VALVES, in common with other components in a receiving set, wear out with continuous use, and it is found that a marked improvement in the quality of reception is obtained if valves are renewed after, say, two years’ continuous employment.
If a set is several years old, there may be some difficulty in obtaining a new valve of exactly the same type, number and manufacture as that already in the set. The original type may no longer be made. But that does not mean the end of the set’s usefulness. Manufacturers have superseded many of the old valves with others of new design, to secure better performance, and replacement of the old type by a new valve may also give better results for that reason alone.
Before purchasing a new valve it is necessary to know the type of set: battery, alternating current or direct current, all-mains. The valve usually has the manufacturer’s name, the type and number on the side or base. But, other details agreeing, it is not essential that the replacement be manufactured by the same firm.
Most valves have alternative equivalent made by one or other of the leading manufacturers; for example, should a power valve of the pentode type need replacing in an alternating current mains set, it could be obtained from at least four sources: Mazda AC/PEN; MullardPEN4VA;OsramM.P.T.4; Cossor MP/PEN. These may not have characteristics exactly equivalent but they are approximately similar. By characteristics is meant the figures that indicate the performance of the valve used during ideal conditions, under the headings of the filament volts, filament current, amplification factor, impedance, mutual conductance, anode volts, screen volts, grid bias, anode current and optimum load resistance. The figures for these items are usually given on a valve data sheet sold with the valve, or they can be found in manufacturers’ valve booklets.
One important feature of the valve is the type of base. It may be found that a valve needing replacement has an equivalent type in all the characteristics but has a different type of base, that is, more or less pins or a different fitting for the holder. At one time most valves had cither four or five pins, and in the case of pentodes and screen grid valves an additional cap connection on the top or side. Today, the valve-base may have from four to nine pins, with top and side connections as well. The type of base, therefore, is of prime importance as the valve, although similar in all other characteristics, will be useless if it will not fit into the valve-holder in the set.
If it is not possible to obtain a valve with a similar base, the only alternative is to replace the valve holder with a new one capable of fitting the replacement valve: this means the purchase of a new holder at the same time as the valve. An example of a change of this kind is when a 5-pin screen grid valve with a top connection has to be replaced and the only available equivalent is a simple type having a 7-pin base. When a suitable valve holder is obtained it can be put in the place of the holder already in the set.
This change presents a difficult job, but with the aid of a soldering iron, and due care exercised with the connections, the alterations can be made. Make a note of all wires as these are removed from the old holder, fit the new holder into position and connect the wires to the solder tag with the same indicating letter as on the old holder-, the top connection, if it be a grid connection, will then connect to the socket marked ‘grid’. The sockets may not be marked on the new type of holder, but these can be identified from the valve maker’s booklet.
A valve may give considerable trouble although not being at fault itself. As it fits into a holder by a number of pins, all push contacts, this is the only means of connection with the rest of the components in the set, except in the case of the cap connections, and often these also are push fits. It is possible for one of these contacts to be either dirty or loose, the valve in consequence failing to function.
If the pins are dirty they should be cleaned with fine emery paper. If the valve fits loosely in the holder, the set will proclaim the fact with crackling noises; the valve should be taken out and the split of the pin eased open with the tip of a knife . The holder should also be inspected, and if necessary cleaned. If the sockets are very loose these can be improved by pinching-in each with small pliers. Removal and replacement of a valve should be done by gripping the base firmly and gently rocking it until the valve pulls out. It should not be pulled by the glass, or this may break away from its base.
It is possible to have a complete valve test at a radio dealer’s. It is useful, however, to be in a position to be able to make various simple tests at home. They can be carried out with very little equipment, such as a voltmeter for reading direct current voltage, a milliammeter with three dial readings of o to 5, o to 50 and o to 150 milliamps for reading the current values, a split anode adapter as, and a battery. This equipment allows for a continuity test, which is a test to ascertain if the circuit is continuous throughout, and the measurement of the anode current to be found.
A filament continuity test is a test to see if the filament wire of the valve is intact, and is quite simple to undertake with the aid of the direct current voltmeter and battery connected. If a battery valve is being tested, the battery in circuit should be 2 volts. If it is an A.C. Valve, a 4-volt battery must be inserted. Should the filament of the valve be intact, the circuit will be closed and a reading shown on the voltmeter.
The anode current can be measured by using the split anode adapter and the milliammeter. The split anode adapter is in the form of a removable plug and is an easy means of making a break in the anode wiring, all the pins being continuous except the anode pin, which is broken inside the adapter and the ends brought out to two terminals for connection to the milliammeter.
With the adapter fixed in the valve socket in the wireless set, the valve is placed in the top and the milliammeter connected to the terminals of the adapter, and the set then switched on. A reading will be indicated on the meter; if the valve is working in the correct manner the reading should compare favourably with that shown on the manufacturer’s’ valve data for the particular valve under test. If the reading indicated is lower than the manufacturer’s figures (obtained from the chart included in the valve carton) something is at fault; the voltages, both and grid bias, must be checked, and if these are found to be correct then the trouble lies with the valve and the latter must be replaced.
Another trouble sometimes experienced is microphony; this is a condition caused by the electrode, I.e., the filament, grid and plates of the valve not being quite rigid and vibrating in sympathy with certain loud passages of music. It is found in the type of receiver that has the loudspeaker mounted directly over the valves. There is no absolute remedy for this, but temporary measures can be taken to improve matters until such time QS the valve can be replaced.
To ascertain which valve is responsible for this trouble, switch on the receiver and gently tap each valve ‘in turn. When the faulty one is struck it will ring loudly; in some cases this noise will be amplified a hundred times. These conditions must not be allowed to continue, or serious trouble may be caused to the other valves and the receiver itself.
Conditions may be improved by fitting an anti-microphonic valve holder in place of the existing standard type, or by winding a rubber ring around the bulb of the valve. In some instances the placing of a jacket of cotton wool around the valve will relieve the trouble. If the set has another valve which is exactly similar to the microphonic valve, improvement will result if the positions of these two are changed.