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VINTAGE RADIO
By JOHN HILL
A look at high tension filtering
Valve radios require a high tension DC power
supply for the valve plates (anodes). This high
tension supply is a frequent source of problems
& must be carefully restored.
Most mains-operated valve radios
obtain high tension DC using a transformer and rectifier valve (usually fullwave). However, the DC output from
such a setup has a high ripple content
(at 100Hz from a full-wave rectifier)
and must be filtered before it can be
used to power a receiver.
Inadequate filtering will produce a
100Hz hum in the audio output. While
low levels of hum are tolerable, high
levels are not and the hum must be
suppressed as much as possible.
High tension (HT) filtering can be
achieved in several ways and usually
involves either chokes (inductors) or
resistors, and electrolytic capacitors.
Let’s take a look at some of the methods used.
The most common high tension
filtering arrangement used in prewar
receivers is the filter choke type – see
Fig.1. This filtering arrangement is
very effective and leaves little to be desired. High tension supplies designed
around a filter choke have quite low
hum levels. The inductance of the
filter choke opposes any change in
current flow, whether this change be
an increase or a decrease.
High tension filter chokes come in
two physical forms. Either the field
coil of an electrodynamic loudspeaker
can be used or it can be a separate unit
This photo shows the field coil (inside metal housing) of an electrodynamic
loudspeaker from the mid-1930s. The field coil played a dual role: (1) it was
used as an electromagnet for the loudspeaker; & (2) it was used as a high tension
filter choke.
56 Silicon Chip
bolted to the chassis at some convenient place. This latter looks like a small
transformer.
In the first case, the cost of a choke
is saved by making the field coil of
the loudspeaker do double duty. A
filter choke is nothing more than a
large coil of fine copper wire wound
on an iron core. In the case of a field
coil, the iron core, when energised,
becomes the speaker magnet. At the
same time, the field coil filters the
power supply current. Electrodynamic loudspeakers were used on most
early AC receivers.
Hum problems
This arrangement does have one
disadvantage, however. Because the
speaker field is being energised by
only partially filtered current, a small
amount of hum can be generated in
the speaker itself. This was overcome
by fitting the speaker with a “hum
bucking coil” in series with the voice
coil. It was magnetically coupled to
the field coil and cancelled out most
of the hum generated in this manner.
Speakers using permanent magnets
instead of a field coil were also available in pre-war sets. However, they
required very large and expensive
magnets, and were not very efficient.
They were mainly used with batteryoperated sets.
That situation changed after about
1948 when much more powerful magnets became available – as a result of
wartime re
search – and permanent
magnet (permag) speakers were subsequently used in all types of receivers.
Because a field coil was no longer used,
these receivers required a separate
high tension filter choke.
Whether the filter choke is a speaker
field coil or a separate unit, its function is much the same; it opposes any
change in the current flowing through
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A selection of high tension filter chokes. These chokes perform the same
function as a field coil in smoothing the high tension supply & are usually
mounted at some convenient spot on the chassis.
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into a piece of equipment rather than
added as an afterthought.
Capacitor values
Either a field coil, a choke or a resistor form the central component of most high
tension filters. In conjunction with high voltage electrolytic capacitors, they
provided adequate HT filtering for most valve radio receivers.
it. Effective though it is in this role, it is
not sufficient by itself. But when large
electrolytic capacitors are connected
from each end to ground, the result is
a smooth DC current.
Capacitors have the ability to store
an electrical charge and this is their
main role in a high tension filter. By
taking on a charge when the rectifier voltage rises and giving up that
charge when the voltage drops, capacitors supplement the filter choke
constant-current action by tending to
maintain a constant voltage.
In some applications, where heavier
DC currents are re
quired, a second
filter choke and an additional electrolytic capacitor can be added to produce
an even smoother supply. As far as domestic radios of the four or five-valve
Fig.1: a typical HT power supply for a
valve radio receiver. In some circuits,
a resistor is used instead of a choke
or loudspeaker field coil. HT supplies
are a common source of trouble.
type are concerned, this extra filtering
stage is unnecessary.
It must be remembered that an additional choke will also lower the output
voltage and thus needs to be designed
The size of the electrolytics also
has an effect on the effectiveness of
the filter. Hum can often be reduced
simply by installing larger capacitors,
particularly on the output side. A larger output capacitor also gives better
regulation.
However, if one cares to check the
valve specification manuals, recommended maximum capacitor values
for the input side of the filter are
usually listed for various rectifiers. It
is inadvisable to fit larger than recommended capacitors in this position.
The objection is the excessive peak
current that these will draw through
the rectifier. Large input capacitors
should not normally be necessary for
hum free results.
If larger input capacitors do need
to be fitted, limiting resistors should
be added in series with the rectifier
plates to protect the valve. The values
of these resistors are usually listed in
valve characteristics manuals.
If a filter system incorporates a
June 1993 57
These 20W wirewound resistors can dissipate quite a lot of heat and should be
mounted well away from any heat-sensitive components. Note that the resistor
on the left is adjustable.
This receiver uses three parallel 1W resistors in its high tension filter. They are
used in conjunction with two 24µF electrolytic capacitors to provide a wellfiltered HT rail for the valve anodes.
speaker field coil, 8µF electrolytic filter
capacitors would typically be used.
Troubleshooting
A distinct hum in an old receiver
is very often the result of electrolytics
losing their capacitance. Replacements will usually solve the problem.
Another cause of hum could be the
bypass capacitors across either of the
grid bias (cathode) resistors.
Electrolytics with electrical leakage
problems are also a matter for concern,
as they can have two effects on a high
tension filter. Leakage will not only
lower the filter’s output voltage but
will also overload the rectifier valve
and shorten its life. However, the
worst aspect of high tension leakage
is the fact that it often overloads the
58 Silicon Chip
choke itself and results in a burnt-out
winding.
By the 1950s, radio manufacture
had become very competitive and
more and more receivers where being made to a price rather than to
specifications. As a result of this cost
cutting, the overall number of parts in
many receivers was reduced to a bare
minimum. One of the components
found to be dispensable, by careful
circuit design, was the high tension
filter choke.
The choke was replaced with a resistor and larger electrolytics used to keep
hum at an acceptable level. Many of
these sets have filter capacitors ranging
from 16µF to 32µF. But the main trick
with these designs was to take the high
tension for the output valve directly
from the input side of the filter. Since
there is no amplification following
this stage, the hum remained within
acceptable limits.
Only the current for the front end
of the receiver passes through the
remainder of the filter system. And,
since this current is relatively small,
it can be quite adequately filtered by
the second electrolytic. The resistor
is not a filter component as such but
serves mainly to isolate the second
electrolytic from the heavy current
demands of the output stage, so that
it serves only the front end.
The resistor used in these filters is
often made up of two or three carbon
resistors connected in parallel, in
order to provide an adequate wattage
rating. Typically, the resistance varies
from 1.5kΩ to 10kΩ and is rated at
around 2-3W for small receivers.
So while a high tension filter that
uses a resistor may seem to be a crude
alternative, it is reliable, effective and
does have some good points.
Filter systems employing resistors
rarely gave trouble. Field coils and
filter chokes, on the other hand, were
common breakdown items and they
could be costly to repair or replace. If
a high tension short circuit causes a
filter resistor to blow, it is both cheaper
and easier to repair than a loudspeaker
with a burnt out field coil. So its use
makes a receiver a little more trouble-free over a long period. What’s
more, most people would never know
the difference when listening to it.
Field coil substitution
Using a resistor type filter system
as a substitute for a choke system becomes a tempting proposition when
a vintage radio repairer is faced with
a serious loudspeaker problem. An
open field coil is not the only thing
that can go wrong with an old speaker,
however. The speaker cone can be out
of shape, split or completely in tatters.
In such instances, the easiest way
out is to fit a “permag” speaker. When
doing so, a suitable substitute for the
field coil/choke must be made and a
resistor may be the logical way to go.
Better still, a combination of choke
and resistor can be used, provided
they add up to the same DC resistance
of whatever it is they are replacing.
Some old radios draw a fair amount
of current through the field coil and
this needs to be taken into account
when selecting a suitable resistor. A
Possible faults
If high tension current is excessive,
it can be caused by a number of factors:
(1) a faulty valve could be drawing
too much current; (2) there could be
electrical leakage through the high
tension capacitor on the output side
of the filter; or (3) the receiver could
have a grid bias problem whereby the
output valve draws more plate current
than it should.
A defective coupling capacitor is
a prime suspect with this particular
problem. But whatever the cause, it
needs to be corrected to avoid damaging expensive components.
In summary, the high tension filter
is an important part of any mains-powered valve receiver and it requires
periodic maintenance to keep it in
good working order. While the physical arrangement differs from set to set,
they all serve the same function –to
produce a smooth DC supply. A ripple
free high tension current is essential
SC
for hum free operation.
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1930’s set with six or seven valves
can draw about 60mA of high tension current and this requires a high
wattage wirewound resistor to handle
the load. I prefer something with a
20W rating – one of those big hollow
resistors with a brass core that can be
bolted to the chassis.
This convenient mounting method
also helps the resistor to dissipate
some of the heat, since the chassis can
act as a heatsink.
Field coils and high tension chokes
should not run hot. Their normal
working temperature is moderately
warm; hot is abnormal and indicates
a fault somewhere.
An average 5-valve receiver will
draw approximately 50mA if it is
operating correctly. If the current consumption exceeds that (eg, 60-65mA)
there will be a considerable increase
in the operating temperature of the
central filter component, whether it
be a choke, a field coil or a resistor.
Overloads of this nature can eventually lead to the over-stressed component
breaking down.
Simply touching a field coil or choke
after a half-hour operating period
will give a reasonable indication of
working temperature (but make sure
that the receiver is unplugged first)!
Connecting a milliammeter in the
high tension line will give an accurate
assessment without the half-hour wait.
June 1993 59
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