Vacuum Tube (Valve) Radio and Audio Repair Guide: Repairing battery operated receivers

Conventional domestic and portable types

The first thing the reader may well ask is how does one go about obtaining the necessary HT, GB and LT voltages now that the batteries used have long since ceased to be produced?

Taking HT batteries first, there are several ways around this, one being to make up your own from the small ‘PP3’ type 9 V units connected in series. Obviously ten PP3s are needed for 90V while fourteen will make 126V, suitable for use in sets using the nominal 120 V HT battery. Intermediate voltage tappings in multiples of 9 V may be made as and when required. With intermittent use PP3s will last a considerable time, but if it is anticipated that a set will be used for long periods at a time it would be better to employ an HT ‘eliminator’. These were made from the late 1920s up until the 1950s and are to be found at most vintage radio auctions and other events for reasonable sums.

Beware: make sure that you obtain one intended for AC mains operation, as they were made for DC mains as well. If nothing else the weight should give the game away as AC types contain a mains transformer, rectifier and largish smoothing capacitors, whilst the DC versions have only a few resistors and some moderately sized capacitors. Most eliminators gave just three or four different HT voltages but the better types included an LT source for ‘trickle charging’ an accumulator overnight when the set was out of use. Note that these charging devices are not suitable for running tubes (valves) filaments (but see later!).

It may not be widely realised that an HT battery in good condition has low internal resistance whilst an HT eliminator in good condition has a high internal resistance. The low internal resistance of the battery serves as effective RF and AF decoupling and when an eliminator is used in its place this is lost and instability may result. The effect will vary from receiver to receiver and from eliminator to eliminator but if it should occur it will be worth while trying a 2 uF or 4 uF capacitor from HT+ to chassis and/or a 25 uF from HT— to chassis

GB batteries, which almost always means 9V tapped at every 1.5V, may be made up by wiring six 1.5V cells in series. An alternative method is to use another PP3 in conjunction with a chain of six 15 k-Ohm resistors wired across it to provide the intermediate tappings. The drain on the battery will be only about 100 uA, which means it will last for a very long time.

On the LT side it is not really practical to consider using dry batteries as the current consumption of even a small receiver would run them down quite quickly. It is far better to use the 2 V lead-acid cells that are still available under the generic name of ‘Cyclon’ cells, in various sizes that should suit almost any receiver, large or small.

In the late 1940s and early 1950s the then well- known firm of Amplion made some special eliminators under the name ‘Convette’ which delivered both HT and LT. To cater for different LT loadings a variable output control was provided along with a small voltmeter to check that it was correct. These Convettes turn up now and again at auctions and other events and are well worth buying. Do not confuse them with another type of Convette which may appear at first glance to deliver LT but which in fact is an HT vibrator pack which needs to be supplied with 2 V from an accumulator. These units can be useful for supplying HT from an LT source but be warned that the consumption from the accumulator is fairly heavy and that they emit a very audible hum whilst working.

The LT voltage must be 2 V within 0.2 V. On no account should the upper limit of 2.2 V (that of a fully charged 2 V accumulator) be exceeded, even for short periods. Once battery tubes (valves) filaments have been subjected to persistent over-running their efficiency and life is reduced sharply. On the other hand, they will not work properly below 1.8V or perhaps a little higher if they have been abused in the past.

Fault finding

In most respects the same techniques apply as for mains operated receivers and the procedures given for these may be used save, of course, that the section on power supplies may be ignored. Servicing data is available for commercially made sets from about 1930 onwards, but for early receivers, especially home-built types, you will need to work things out for yourself. Fortunately most examples were fairly simple TRFs which departed only in detail from the basic receivers shown earlier in this guide. It is good practice to sit down and trace out the circuitry of old sets and thus see the technical features discussed in the guide ‘brought to life’. As regards components, it is surprising how many ‘new’ 70- and 75-year-old items are still in circulation and may be obtained at vintage radio events or from specialist dealers. The same applies to tubes (valves), and it is only the really old ‘pip top’ types that are likely to be very expensive, although the $70 that one may cost nowadays is still cheaper in real terms than it was new at 18/6d back in 1923 for someone earning $4 per week!

‘All-dry’ and mains-battery sets

Generally speaking the maximum HT used in the sets was 90 V in the standard size receivers and 67.5 V in the ‘personal’ sets, although some American sets did work with as little at 45 V. As before, PP3 batteries in series may be used to provide replacements, ten for 90 V while eight will give 72 V, quite suitable to replace a 67.5 V battery. The filaments of ‘all-dry’ tubes (valves) were, of course, intended to be powered by batteries and a couple of high-power ‘C’ cells wired in parallel will be effective. Grid bias batteries were not used in these receivers. Amplion produced ‘Convettes’ especially for ‘all-dry’ portables and personals, in sizes that matched the original batteries. Please note, though, that the types intended to give an LT output to suit the standard 250 mA of the original octal and B7G types should not be used with 25 mA filament tubes (valves).

A third alternative which may well become popular in the future is a small ‘inverter’ capable of delivering both LT and HT from a miniature rechargeable battery. These have been developed for military applications, for tubes (valves) still have their place in the armed forces since they can survive nuclear radiation that would destroy transistorized equipment. It is hoped that these devices will come on to the civilian market in the near future.

As mentioned earlier, the circuitry of nearly all makes of popular ‘all-dry’ sets was very similar. In fact, in a number of cases two or more different brand names would employ a common chassis, made by such firms as British Radiophone or Plessey. As examples, the post-war Every-Ready model ‘C’ and the Cossor model 469 shared a Plessey chassis in which only one resistor, that for the bias in the HT negative, differed. Nearly all the ‘personal’ portables of between about 1946 and 1950 were fitted with an identical Plessey chassis. This commonality is certainly helpful to the repairer as generally speaking the same faults tended to appear on most brands. The same techniques are used as for any battery superhet, starting with a careful check on tubes (valves) electrode voltages. Experience shows that even at the low HT voltages to be found, small paper capacitors were just as vulnerable. Sets using the B7G diode pentodes employed very high values of screen-grid resistor, typically as much as 6.8 M-ohm with anode load resistors of up to 1.5 M-ohm. Service data of the day, based on the use of the AVO ‘Seven’, was apt to refer to the anode and screen voltage simply as ‘very low’ or even ‘not measurable’. An AVO ‘Eight’ will measure them reasonably successfully but even then only a few volts will be registered on each. The safest thing here is to measure the resistance of the two resistors in case either or both has gone very high or o/c. Don’t forget either the screen-grid decoupling capacitor or that coupling the anode to the grid of the output valve. With respect to the latter, it is surprising how much anode current one of those miniature tubes (valves) will draw with only a tiny positive voltage on its grid.