With an eye to preserving our history, there are many enthusiasts who restore early equipment, mostly radio receivers from the 1930s, 40s and 50s. Most popular sets used from four to six valves and many of them were well made and are well-worth keeping as representative of the technology of the era.
During the World War 2 (WW2), all radio communication depended on valves. It wasn’t much use sending up a squadron of fighter planes if they couldn’t talk to each other and coordinate the operation. Initially, valves were the least reliable parts in the equipment used until valve manufacturers in several countries made them much more rugged.
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.
These military valves were produced 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 perform 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 poor performance in old radios and this article will answer some of the questions that are commonly raised by people restoring vintage sets.
But first a word of warning. Most enthusiasts will be aware of the basic safety issues for radios operating from the 230VAC power mains. If you are not confident about dealing with mains-operated equipment, then leave well alone. Even if you are capable and know what you are doing, be careful about doing repairs for acquaintances. There could be legal implications if something goes wrong.
We’ll assume here that we are dealing only with sets that have a mains transformer. If you have such a set, it should be fitted with a good-quality 3-core power lead that’s been properly anchored and has a good earth connection to chassis.
Unless you a very experienced and know exactly what you are doing, don’t touch transformerless (hot chassis) AC/DC sets that have one side of the mains (Active or Neutral) connected to chassis. They are absolute death tra ps for the unwary and should be avoided.
What sort of valves are there?
The simplest electronic valve type is the diode. It has two elements – the cathode and the plate (anode). When the plate is made positive with respect to the cathode, electrons are attracted to it and a current flows. Conversely, if the plate is negative with respect to the cathode, no current flows.
Diode valves are used to rectify alternating current. The larger diodes typically rectify the high-voltage AC secondary of the mains transformer, while the smaller diodes are used to recover the audio modulation from radio frequency (RF) signals. More often than not though, the latter will not be a single diode valve but will instead be incorporated into other valve types. In fact, there will usually be two diodes in the envelope – one to recover the audio and the other to derive the AGC (automatic gain control) signal.
The next valve on the list is the “triode”. It has an element called a “grid” which is placed between the cathode and the plate. This grid usually consists of a fine helix of wire which surrounds the cathode.
In operation, the grid is usually made slightly negative with respect to the cathode and, depending on the voltage applied to it, controls the electron flow to the positive plate. In this way, it can be made to amplify.
As a result, triodes in radio receivers are usually used to amplify audio signals (ie, the audio is fed to grid of the triode stage). However, triodes have problems operating at radio frequencies (RF) because of the capacitance that exists between the plate and the grid (known as Miller Effect).
This problem can be overcome by placing another helix of wire around the control grid, to screen it from the plate. Valves with this feature are known as “tetrodes” and are used in simple circuits to amplify RF signals.