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VINTAGE RADIO
By RODNEY CHAMPNESS, VK3UG
How to substitute for valves
that are no longer available
A big problem for anyone restoring vintage
radios is obtaining new valves, particularly
for very old sets. In this article, we describe
how it is possible to substitute valves with
similar characteristics and even eliminate
rectifiers while still maintaining an original
appearance.
From time to time, vintage radio
buffs find that they don’t have a particular valve that is used in a very
old, little known model made by an
obscure manufacturer. For example,
where would you get an A141 valve?
Never heard of it! They are very rare
and some others are even rarer. A 49
MAY be able to replace it, although its
characteristics are quite different and
so the result of replacing the A141 with
a 49 is uncertain.
Having exhausted all practical avenues to obtain a valve, what do you do?
The only practical alternative is to fit
a close relative of the valve. This may
involve some modifications to the set,
although it is desirable to keep these
Many valves had a host of near equivalents which may have come in a different
envelope and required different heater, plate current and bias settings and so
on. This photo shows a 6V6GT and some near equivalents.
90 Silicon Chip
to an absolute minimum to maintain
originality.
Selecting a replacement
Manufacturers had the habit of designing many types of valves which
had similar but not identical characteristics. One manufacturer would
produce an XYZ valve with specific
characteristics and then another manufacturer would produce valve ABC,
similar but not the same – either the
base wiring was different or the characteristics varied just a little. Toward the
end of the valve era though, manufacturers used a lot more sense and built
virtually identical valves but with
their own label (AWV, Philips, Mull
ard, etc). I imagine they had some sort
of licensing agreements to cover this.
Some valves were still only available from one manufacturer, such
as the 6BV7 from AWV. Even some
“identical” valves from different
manufacturers are not the same under certain circumstances. Take the
6DQ5, for example. This was used
in early American colour TV sets as
the horizontal output valve. Certain
characteristics were designed into the
valve and one of the most important
was its ability to draw many hundreds
of milliamps in pulse mode – which its
function as a horizontal output valve
requires it to do.
Now single sideband (SSB) radio
transmitters operate virtually in pulse
mode due to the peaky nature of the
human voice. An American manufacturer of amateur radio transceivers
decided to use the 6DQ5 as the power
amplifier (PA) valve in some of its early
single sideband (SSB) transmitters.
The 6DQ5 was considerably cheaper
than similar transmitting valves and its
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ability to draw high current for short
periods was ideal for use in SSB transmitters. The transmitters worked well
until the 6DQ5 needed replacement. If
replaced with the same brand all was
well but if another brand of 6DQ5 was
substituted, the output power was low
and the valves had a short life.
If they were “identical” why was
one valve so much better than the
other? The reason for the difference
was that the valves were designed for
horizontal output (flyback) in a TV set
running at roughly 16kHz and each
brand met those design parameters.
But while one particular brand of
6DQ5 might have been adequate for
SSB use (by good fortune), others were
only suitable for TV use.
Valve data books
The best way to find out which
valve can be used as a replacement for
an unobtainable valve is to look up a
valve data book. Preferably, the data
book will have all the performance
graphs so that the characteristics of
all the likely replacement valves can
be compared. To the average vintage
radio buff that is a lot of hard work
and in most circumstances, it is not
necessary.
A valve data book such as the “Miniwatt Technical Data” (7th edition best)
gives abridged specifications and this
data is usually enough to determine
whether any particular valve can
replace another. However, it doesn’t
have much on very old valves. If
you think that you may have to do
much substitution work, get as many
valve data books as you can, particularly early ones or reprints of early
ones.
The 6V6 and equivalents
Now let’s look at some comparisons.
Take the ubiquitous octal-based 6V6,
for example. Checking in the data
book, it can be seen that the 6V6 has
the same ratings as variants 6V6G,
6V6GT, 6V6GT/G and 6V6GTA. These
are plug-in equivalents but due to the
shape of the glass envelope, some
may not physically fit into the set.
(GT stands for glass tubular envelope).
Now consider the 6BW6. This is
a 9-pin miniature type and the data
says “* For data and notes refer to
type 6V6G”. But it is not likely that
you will replace a 6V6G with a 6BW6
as it is less common than the 6V6G,
having been used almost exclusively
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Photo Gallery: The
Radiolette Model 47 Console
The Model 47 Console was manufactured by Amalgamated Wireless
Australasia, (AWA), in 1934 and sold for £21/18/6. The set used the same
chassis as the companion table Model 27 but was fitted with an 8-inch
speaker, in place of the 5-inch unit used in the table model, which resulted
in improved audio quality. A “local-distance” switch was fitted to the rear
of the chassis to reduce overload in strong signal areas.
The 47 was small by comparison with many other console sets of the
day, measuring 910mm(H) x 510mm(W) x 260mm(D). The circuit was a
5-valve reflexed superheterodyne with an Intermediate Frequency (IF) of
175kHz. The valves used were: 78, RF amplifier; 6A7, frequency changer;
6B7, reflexed IF amplifier, detector, AVC rectifier and audio amplifier; 42,
audio output and 80, rectifier.
Picture & text, courtesy Historical Radio Society of Australia (Inc).
August 2001 91
Fig.1: this circuit can be used to replace a defective rectifier valve. If
the defunct valve has no faults other than loss of emission it may be
left in the socket to preserve the original appearance.
by STC. The 7C5 is also identical in
characteristics to the 6V6G – but where
would you get a valve socket for it?
Then we could consider the 6AQ5
and 6AQ5A. The first few lines of information are the same as for the 6V6G
but following data is different. The
differences are mainly due to the fact
that the 6AQ5 is physically a smaller
valve and is derated so that it doesn’t
get too hot and melt down.
So there we have four physically
different valves that are electrically
identical in many ways but may or
may not be suitable for substitution,
depending on the set and its circuit.
Most restorers would rather use
valves that look similar to the originals if at all possible. Let’s see what
octal-based valve could be used to
replace the 6V6. The following could
be considered:
• 6EY6 plug-in, greater heater current and requires more bias;
• 6F6G plug-in, greater heater current, requires more bias and a higher
impedance plate load;
• 6K6G plug-in, less heater current,
requires more bias and a higher impedance plate load;
• 6L6G plug-in, greater heater and
plate current, lower impedance plate
load;
• 6W6GT plug-in, considerably more heater current but
approximately the same bias and
plate impedance;
• 6Y6G plug-in, considerably
higher heater current, higher plate
current, lower plate voltage and
plate load impedance.
So there are six possible plug-in
valve types that could possibly be
used in place of the 6V6GT. Some
would have to be crossed off the list
as the extra heater current may stress
the power transformer too much.
92 Silicon Chip
Bias would need to be altered in most
cases and the audio output speaker
transformer impedance may need to
be altered.
In another example, the 42 valve can
be replaced by the 6F6G if the socket
is replaced. And with slight alteration
of the operating conditions, the 6V6GT
could replace the 42. So by doing a bit
of research backwards and forwards
through the data, it is possible to determine if and what valve could replace
that “hard to get” one.
Alternative RF and IF valves
RF (Radio Frequency) valves are
another area where alternative types
may need to be used. This time we’ll
look at possible octal replacements
for the remote cut-off 6U7G. For those
new to valves, “remote cut-off” means
that the gain of the valve is gradually
reduced as the negative bias on the
grid is increased. The 6U7G valve
can be considered to be cut off and
not amplifying when a bias of -50V is
applied to the grid. Valves such as the
6U7G are very suitable for use in circuits that have automatic gain control
(AGC).
The 6K7G is a plug-in closely
similar to the 6U7G, with the 6S7G
being somewhat similar. The 6AR7GT,
6B8G and 6G8G are all duo diode RF
remote or semi-remote pentodes but
the socket would need rewiring to suit.
The 6SF7, 6SG7, 6SK7 and 6SS7 are
all single-ended valves but could be
worth considering too.
Of course, the large 7-pin 2B7 and
6B7 valves may need replacement
too. They are electronically the same
as the 6B8G but the heater of the 2B7
is 2.5V. If you are really hard up and
have suitable valve sockets the 7V7,
7R7, 7H7, 7E7, 7B7, 7A7 and 7AH7
would also be possible replacements.
Perhaps the set uses one of the
duo-diode/pentodes with all elements
in use and you only have a straight
pentode to replace it. The straight
pentode can be put in the rewired
socket but it would then be necessary
to use either a couple of germanium
or silicon signal diodes to do the work
of the diodes from the defunct valve.
Sharp cut-off octal and large 7-pin
base RF pentodes are sometimes used
in receivers but are not usually connected to AGC lines. Once again, by
going through the data book, various
re
placement valves will be found.
The 6J7 and the 6SH7 octal valves
are probably the most common of this
type, although there are a few others to
consider such as 6SE7GT and 6W7G.
Replacement converter valves
Converter valves are probably more
of a worry, as they tend to become
faulty more regularly, being a family
of valves with quite complex internal
structures. A common converter valve
is the 6J8G and 6J8GA triode-heptode.
Note that the GA version has a 0.45A
heater filament whereas the G version
requires only 0.3A.
The only other relatively common
triode-heptode is the 6AJ8 9-pin miniature, arguably the best of the converter valves. The 6K8G triode-hexode can
be plugged in place of the 6J8G and
works satisfactorily even though it is
slightly different internally. In 9-pin
miniature triode-hexodes, the options
are 6AN7(A), 6AE8 and 12AH8. The
12AH8 may be used on 6V or 12V
heater lines.
Pentagrid converters are noticeably
different to the tri
ode-heptode and
triode-hexode converters and therefore use dif
ferent circuitry. If you
compare the circuit around a 6BE6
pentagrid converter and that of a 6AN7
triode-hexode and you will see what
I mean.
Here is a list of common and not
so common pentagrid con
verters.
2A7*, 6A7*, 6A8G+, 6D8G+, 6L7G+,
6SA7GT+, 6SB7+, 6BA7# and 6BE6!.
* = Large 7-pin base, + = octal base, #
= 9-pin miniature base and ! = 7-pin
miniature base.
Electrically, the 2A7, 6A7 and 6A8G
are identical. If you cannot get a 2A7
or a 6A7, a 6A8G will do the job if
you change the valve socket. However, when replacing the 2A7 it will be
necessary to fit a 6.3V filament transformer as the 2A7 has a 2.5V heater.
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The 6SA7GT and the 6SB7 are somewhat similar and have the same socket
pinouts. The 6BE6 miniature is similar
to the previous two valves. Finally,
the 6BA7 has similar characteristics
to the 6SB7.
So while the list of converters is
not extensive, it should be possible
to find one that will do the job with
some modification. If an octal or large
7-pin socket converter has a valve
shield around it, it is always possible
to substitute a miniature equivalent
inside the shield if you are really stuck.
The same applies to RF/IF valves too.
Photo Gallery: The Astor GR “Football” Mantle Radio
Duo-diode/triode substitutes
There is quite a variety of octal and
pre-octal valves that are duo-diode/
triodes as used in the second detector
and first audio stage of most receivers.
Electrically, the 2A6, 6SQ7GT, 6B6G
and the 75 are the same except for their
bases and some heater voltages. A few
others that can be considered are 6Q7,
6R7, 6SR7GT, 6ST7, 6SZ7, 6T7G, 55
and 85. In each case, it will be necessary to check their characteristics,
particularly whether they are high or
low-gain valves as the bias and plate
resistors may need to be changed. If
a higher gain valve is fitted, feedback
may occur.
Miniature duo-diode triodes that
are somewhat similar are the 6AQ6,
6AT6, 6AV6 in 7-pin and 6BD7 in 9-pin
form.
Rectifier replacements
Replacement of rectifier valves is
in some ways easier than any other
valve because if a replacement cannot
be found, you can always resort to
silicon rectifier diodes. The circuit of
Fig.1 shows how two 1000PIV diodes,
such as 1N4007s, can be wired across
the valve plates and common cathode.
If the transformer secondary voltage
is above 250V per side, two 1000PIV
diodes in series must be used to
replace each section of the valve
rectifier, otherwise the peak inverse
vol
tage (PIV) rating of the diodes
may be exceeded, particularly if the
mains supply has any spikes on it.
Catastrophic failure may occur with
the diodes shorting and causing the
transformer to burn out. A 6X4 has a
peak inverse voltage rating of 1250V
and a 5Y3GT a rating of 1400V.
Since silicon diodes are much more
efficient than rectifier valves like the
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This is the highly sought after Astor “Football” radio. It is commonly
called this because its size, shape and colour is suggestive of a football. Its
model number is GR or GRP and was a basic TRF circuit produced around
1948. It has three valves including the rectifier, so has a rather limited
performance despite its use of a reflex circuit. They sell at auction for up
to $300, depending on condition and colour.
5Y3GT and 6X4, it is necessary to put a
resistor in series with the diodes. This
keeps the DC voltage of the supply at
much the same level as that produced
by the valve rectifier. The value of the
resistance will need to be determined
by experiment but 300Ω at 5W is a
good starting point.
Provided the defunct valve has no
faults other than loss of emission, it
can remain in circuit and the set will
then look to be in original condition.
Surge voltage ratings
There is one more point to be
considered if you substitute silicon
diodes for a valve rectifier. Because
they don’t have filaments which take
time to heat up before the cathodes
can start to emit electrons, when a
silicon diode rectifier is used, the HT
(high tension) voltage is immediately
applied to the filter and bypass capacitors at switch-on.
And because it takes time for the
other valves to heat up and start
drawing current, the initial HT may
be substantially higher than when all
the valves are drawing current. It is
therefore prudent to check that the
voltage at switch-on does not exceed
the voltage ratings of any of these
capacitors.
In this case, we can take advantage
of the “surge voltage” ratings of the
capacitors, provided they are new.
Typically, the surge voltage rating of
a new electrolytic capacitor is 15%
higher than the “working voltage”
(WV or VW). However, if the set has
the original capacitors in it, they will
have to be “formed up” to the higher
voltage, with no valves in the set, as
described in the December 2000 issue.
August 2001 93
Vintage Radio – continued
Note that, if you exceed the surge
voltage rating of any electrolytic capacitor, even for a very short period, it is
likely to break down immediately and
spray its contents all over the inside of
the set, with disastrous results.
Miniature valve equivalents
I deliberately haven’t discussed
miniature valve equival
ents to any
extent as they are much more common
than octal and pre-octal valves and
shouldn’t be too hard to obtain. One
valve that appears to be not particularly common is the 6M5. It can be
replaced with the 6BQ5, with a slight
rewiring of the socket. The pentode
section of a 6BM8 could also be used
in place of a valve such as the 6M5.
Rewiring of the socket and alteration
of the bias will be necessary. 6BM8
valves are as common as mud after a
flood.
A common problem with 6M5
valves concerns migration of metal
between the screen and grid (pins 2
and 1) which causes them to appear
to be gassy. Scrape between these two
pins with a metal scriber and in many
cases the valve will again work well,
saving it from the rubbish bin.
Battery valves
Battery valves from the early to mid
1930s onwards usually had 1.4V or
2V filaments. Prior to this, a variety
of voltages were used. A number of
octal and pre-octal valves are identical
except for the base. Several valves had
sub-models like the 1B4, 1B4P and
1B4T pre-octals. Electrically they are
not sufficiently different to warrant a
different number and are similar to the
1E5GT and 1E5GP octal valves.
Valves with 2V filaments draw
0.06A, 0.12A or 0.24/0.26A filament
current. In some cases, where parallel filament wiring is used with a 2V
battery, the amount of filament current
drawn is not critical. However, where
series/parallel wiring is used from a
6V battery, replacement valves need to
draw the same filament current unless
a careful redesign of the filament cir
cuit is undertaken.
The same requirement applies to
1.4V valves, although they tend to
draw either 0.05A or 0.1A of filament
current. Output valves such as the
3S4 have a 3V filament that is centre-tapped, which makes it easy to use
on either 1.4V or 2.8V.
The typical valve line up of a
5-valve battery set using 2V valves is
as follows, with possible alternatives
in brackets. They are certainly not all
direct equivalents:
• Converter 1C7G (1C6, 1A6,
1D7G);
• RF/IF remote/medium cutoff
1M5G (1D5*, 1A4*, 1C4, 34);
• Sharp cut-off 1K5G (1K4, 1B4*,
1E5*, 15, 32);
• Detector/audio pentodes 1K7G
(1K6, 1F7*, 1F6);
• Triodes 1H4G (30, 1H6);
• Audio output 1L5G (1D4, 33, 49,
1G5G, 1F4, 1F5G);
• Twin triodes 1J6G (19, 31);
• Twin pentode 1E7G.
* means that there are several minor
variants of this type number.
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The equivalents for a receiver using
1.4V valves is as follows:
• Converter 1R5 (1A7GT, 1AC6,
1L6);
• RF/IF amplifiers, sharp and remote cutoff 1T4 (1L4, 1N5G, 1P5G,
1U4);
• Diode first audio 1S5 (1U5, 1G4G,
1H5G);
• Audio output 3V4 (3S4, 1S4,
1Q5GT, 1T5GT, 3Q4, 3Q5GT, 1C5G,
1A5G).
One valve that has no equivalent
is the 1D8GT, diode/tri
ode/pentode
(output). Miniature dry battery valves
universally used the miniature 7-pin
base, whereas a miniature 1D8GT
would have needed a miniature 9-pin
base. What a shame, as I believe it
would have been a popular valve.
Summary
While I haven’t given you a blow-byblow description of which valves can
replace which, I trust I have given you
some ideas on how to find a substitute
when the correct one is unavailable.
If you are in a quandary of not being
sure if a particular valve will do what
you require, consult a fellow collector
– particularly if they have a valve data
book – and work out what valve will
do the job. It can be a lot of work but
it can also be fun figuring out how to
solve the problem and save money
into the bargain.
When checking the compatibility of
various valves, it is necessary to look
at the heater/filament volts and current, the HT volts and current, screen
volts and bias voltages. For example,
if a possible replacement valve draws
too much HT current, it may be made
to suit by lowering the screen volts
and/or in
creasing the negative bias
on the grid.
It is also important not to replace
a low gain valve with one that has
considerably higher gain, as oscillation and other strange symptoms
may appear which may not respond
to remedial attention. Some valves
have an internal shield – others don’t,
so if replacing a valve with an internal
shield with one that hasn’t a shield,
an external earthed shield may be
necessary.
An example of this problem is discussed in my article in the February
2001 issue. I found that a 6BE6 (un
shielded construction) needed to be
shielded for optimum performance
SC
of the receiver.
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