A Look at High Tension Filtering

A look at high tension filtering Valve radios require a high tension DC power bolted to the chassis at some convenient place. This latter looks like a supply for the valve plates (anodes). This high small transformer. tension supply is a frequent source of problems In the first case, the cost of a choke is saved by making the field coil of & must be carefully restored. the loudspeaker do double duty. A filter choke is nothing more than a Most mains-operated valve radios Let's take a look at some of the meth- large coil of fine copper wire wound obtain high tension DC using a trans- ods used. on an iron core. In the case of a field former and rectifier valve (usually full- The most common high tension fil- coil, the iron core, when energised, wave). However, the DC output from tering arrangement used in prewar becomes the speaker magnet. At the such a setup has a high ripple content receivers is the filter choke type — see same time, the field coil filters the (at 100Hz from a full-wave rectifier) This filtering arrangement is power supply current. Electrodynamic and must be filtered before it can be very effective and leaves little to be loudspeakers were used on most early used to power a receiver. desired. High tension supplies de- AC receivers. Inadequate filtering will produce a signed around a filter choke have quite 100Hz hum in the audio output. While low hum levels. The inductance of Hum problems low levels of hum are tolerable, high the filter choke opposes any change This arrangement does have one levels are not and the hum must be in current flow, whether this change disadvantage, however. Because the suppressed as much as possible. he an increase or a decrease. speaker field is being energised by High tension (HT) filtering can he High tension filter chokes come in only partially filtered current, a small achieved in several ways and usually two physical forms. Either the field amount of hum can be generated in involves either chokes (inductors) or coil of an electrodynamic loudspeaker the speaker itself. This was overcome resistors, and electrolytic capacitors. can be used or it can he a separate unit by fitting the speaker with a "hum bucking coil" in series with the voice coil. Jt 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 roil 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 battery- operated sets. That situation changed after about 1948 when much more powerful magnets became available — as a result of wartime research — and permanent magnet (permag) speakers were sub- sequently used in all types of receivers. Because a field coil was no longer used, these receivers required a separate high tension filter choke. This photo shows the field coil (inside metal housing) of an electrodynamic Whether the filter choke is a speaker loudspeaker from the mid-1930s. The field coil played a dual role: (1) it was field coil or a separate unit, its func- used as an electromagnet for the loudspeaker; & (2) it was used as a high tion is much the same; it opposes any tension filter choke. change in the current flowing through 56 SILICON Cull' 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. Signed into a piece of equipment rather than added as an afterthought. , ,Capacitor values The size of the :electrolytics :also has an effect on the effectiveness of Either a field coil, a choke or a resistor form the central component of most high the filter. Hum can often be reduced tension filters. In conjunction with high voltage electrolytic capacitors, they simply by installing larger capacitors, provided adequate IIT filtering for most valve radio receivers. particularly on the output side. A larger output capacitor also gives bet- it. Effective though it is in this role, it ter regulation. is not sufficient by itself. But when However, if one cares to check the large electrolytic capacitors are con- valve specification manuals, recom- nected from each end to ground, the mended maximum capacitor values result is a smooth DC current. FIELD COIL OR CHOKE for the input side of the filter are Capacitors have the ability to store FILTERED usually listed for various rectifiers. It an electrical charge and this is their DC is inadvisable to fit larger than recom- ELECTROLYTIC main role in a high tension filter. By CAPACITORS _I mended capacitors in this position. taking on a charge when the rectifier I The objection is the excessive peak voltage rises and giving up that charge Fig.1: a typical HT power supply for a current that these will draw through when the voltage drops, capacitors valve radio receiver. In some circuits, the rectifier. Large input capacitors supplement the filter choke constant- a resistor is used instead of a choke or should not normally be necessary for current action by tending to maintain loudspeaker field coil, HT supplies hum free results. a constant voltage. are a common source of trouble. If larger input capacitors do need to In some applications, where heavier be fitted, limiting resistors should be DC currents are required, a second valve type are concerned, this extra added in series with the rectifier plates filter choke and an additional electro- filtering stage is unnecessary. to protect the valve. The values of lytic capacitor can be added to pro- It must be remembered that an ad- these resistors are usually listed in duce an even smoother supply. As far ditional choke will also lower the out- valve characteristics manuals. as domestic radios of the four or five- put voltage and thus needs to be de- If a filter system .incorporates a . JuNE 1993 57 directly from the input side of the filter. Since there is no amplification following this stage, the hum re- mained within acceptable limits. Only the current for the front end of the receiver passes through the re- mainder 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 These 20W wirewound resistors can dissipate quite a lot of heat and should be resistors connected in parallel, in or- mounted well away from any heat-sensitive components. Note that the resistor der to provide an adequate wattage on the left is adjustable. rating. Typically, the resistance varies from 1.51d2 to 101(S1 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. This receiver uses three parallel 1W resistors in its high tension filter. They are used in conjunction with two 240' electrolytic capacitors to provide a well- Field coil substitution filtered HT rail for the valve anodes. Using a resistor type filter system as a substitute for a choke system speaker field coil, 8uF electrolytic fil- the choke itself and results in a burnt- becomes a tempting proposition when ter capacitors would typically be used. out winding. a vintage radio repairer is faced with a By the 1950s, radio manufacture serious loudspeaker problem. An open Troubleshooting had become very competitive and field coil is not the only thing that can A distinct hum in an old receiver is more and more receivers where being go wrong with an old speaker, how- very often the result of electrolytics made to a price rather than to specifi- ever. The speaker cone can be out of losing their capacitance. Replace- cations. As a result of this cost cut- shape, split or completely in tatters. ments will usually solve the problem. ting, the overall number of parts in In such instances, the easiest way Another cause of hum could be the many receivers was reduced to a hare out is to fit a "permag" speaker. When bypass cap.,:itors across either of the minimum. One of the components doing so, a suitable substitute for the grid bias (cathode) resistors. found to be dispensable, by careful field coil/choke must be made and a Electrolytics with electrical leak- circuit design, was the high tension resistor may he the logical way to go. age problems are also a matter for filter choke. Better still, a combination of choke concern, as they can have two effects The choke was replaced with a re- and resistor can be used. provided on a high tension filter.

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