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Rmionic Valves Ubes By MAURIE FINDLAY, MIEAust., VK2PW A10 about rmionic valves as told by a veteran (or yaw ubes) radio-- designer duced using the latest technology available and in versions equivalent to ordinary valves. Most of them, if not abused, will meet their original specifications after 60 years. Not all people restoring early radios will be trained technicians or engineers. They may be able to do a great job of polishing the cabinet, replacing the dial cord and even repairing the speaker cone. However, when switched on, the set just doesn't per- form as well as it should. Maybe the maximum volume is limited or it will pull in only strong local stations. If so, does the set need realignment or does it have a faulty valve? Or could something else be wrong? Valves are a common reason for Valves reigned supreme in the electronic poor performance in old radios and world for more than 40 years, being the this article will answer some of the questions that are commonly raised by essential components of radio receivers, people restoring vintage sets. transmitters, early computers and many But first a word of warning. Most enthusiasts will be aware of the ba- other devices. When television first came sic safety issues for radios operating to Australia, it was black & white and the from the 230VAC power mains. If you are not confident about dealing with sets used valves. It has now been 50 years mains-operated equipment, then leave well alone. Even if you are capable and since the end of the valve era. know what you are doing, be careful about doing repairs for acquaintances. There could be legal implications if TH AN EYE to preserving our radio communication depended on something goes wrong. Wiistory, there are many enthu- valves. It wasn't much use sending We'll assume here that we are deal- siasts who restore early equipment, up a squadron of fighter planes if they ing only with sets that have a mains mostly radio receivers from the 1930s, couldn't talk to each other and coor- transformer. If you have such a set, it 40s and 50s. Most popular sets used dinate the operation. Initially, valves should be fitted with a good-quality from four to six valves and many of were the least reliable parts in the 3-core power lead that's been properly them were well made and are well- equipment used until valve manufac- anchored and has a good earth connec- worth keeping as representative of the turers in several countries made them tion to chassis. technology of the era. much more rugged. Unless you a very experienced During the World War 2 (WW2), all These military valves were pro- and know exactly what you are do- 84 SILICON CHIP siliconchip.com.au Taken in February 1954, this picture shows "Radio & Hobbies" staff members Raymond Howe, Neville Williams, John Moyle (Editor) and Maurice Findlay on the roof on "The Sun" newspaper building in Elizabeth St, Sydney (there to watch the Queen proceed down nearby Macquarie St during her 1954 visit to Australia). Both Raymond Howe & John Moyle served in the RAAF during WW2, specialising in signals and radar. ing, don't touch transformerless (hot (automatic gain control) signal. screen grid. This improves the effi- chassis) AC/DC sets that have one side The next valve on the list is the ciency of the valve which is now called of the mains (Active or Neutral) con- "triode". It has an element called a a "pentode". Like the screen grid, the nected to chassis. They are absolute "grid" which is placed between the suppressor grid also usually takes the death traps for the unwary and should cathode and the plate. This grid usu- form of a helix but the turns are more be avoided. ally consists of a fine helix of wire widely spaced. which surrounds the cathode. A special case for power valves is What sort of valves are there? In operation, the grid is usually the "beam tetrode". It employs a spe- The simplest electronic valve type made slightly negative with respect cial construction technique that does is the diode. It has two elements — the to the cathode and, depending on away with the need for a suppressor. cathode and the plate (anode). When the voltage applied to it, controls the Pentodes are commonly used for both the plate is made positive with respect electron flow to the positive plate. In audio and RF amplification. to the cathode, electrons are attracted this way, it can be made to amplify. to it and a current flows. Conversely, As a result, triodes in radio receivers Special valves if the plate is negative with respect to are usually used to amplify audio sig- To make things more complicated, the cathode, no current flows. nals (ie, the audio is fed to grid of the there are a number of special valves Diode valves are used to rectify triode stage). However, triodes have that are frequently used in super- alternating current. The larger diodes problems operating at radio frequen- heterodyne receivers to convert the typically rectify the high-voltage AC cies (RF) because of the capacitance tuned RF signal to the intermediate secondary of the mains transformer, that exists between the plate and the frequency (IF). One such valve is the while the smaller diodes are used to grid (known as Miller Effect). "triode heptode" recover the audio modulation from ra- This problem can be overcome by In this type of valve, the triode dio frequency (RF) signals. More often placing another helix of wire around element forms part of an oscillator than not though, the latter will not be a the control grid, to screen it from the and it injects the oscillator signal single diode valve but will instead be plate. Valves with this feature are into a screened grid which is in the incorporated into other valve types. In known as "tetrodes" and are used in main electron stream to the plate. As fact, there will usually be two diodes simple circuits to amplify RF signals. a result, the intermediate (IF) signal in the envelope — one to recover the Another grid called the "suppres- appears at the plate and is then fed to audio and the other to derive the AGC sor" is often also placed around the the receiver's IF stages. siliconchip.com.au SEPTEMBER 2010 85 ry;, icAL nAiTERY VALIal TYPICAL 4 - C vALVLS 0 Trioses Te!rottrs Haii•wave Reclitiers Full-writ RtOPen Duo•Dkock•friotsk5 Tricor Twin Triode Tekluart Pintapro Conoirrrit4i. Di4P.Diotle•Tnutlit Duo-Drode•Pentode Co -1W le Fig.1: reproduced from the April 1949 issue of "Radio & Hobbies", this diagram shows the various valve types that were available. The valves designed for battery sets used directly-heated cathodes, while valves designed for use in mains- operated sets generally used indirectly-heated cathodes to eliminate hum problems. Most valve superhet receivers use emission and eventually making the aerosol contact cleaner may fix this a triode-heptode for the input stage. valve unserviceable. problem but if the control is worn, However, more elaborate receivers replacement is the only answer. may use a pentode RF amplifier stage Other parts Valve sockets and the pins of the before the frequency converter. This Now for a brief look at other com- valves themselves can also cause amplifies the tuned RF signal before it ponents. First, the electrolytic ca- problems if the radio has been stored is fed to the converter and so provides pacitors on the HT line in the power in damp conditions. Look carefully at better performance on weak signals. supply don't last for 50 years and if the general condition of all metal parts the originals are still there, they will —if they are corroded, this gives a good Cathode construction need replacing. You can often tell from indication of valve socket problems The cathodes for all the above valve their appearance that they have failed, Power transformers are generally types can take two different forms. For especially if they are leaking. reliable, even after many years. The battery operation, strands of wire are If there is any doubt, replace them primary winding can be checked used and valves with directly heated with modern capacitors with a voltage with a multimeter by measuring the cathodes suitable for operation from rating of, say, more than 400V. The re- resistance between the Active & Neu- both 1.5V and 2V were produced. placement values should be equal to or tral pins on the power plug with the By contrast, most of the valves only slightly higher than the originals. power off and the radio's on/off switch encountered by restorers in mains- In particular, note that substituting (if fitted) in the ON position. The pri- operated sets use indirectly-heated much larger value capacitors in the mary winding will typically have a DC cathodes. This type of cathode consists position immediately following the resistance of several hundred ohms. of a fine metal tube with the heater rectifier will invariably shorten the life If you have an insulation tester, wire inside and insulated from it. A of this valve, so don't do this. check that the primary isn't breaking big advantage of indirectly-heated Note also that the negative leads of down to the transformer frame. valve cathodes is that the heater can be these capacitors are sometimes con- operated from low-voltage AC without nected to positions other than to the Valve testers introducing hum. chassis. This means that you must During the heyday of valve radios, An exception in mains-operated check the lead connections carefully valve testers were readily available.
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