How Do You Power A Battery Tube Portable Radio?

Using a battery powered, tube portable radio, in the way it was supposed to be used and enjoyed, is no longer an easy thing to do in the twenty-first century. The dual-voltage 9v/90v and single-voltage 90v batteries went out of production in the late 1960's. Although zinc-carbon batteries are still available in 22.5, 45 and 67.5 volt types, they are very expensive and not a cost-effective solution to the problem of 'providing power'.

The mains-powered 'battery eliminator' has always been available both as a commercial product and as an easily-constructed design for the hobbyist. However, they tie the radio to the nearest power outlet and turn a portable radio into a non-portable one. Additionally, now that radio tubes are no longer in production, it makes good sense to provide close regulation of the low-voltage 'A' supply to the tube filaments so that their life is not brought to a premature end - something that older designs did not have.

Should the enthusiast wish to build a simple battery eliminator then I suggest that designs using a LM317 family of voltage regulators are used for the low-tension heater supply (the 'A' supply). Paul Stenning has a well-developed design (on his website) here for a system that offers regulated 1.5v and smoothed 90V. It is easy to modify the LT supply to give 7.5V or 9V and it uses widely available components. I have a home-built Stenning eliminator on my workbench and it is an well-used piece of kit. The whole thing can be built for about £25. Tony Thompson features another design on his website and this one uses a slightly different approach. Tony's transformer uses slightly odd windings but should be readily available.

But if we wish to run our portable radio as a true portable we must find another solution. This solution may be found in one of the following two approaches...

Batteries

Our radio is in need of two power supplies: a high tension 'B' supply for the circuitry and a low tension 'A' supply for the tube filaments ('valve heaters' in Brit.English). The 'A' battery is easy: simply combine modern 'D' or 'C' size alkaline cells in series for the required voltage. Even a single 'D' cell powering four low-current battery tubes will last for many hours. By using a combination of series and paralell batteries, many hours of use can be obtained within the size allowed by the radio case.

For the higher voltage 'B' supply, the simplest answer is to combine the required number of 9v batteries in series. This may be done by wiring the snap leads together or snapping the batteries one to the other. The problem here, though, is twofold. Firstly, a block of 10 batteries may not fit into a smaller radio and, secondly, even if the cheapest type of battery is purchased (usually from an 'Everything For A Dollar/Pound' type store) one will spend a significant sum of money for a limited battery life.

Despite this, it is an idea that can work well. A good sense of how well can be seen at www.batpack.com

An Alternative - The 'Electronic Battery'

In times past, there was an electromechanical device - called a 'vibrator' - that was capable of producing the high-voltage power required from a comparatively lower-power input. Usually, these devices were used in vehicle-powered equipment, most notably by the military, and although they were not terribly efficient they were fine when used with high-current supplies, such as those found in a car or truck.

Fortunately, there are now modern electronic equivalents of the vibrator that may be used with smaller, more common (and cheaper) cells such as the ubiquitous 'AA' or 'AAA'. Although these devices can be initially quite expensive, they pay for themselves over time by reduced battery replacement costs. Additionally, they may be more compact.

So far, I know of three versions of the 'electronic battery' that may be used for most portables.

In Germany, Hans Borngraber has developed an transformerless design that produces 60-90V at 11mA using a 6v battery. A translated version of his article has been mirrored HERE

In Australia, Tony Maher, is an electronics engineer and vintage wireless enthusiast.He has contributed articles to 'Radio Waves', the journal of the Historic Radio Society of Australia, and has designed two types of 'electronic battery'. We will concern ourself with his design for the 'B' battery - and Tony's original article describing this unit (with parts list and schematic) can be seen by clicking HERE. Tony is able to supply the PCB and components for this unit, but the constructor will have to source their own case and battery holders.

Tony's design is a very compact power source that can produce 67 or 90 volts depending on the number of supply cells used. The whole unit is contained on a screen-printed PCB that only requires external connection to the supply cells. Although the unit produces an audible whine when in operation, there is no RF noise.

The Kit Radio Company 'KRC-A-2' was featured in 'Practical Wireless' magazine. It is a modern replacement for the British Ever Ready B126 90V battery and is supplied in kit form. It contains absolutely everything needed to build an 'electronic battery' that produces 85V from six 'AAA' cells. It can also be supplied ready-built but this is a little more expensive.

The KRC-A-2 is constructed using stripboard ('Veroboard' in the UK) and the board is supplied with a printed paper overlay stuck on to show the position of the components. I must admit that this struck me as being somewhat 'amateur' - I was transported back to my teenage years at the instruction to to 'cut the tracks with a drill bit' - and I would have expected a screen-printed PCB. However, the unit is designed to replace a specific battery and so compactness for its own sake is less of an issue.

Similarly, there is no option to vary the output voltage by adding or subtracting supply cells. Although this isn't strictly necessary I believe that the 85V output may be a touch low for weak or marginal tubes.

In operation, the KRC-A-2 is completely silent and there was little RF noise across the standard medium waveband (the 'broadcast band'). The kit is supplied with a flattened case of heavy cardboard, printed with the original Ever Ready artwork - this is a nice touch and a good finish to a solid product.

The kit may be purchased from: Tony Westbrook, Kit Radio Company, Unit 11, Marlborough Court, Westerham, Kent TN16 1EU Great Britain. Tony may be reached at (44) 01959 563023 and KRC's website is at http://hometown.aol.co.uk/kitradioco/uk.htm

To summarise: If you need 'authenticity' or a unit to power your 'shelf queens' then the KRC-A-2 is the best unit for the job - if it is appropriate. If you want a unit with a little more flexibility, then speak to Tony Maher about one of his kits. That's my opnion but Your Mileage May Vary...good luck!