impedance, so that an output transformer is necessary, and the ratio is
readily calculated by dividing the optimum load impedance by the speech
coil impedance and taking the square root of the result. When the speech
coil impedance is less than the optimum load impedance, the transformer
ratio is step-down.
With triodes in Class A the matching is not very critical. It is much
more so with tetrodes, pentrodes and Class AB and Class B stages
generally. Tetrodes and pentrodes also have a high output resistance
which leaves the loud speaker substantially undamped, and it is
consequently often advantageous with these valves to use negative
feedback to reduce the effective output resistance.
Class AB1 and Class B1 stages demand a more carefully designed output
transformer than Class A types. A low DC resistance for the primary is
needed and it is usually important that the two half-primaries be
sectionalised and interleaved with one another to keep the leakage
inductance between them at a low figure. Class AB2 and Class B2 stages
have the same output transformer requirements, but as they have a low
input impedance a driver valve and well designed driver transformer are
needed.
Rectifiers
Few remarks are necessary on rectifiers, but it is as well to point out
that the reservoir condenser must be rated for working at not less than
1.4 times the RMS AC input to the rectifier. Thus for a full-wave
rectifier with an input of 500-0-500 volts the condenser must be at least
700v working.
A number of rectifiers rated for very high voltages are to be found.
These are of the half-wave type and are intended for providing the very
small current taken by a cathode ray tube. They are primarily television
valves. The reservoir condenser with these is usually 0.1 mfd.
The data for metal rectifiers is essentially the same as for valves. The
capacities of voltage-doubler condensers, however, depend on the mains
frequency, the values given being for 50 c/s. With 100 c/s mains the
capacities must be one-half the listed figures and with 25 c/s supplies
double.
Each valve has its base connections definitely identified. In every case
the figure opposite a valve in the "Base" column denotes the number of
pins in the base, while the following letter denotes the connections for
that number of pins. A preceding letter is used to distinguish between
different arrangements of the same number of pins. Thus a conventional
4-pin triode is listed as Base 4A while a 4-pin screen-grid valve has
Base 4B. The Midget valves have different pin arrangements and a triode
is listed as Sm4A being an abbreviation for small 4-pin base, A
connections. Similarly an American valve base has the prefix A, and
Continental types the prefix C. Side-contact types are distinguished by
Ct.
It should be noted that the same drawing is used for the British and
American octal base. Actually the pin spacing is slightly different in
the two and the centre spigots are not the same size. The British octal
base is denoted by 8 and the American by A8.
The code is an arbitrary one, but is easy to remember, for the numerical
and any preceding letters show at a glance the number of pins in the base
and the type of base, while the following letter refers to the
connections for the particular valve.
In connection with the small 4-pin base for Midget valves, this is used
by several valve makers, and it should be pointed out that the different
makes are not all interchangeable. The bases appear the same at a
glance and the connections are the same. Actually, however, the pin
spacing is slightly different in the Hivac valves from that adopted by
Marconi, Mullard and Osram.
In few cases it will be noticed that two or more valve type numbers are
quoted for the same characteristics and that two base codes also appear.
This indicates that the same valve is available under different type
numbers with different bases. As an example, we find AC/5 Pen and
Pen.45 under the type column 7J and 8J. This indicates that the AC/5
Pen has type 7J base and the Pen.45 the 8J base. Otherwise the valves
are identical in all characteristics except possibly interelectrode
capacities. Slight differences in capacities are sometimes found with
different bases.
Back to 1938 Supplement Index