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VINTAGE RADIO
By JOHN HILL
The d.iff erent types of radio valves
This month, we shall be looking at valves a
little more closely. Instead of discussing them
collectively, it's now time to discuss them
individually and to sort them into categories
depending on their applications.
In the early days of valve radio,
everything was wonderfully simple.
The 3-element or triode valve reigned supreme and was the only type
in use. What's more, the receivers
were all battery operated and had
from one to five valves (sometimes
more), depending on price.
Those old triode valves weren't
all the same. Most were made to
suit special applications such as
radio frequency amplifiers, detectors, audio amr>lifiers and audio
output. Each manufacturer produced their own range of products
which tended to complicate things a
little, but the decision as to what
should be used for a particular job
was fairly straightforward in the
mid 1920s.
Early radios had a few problems,
one such problem being the high
cost of batteries. The purchase of
dry cell "B" batteries at six to
12-month intervals was a considerable expense, to say the least.
There was, therefore, a large
market just waiting for mains
operated radio receivers and this
type of set became increasingly
popular from around 1927 onwards.
These first "all electric" radios
had two types of valves: the usual
complement of triodes, plus a double diode or 2-element valve which
was used to rectify the AC mains
current to DC.
The late 1920s to mid-1930s saw
a period of intense development for
both radio valves and the receivers
they were used in. Mains-operated
superhet receivers were taking
over from the old squealing reaction sets and radio improved so
much in such a short time that the
period can only be described as
"remarkable"!
Within this rapid developmental
period, numerous new types of
valves evolved and most were
tailored to suit the AC-powered
superhet receiver. These new types
included tetrodes, pentodes, pentagrid converters and variable mu
valves that worked in conjunction
with diodes for automatic gain control. Beam power tetrodes and pentodes were also developed for
power output applications.
The 5-valve receiver
In the mid 1920s, battery operated triode valves were the only radio valves in
common use. Things really w ere simple in those days !
16
SILICON CHIP
It wasn't long before a standard
pattern emerged that was to be
adopted by all manufacturers for
many years to come. While commercially made radios ranged from
cheap 3-valve sets to monsters with
a dozen or so valves, the industry
standard was the 5-valve receiver.
The rectifier valve was usually
included in the count even though it
did nothing more than convert AC
current to DC current for the high
tension supply. In other words, a
5-valve receiver was really a
4-valve receiver and the majority of
The valve complement of this old
Radiolette was common for many
1936 receivers. The line-up was 80,
6A7, 6D6, 6B7 and 42.
This photo shows some early AC valves. They are (from left) 224, 227 and
E406. Note the top cap on the 224 (left).
These three valves - 2A5, 42 and 6F6 - have the same electrical
characteristics, the only differences being heater voltages and base pin
configurations.
sets made at the time fitted into this
four plus one or 5-valve category.
Popular valve types
Let's take a close look at the
valve complements of some of those
old receivers and, by so doing,
establish popular valve types. This
information will be a good start to
compiling a list of common valves;
the types that radio restorers
should have on hand.
Domestic superhet radio receivers used only a few types of valves.
These can be categorised as: power
rectifiers, frequency changers,
radio frequency amplifiers, detectors, audio amplifiers and audio
output valves. The detector stage
consisted of a diode (much smaller
than the ones used in power rectifiers) and this was normally [but
not always) incorporated into one of
the other valves. Some old sets used
a 6H6, a double diode which performed the roles of detection and
automatic gain control.
I have in my collection of radios
several 1936-37 mantle models. The
valve complement used in these sets
is as follows: 80, 6A7, 6D6, 6B7 and
42.
In order, these valves perform
the following functions:
• 80: supplies high voltage DC to
the other valves;
• 6A7: mixes incoming RF signals
with the oscillator frequency to
form an intermediate frequency;
• 6D6: amplifies the intermediate
frequency;
• 6B7: amplifies the audio signal
from the detector (the detector
diode is within the same envelope);
• 42: additional audio amplification prior to loudspeaker.
Any radio that is about two or
three years older than the previous
example would most likely have
2.5-volt valves instead of the
6.3-volt types just listed. One could
expect to see the following valves in
such a receiver: 80, 2A7, 58, 2B7
and 2A5. In this case, all these
valves are electrically the same as
the first example except that they
have 2.5V heaters instead of 6.3V
heaters, with the 5V 80 being the
only exception.
However, there could be other
variations for there are many types
of 2.5V valves. The output valve
need not be a 2A5; it could well be a
47 or a 59, so what you expect to
see and what might be there are
often two different things.
Going back once more to the first
example, the valves in these particular radios were becoming obsolete while the sets were built. The
"new" octal-based valves were
coming into use at the time and if
OCT0BER1990
17
separate heater transformer or by
using different valve sockets.
European valves
A valve equivalents manual is a handy book for the vintage radio enthusiast. If
you cannot locate the original valve type, it can nominate a suitable substitute.
these radios had used octal valves,
then the valve complement could
have been: 5Y3, 6A8, 6U7, 6B8 and
'6F6. Once again, the valves are
electrically the same as those in the
first example but the octal valves
have different bases.
Therefore, in the 1930s, a good
many apparently different valves
were basically the same, the only
real differences being their bases
and heater voltages. Hence, one set
of valves can be interchanged with
another by the addition of a
Unfortunately, the picture is not
quite as clear as it may appear to
be at this stage . . The valve types
mentioned so far have been
American types; the European
varieties complicate matters somewhat.
European manufacturers made a
range of valves that were not
generally interchangeable with
American types. For a start, the
bases were different. British 4 and
5-pin types used a different arrangement to American 4 and 5-pin
types and so on. There was also a
series of 4V valves and from 1936
many Philips and Mullard valves
used the side contact "P" base,
making substitution totally impossible.
In addition, English Mazda valves
had their own non-standard version
of the octal base just to make things
even more difficult. There were
many differences and variations
between American and European
Vintage Radio Service Tips (From Resurrection Radio).
This is the start of a regular series briefly detailing the day-today experiences of a vintage radio service technician. Some
problems are unique to specific brands and models but many
are commonplace faults found in many radios.
Silver Mica Capacitors
• Alignment drift - sometimes
the IF gain can drop with a click
when final alignment is being performed . This is often caused by a
faulty silver mica capacitor across
one of the IF windings. Try to repeak each slug until one has a
noticeable improvement in gain.
Note the slug, disassemble the IF
transformer and replace the
capacitor across the corresponding winding.
• IF slug won't peak - open circuit silver mica capacitor across
the appropriate winding.
• HMV Little Nipper - crackles
and popping noises in late · model
Little Nippers can be caused by a
defective silver mica bypass
capacitor on the plate of the 6AV6
audio valve (see Fig.1 ).
18
SILICON CHIP
100pf
SM
I..,.
.02
50k
1M
270k
.,.
HT+
points tor the new capacitors.
They may eventually short and
take out the rectifier and power
transformer.
• 50Hz hum in early sets is often
due to long unshielded leads in the
audio section. Replace these with
shielded cables where necessary.
• Can't track down an elusive
hum? - when all else fails, try connecting a large electrolytic
(25-1 00uF) across the back bias
resistor (be sure to observe the
correct polarity).
Fig.1
Hum Problems
• For those that don't already
know, most hum in vintag e
receivers can be traced to old
dried-out electrolytic capacitors,
both tubular and chassis mounted .
Replacement usually cures the
problem. If you leave the chassis
mounted capacitors in the set (and
most people do). check that they
are not leaking . Also, don't leave
them in circuit as convenient tie
Audio Distortion
• Audio distortion is the most
common symptom in vintage
radios that are still operating but
have not been serviced for many
years. It's generally the result of a
positive voltage on the control grid
of the output valve. A leaky coupling capacitor from the plate of the
audio amplifier is the likely culprit
and this should be replaced. An
output valve with low emission can
also exhibit similar symptoms.
Resu:r:reetion
Radio
The original vir.itage
wireless specialists
Our skilled technicians offer QUALITY
repairs & restoration.
We have a large stock of bakelite and
timber radios fully restored and for sale.
Valves, high voltage capacitors and
other specialised parts available.
ADDITIONAL SERVICES
European side contact or P-base valves were produced by some manufacturers
from the mid-1930s. P-base valves are most commonly encountered in sets
made by Philips and Mullard.
58,78,235,6D6,
6U7 ,6G8,EBF35,6AR7,
6BA6,6BH5,6N8
MIXER
IF
AMPLIFIER
6H6,EB4 *
55, 75,85,227,
2A6,2B7,2B6,2B8,
6SQ7,6BD7,6AV6
41,42,45,47,
59,2A3,2A5,6AG6,
6F6,6V6,EL3,EL33,
6AQ5,6M5,6BV7,6BQ5
DETECTOR
AF
AMPLIAER
OUTPUT
COPIES OF:
CIRCUIT DIAGRAMS $5.00
SERVICE MANUALS $10.00
AVAILABLE FOR MOST MAJOR AUSl .
MAKES FROM 1934 ONWARD.
WANTED - valves, radios, etc.,
purchased for cash .
Call in to our showroom at:
51 Chapel Street, Windsor, Vic 3181
PO Box 1116
\
57 ,AK1,AK2,6AN7,6BE6,
,_____.~.._, ECH33,ECH35,EK2,EK32,
6J8,6K8,6SA7
LOCAL
OSCILLATOR
80,5Y3,5V4,6X4,
6X5,6V4,EZ3 EZBO
HIGH TENSION D C ~ W E R
;UPPL y
240VAC
LOW TENSION AC
(VALVE HEATERS)
*mooE DETECTION: DIODES USUALLY CONTAINED IN OTHER VALVES
Fig.1: this diagram lists most of the valves likely to be encountered in the
average Australian-built receiver and shows where they are used. About 60
different types just about covers everything.
valves and while some may be interchanged, most can not.
Fortunately, the majority of
Australian manufacturers went
with the American system and most
of our locally made sets have
American type valves in them.
However, P-base valves do appear
in some radios, particularly those
made by Philips and Mullard.
Personally, I make a point of
avoiding radios with P-based
valves. Unless exceptionally appealing, I prefer to collect receivers
that have what I consider to be
"standard valve types".
Now I know that last statement
will stand on a few toes but as I see
it, P-based valves are non-standard
varieties and I avoid them where
possible. Although I have a few sets
with the odd P-based valve in them,
I prefer the standard American
type valves because of their better
availability.
The advent of the miniature
baseless valves saw the end of an
era where so many manufacturers
and countries had their own special standard. Miniature valves
valves have either seven or nine
pins and no topcap connection to
come loose and fall off. It was not
until this last generation of valves
that some form of international
standardisation came about.
Frequency converters
Frequency converters warrant
some discussion as they come in
many shapes and forms. There are
pentagrid converters, octodes,
triode heptodes and triode hexodes
- whether they be in one envelope
Telephone: (03) 529 5639
CHRISTIAN
BLIND
MISSI(~, ·J
Re$tot"iK9' {:~~,, M1K
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Please cut and send to:
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Phone: (03)817-4566
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Please send me further information
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As long as it is possible for me, I will help:
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OCT0BER1990
19
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SILICON CHIP
American and European valve bases were of different designs which
prevented convenient substitution. At left is the European 4-pin style while at
right is the American 4-pin base.
or two. There are a number of ways
to produce an intermediate frequency but this particular story is
not going into those details at this
point.
The mixer and the oscillator
stages of a superhet receiver can
be designed to work with one valve,
two valves or two valves in the one
envelope. In the block diagram
(Fig.1 ), all the frequency converter
valves, regardless of what type or
where or how they are used, are
listed under "mixer" .
Fig.1 lists most of the valves the
vintage radio enthusiast is likely to
encounter in the average Australian built receiver. About 60 different valve types just about covers
everything.
Note that the last statement says
"just about covers everything".
There is always some obscure set
that has something different in it.
Also note that the list covers from
the early 1930s on. Mains operated
radios from around the 1928-1930
era used entirely different valves
but these have been deliberately
left out because of the rarity of
such sets and suitable replacement
valves.
Astor Mickey
I recently picked up an Astor
Mickey of mid-1950s vintage only to
find that it had a 6AD8 in it, a valve
that was not in my collection.
However, a study of the valve
manual showed that the 6AD8 is a
very close relative of the 6N8. Slipping a 6N8 into the socket had no effect on the set's operation which
goes to show that a ring-in valve
will often do the job just as well.
Astor seems to be the odd one
out. I also have some early 1930s
Astors which use 41 and 43 output
valves instead of the normal 42 .
The 43 seems most odd for it has a
25V heater, while the remaining
valves in the set are of the usual
6.3V variety. No, it's not an AC/DC
set and the heaters are not connected in series .
Many valves can be recognised
simply by looking at them. Rectifiers are easily identified by their
internal construction. Anyone
familiar with valves can instantly
pick an 80 from a 5Y3 because of
the different bases. Likewise with a
6X4 and a 6V4. The 6X5 is different
again.
Valves such as the 6B6, 6SQ7, 55,
75 and 85 all look the same inside.
However, the size and shape of the
glass envelope, the type of base and
the heater voltage will soon sort
most of them out. Anyone familiar
with these valves will be able to
distinguish between them.
Getting to know all these old
valves takes considerable time and
effort but it is very necessary if one
takes vintage radio seriously.
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