Old Radio Substitution pageWelcome to my Valve/tube Substitution page. Last
updated 8th November, 2004..
Sometimes when you have a wireless set [please, use appropriate terminology 8*)]
under restoration or repair, you can find that it simply is not possible to
locate one or more of the valves that were used in the original design and you
may want to substitute a different valve in it's place in order to get the set
in a 'real' working condition. Service engineers of the period used to do
exactly the same thing when faced with an 'older' set for which the valve
type(s) were obsolete; it was quite common practice. Here's a few hints and tips
taken from both experience, childhood memories and a few of the data books and
diagrams I have around. For those of you who remain unconvinced that this was
'normal practice' I show you here a page from a valve data book from a few years
ago intended to assist service engineers of the time of this very practice:
In general, the substitute valve might need a different biassing-point, or a
different screen-voltage, for optimum results: always make sure that these
voltages are within the range of the valves maximum operating envelope. A valve
may give good performance in conditions and applications for which it was never
intended, provided that it's operating limits are not
exceeded. Just because a valve works well with modified
conditions does not mean that it will do so for very long!
R.F. PentodesYounger types of H.F. pentode valves have in general a
higher mutual conductance and a higher maximum operating frequency: this means
that they may well oscillate or become unstable due to such things as wiring
lengths, stray capacitances, etc. Always check that any screening, metallizing
or extra grid/cathode connections are connected properly and with as short a
length of wire or component lengths as is possible.
Remember that bad contact resistance, or a resistance or reactance of just a
few ohms can cause enough unwanted signal coupling to create instability. In
particular, ensure that anode and gris lines are kept as far apart as possible
(especially if you are replacing a 'top-cap-grid' type with a type which uses
just the base pins). If the substitute valve has significantly higher gain than
the original, then either reduce the screen voltage, or add some damping on the
input grid, or where appropriate add some un-decoupled resistance into the
cathode connection. Lots of hiss doesn't mean great sensitivity!
Mixers/Frequency-changersMany of the older types of heptodes and
octodes can in fact be substituted for each other, especially when the only real
difference is the base (many 'newer' valves were just the same electrode
structure in the same or smaller glass envelope with a 'newer' base or pinout!).
However, be very careful if you plan to replace an older-type (e.g. octal) valve
with a miniature, or younger, version: some of these younger types do not have a
separate triode oscillator anode (they use g3) and serious modification of the
oscillator section could be necessary. Much less current went into g3 than in
the older 'real' triode anodes.
Triode-hexodes and triode-heptodes can in general be interchanged in a mixer
stage, the difference being that the heptode has a supressor grid to reduce I.F.
transformer loading by increasing impedance. Designs using a triode-pentode with
cathode coupling can often be replaced by triode-hexodes, or even heptodes, with
changes of anode and screen resistors to match the valves' ratings. Older sets
using R.F. tetrodes as self-oscillating mixers can be treated to a substitute
using a younger R.F. pentode, but do not use vari-mu types since they have a
lower conversion factor and can give rise to non-linearities which results in
harmonics being generated.
Whenever the mixer stage valve is substituted, it is essential to re-align
the stage ('trim' the oscillator padding capacitors, etc) and to check that
oscillator signal levels are not too high (an indicator is loads of extra noise,
or 'whistles' as stations are tuned across).
Audio preamplifiersIn many sets, a small change in the performance or
gain of an audio stage is not a problem at all, and substituted valves can be
matched to the set by changing of anode load resistors, screen voltages (in the
case of pentodes), and where necessary coupling capacitors. Always take care
when examining an audio pre-amplifier that any electrolytic capacitors used in
cathode decoupling are not 'dry' - temporarily parallel a new capacitor of the
same value across such a component and check that no massive level change
occurs. If it does, the electrolytic most certainly needs replacing. If the
stage uses a pentode valve, or you wish to substitute a pentode there, a good
guide is to arrange for the anode voltage to be half the H.T. voltage and the
screen about a quarter of the H.T. voltage; you should also in general see a
bias voltage of some 1.5 volts (often the cathode voltage above ground). If the
stage is a transformer-coupled stage, a lower-mu valve should be employed; for
resistance-capacitance coupling, use a high-mu valve.
Some younger sets with 'phono' (record player) inputs or those which had a
record deck included, used low-noise designs for the cartridge-input
pre-amplifier. Here, the main problem encountered in substitution is hum pick-up
due to wiring layout and due to coupling from A.C. heater circuits. If you have
to substitute a valve in such a stage, try to use a pentode as similar as
possible to the original, and consider using extra screening of wiring where the
base layout of the replacement valve is different from the original. Some
younger sets employed a specially-designed pentode for this purpose, the EF86,
often mounted in a socket with rubber anti-vibration stubs to avoid microphony.
A.F. Output stagesIn older sets which used no intermediate audio
pre-amplifier, the output valve had to provide a high gain. Any substitute valve
that you intend to place in such a stage must therefore also be capable of
providing significant gain. Good, linear gain is more important than absolute
output power capability, but there are sometimes cases where there just isn't a
valve available which matched the original in performance (or only ones with too
many years of service which can't do it any more). In such cases, where there's
no alternative, it is sometimes possible to replace (e.g.) an output pentode
with a triode-pentode, thus adding an audio preamplifier to the set without
wrecking the chassis and adding a valve. Again, in this stage of a set, always
check the coupling / decopuling capacitors - including the supply reservoir
capacitors - when contemplating substitution. If you have a stage which
originally used an output triode and have to
replace it with a pentode, remember that just triode-connecting the pentode
(connecting g2 to anode) may not be possible, since the rated screen dissipation
may be exceeded. In this case, include an extra screen dropping resistor and
In younger 'push-pull' output stage designs, do not replace only
one of the valves with a significantly different valve; replace
both of the valves (or double-valves as appropriate) otherwise you may
end up damaging the output transformer or even one of the valves. In any case,
ensure balance throughout the stage is maintained.
RectifiersSubstituting / replacement of a rectifier valve must always
be very carefully considered, since the stress that power-supply components,
including these valves, endure is considerable. Older-type rectifier valves had
significant voltage 'drop' across them, and replacement of a younger type may
well result in higher H.T. voltages everywhere else in the set due to the lower
drop across the new-type valve. Also, remember to check that no heater-cathode
voltage problems might arise with a substitution, since many designs used
rectifier valves with a separate heater winding on the supply transformer, and
if using valves intended for such operation with a non-isolated heater supply,
the heater-cathode voltage rating could be exceeded. At the risk of labouring on
the subject, remember again that the most-likely-faulty component in the power
supply stage of a set can be the electrolytic reservoir and smoothing
capacitor(s), so check that they are not 'dry' or leaky.
One other thing to be careful about is the warm-up time of the valve
to be used as a replacement: if most of the other valves used in the set have
long warm-up times, then be careful not to use a 'newer' type rectifier valve
which warms up significantly faster - this could mean that after switch-on there
is excessive H.T. in the set until the other members of the gang start to get
serious. Either use a longer warm-up rectifier or use a scheme to delay the
application of H.T. voltage.
Other StuffIf anyone has useful input and/or corrections then please do
e-mail me, I'll be happy to include your information (creditted) and also to
place extra links (at my discretion) into the page.
You can E-mail the author of these pages
(Trevor Gale) by using this link. on the
Dutch Internet service provider XS4ALL.
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