FCTG's Radio Valve Swap Page.
Radio Valves/Tubes Substitution
Welcome to my valve substitution page.
Last updated 14th November 1996.
Sometimes when you have a wireless set [please, fitting 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.
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. Pentodes
Younger 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-changers
Many 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 preamplifiers
In 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 stages
In 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 decoupling capacitor.
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.
Rectifiers
Substituting / 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 Stuff
The above is only a very beginning of what I plan to be 'hint & tips'
about repairing/restoring wireless sets: as I manage to get my various
notes and thoughts and memories onto computer files I will extend the
page. If 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.
Click on this link to go
to the valve data page for some of my old valve list;
click here to go
to FCTG's Home Page. There's also a link
to my Amateur Radio Page available. Or you can send mail
to the author if you wish. Thanks for reading.