Vacuum Tube (Valve) Radio and Audio Repair Guide: Battery and mains/battery portable receivers

The portable receiver is very nearly as old as the domestic set, and as early as 1925 GEC was offering a superhet portable claimed to have a magnificent performance. At its price of 50 guineas, half that of the contemporary small Morris ‘Bull-nose’ motor car, it ought to have been good. By about 1927 portable design had settled down to a near standard of a three- or four- tubes (valves) TRF following the lines of domestic sets in every respect except for the ‘chassis’ — often made of plywood — and the use of a frame aerial. Two volt battery tubes (valves) were used with the LT supplied by a special type of accumulator with celluloid case and jellified electrolyte that was virtually unbreakable and unspillable. In most cases a standard size 90 V dry battery provided the HT. Two main types of cabinet design were used, the upright, with receiver section at the top and the loudspeaker and batteries at the bottom, with the frame aerial mounted around all, and the ‘suitcase’, with receiver and batteries in the lower part and the loudspeaker and frame aerial in the lid. Portable sets on these lines were still being produced ten years later, the only main difference being that they tended to be much smaller. Other than a few isolated examples, superhet portables did not become generally popular again until the late 1930s. As with domestic sets, the four- tubes (valves) arrangement of frequency changer, IF amplifier detector/AVC/AF amplifier and output was the norm.

Fault finding on the above follows exactly the same lines as for domestic battery sets, except for the fact that the frame aerial(s) have to be considered. These were normally of robust construction and unlikely to suffer from 0/cs, but in the ‘suitcase’ portable with the frame in the opening lid there are flexible interconnections with the receiver chassis which may fray with long usage. Another possible cause of trouble is corrosion due to acid fumes from the accumulator, which so often was in a confined space close to the chassis. Keep an especial eye on switch contacts and tubes (valves) pins and holders.

‘All-dry’ portables

In 1939 portable set design was revolutionized by the introduction of a new series of tubes (valves) specifically intended for superhets and having filaments rated at 1.4V and intended to be powered by dry batteries, hence their name of ‘all-dry’ tubes (valves). In this country the ranges produced by MOV and Mullard both comprised a heptode (pentagrid) frequency changer, a variable-mu RF pentode, a diode-triode and an output pentode, but whereas the MOV types had the international octal base, those from Mullard had the ‘P’ side contact base. These latter had only a limited production run and soon were replaced by types with octal bases. Another improvement to both the M-OV and Mullard types was the fitting of a tapped filament on the output tubes (valves) only. With the two halves in parallel the voltage and current rating was the same as that of the previous types, i.e. 1.4V, 0.1 A, but by running both halves in series they were able to operate at 2.8 V, 0.05 A, making it possible to run all four tubes (valves) in a series chain. This in turn made possible the introduction of true ‘universal’ receivers that would work on either batteries or AC/DC mains. These too started to arrive in Britain in 1939, having been pioneered in the USA. Before discussing these, however, we should first look at what became virtually a standard design of four- tubes (valves) superhet and lasted for at least ten years unchanged.

Coverage was invariably restricted to MW and LW, small frame aerials being provided for both bands. The heptode frequency changer was followed by a pentode IF amplifier with the single diode of the diode-triode used as detector and the source of AVC bias and its triode section as first AF amplifier resistance-capacity coupled to the output pentode. It was usual in battery-only sets for the filaments to be run in parallel with a total consumption of 0.25 A, giving an acceptable number of hours’ operation from an LT battery. HT consumption was usually about 10 mA from a 90 V battery but the tube (valve) would in fact give a reasonable performance at no more that half that voltage.

The way in which AC/DC mains operation was achieved was to use a conventional tubes (valves) rectifier such as a 25Z6 to give both the HT and LT voltages via suitable dropping resistors. The rectifier heater had to have its own dropper, usually in the form of a resistive line cord. In the original USA sets all this was easy enough to arrange from a mains voltage of only 110/120V but in Britain all the resistors had to be of rather inconveniently high values. For instance, the rectifier heater alone at 25V @ 0.3A required a line cord capable of dropping 200V, which at the usual 60fl per foot worked out to just over eleven feet in length. A wire-wound resistor both dropped the DC from the rectifier down to approximately 90 V for the HT and provided reasonably effective smoothing. A further resistor dropped another 83V or so to give a nominal 7.5 V for the tube (valve) filaments, operated in series. For this kind of work the design centre rating of each was no more than 1.3 V (2.6 V total for the output valve) to provide a safety margin against the possibility of voltage surges on the mains.

With some sets the changeover from battery to mains operation and vice versa had to be done by a manual switch, but others had automatic change over by means of a relay. In its normal position, at rest, the batteries were connected as in a normal receiver with only the usual on/off switch to be handled. When the mains plug was inserted, as soon as the rectifier began to give its full output the relay operated, switching in the mains HT and LT supplies and disconnecting the batteries. If the mains should fail or otherwise be disconnected the relay immediately changed the set back to battery operation without any interruption to the program being received. This was so impressive as far as owners were concerned that it is little wonder that one firm (Pilot) called its mains/ battery portable the ‘Twin Miracle’. Be warned that in this particular set the very unusual ploy is used of connecting the two strapped cathodes of the rectifier to the live mains input with negative HT being drawn from its anodes. This means that a lot of the HT and LT dropping and smoothing circuitry looks ‘back-to-front’ and the greatest care is necessary when faults are being traced. Fortunately this eccentricity was not emulated by other manufacturers who stuck to the conventional way of wiring the rectifier.

A close examination of the circuitry of most AC/DC/battery sets will show that various resistors are connected in parallel with the tube (valve) filaments. The purpose of these is to balance out the actual current flowing through each, because that of the HT as well as of the LT passes through them, and this is cumulative along the chain. If not protected by the shunt resistors the filament at the bottom end of the chain would be made to pass about 10 mA more than that at the top — not a great deal but sufficient to affect its long-term life.

When UK production of mains/battery sets resumed around 1946 small selenium rectifiers took over from tubes (valves) types, obviating the need for mains droppers of resistive line cords. For details of how these rectifiers may be replaced when they wear out, please refer to the main section on power supply repairs.

Miniature tubes (valves)

Around 1940 a new generation of ‘all-dry’ tubes (valves) appeared in the USA. Physically very small — about two inches long by three-quarters in diameter — they were of all-glass construction (later known as B7G in the UK) with the seven base pins also supporting the electrode structure. A number of different types appeared but of those which became best known the frequency changer, the RF pentode and the output pentode were electrically the same as the octal types just discussed. Taking over from the diode-triode was a diode-straight RF pentode which offered more gain when used as an AF amplifier. Had it not been for the Second World War these tubes (valves) undoubtedly would have appeared in the UK shortly afterwards, but as it was British set makers had to wait until 1945 before they could use them. Meanwhile they began to arrive in this country in small ‘personal’ portables that had become popular in the USA and were easily carried in the luggage of American servicemen posted over here. The circuitry of ‘personals’ differed very little from that of the standard size all-dry portables except for the extra resistor and capacitor used for the screen grid of the diode-pentode. Many of the components also were miniaturized, such as the tuning capacitor, IF transformers, loudspeaker, etc., while special layer type HT and LT batteries reappeared (they had first been made back in the late 1920s) in very compact form. The LT remained at 1.5 V whilst the most popular HT voltage was 67.5 V. Consumption was 250 mA and 8 mA approximately for the LT and HT, respectively.

Octal and B7G types were used side by side in conventional and personal portables until about 1950, when the octal began to disappear, leaving the B7Gs supreme. This state of affairs continued until around 1954, when tubes (valves) manufacturers introduced a new generation of B7Gs with low consumption filaments that demanded no more than 25 mA for the heptode. RF pentode and diode-pentode, and 50 mA for the output pentode. Although the HT consumption remained the same, the considerable saving in LT current gave much better battery life.

Both the original 5OmA types and the new 25 mA tube (valve) were used in mains/battery receivers including revised types suitable only for AC mains operation. In these the tube (valve) filaments were operated in parallel and supplied with LT by a small mains transformer and full-wave rectifier. Another winding on the transformer gave around 90 V for the HT via another rectifier. The advantage of the AC-only arrangement was that heat-generating and vulnerable dropping resistors were no longer required, and in addition the chassis could be isolated completely from the mains.