Transistor Radio Power Supply Arrangements

With the valve radio discussion, power supplies are a major section. With transistor radios things are much simpler. The vast majority of transistor radios within the era of interest operate from batteries.


The most common battery voltage is 9V. This probably came about because it was sufficient to allow the types of audio output circuit and transistors available to deliver a few hundred milliwatts to the speaker, giving a set that is loud enough to make itself heard.

Some higher quality sets such as some Hacker, Dynatron and Roberts models operate from 18V (two 9V batteries in series). The higher voltage allows more power (two or three watts) to be delivered to the speaker. Since bass uses a lot of power, the bass response of these sets can be improved. The connection between the two batteries can also be used to simplify and improve the output stage, as we will see later.

Some sets used 6V batteries. In some cases these have the same press-stud connectors as the more common 9V types so care needs to be taken not to connect a 9V battery to a 6V set.

Some far-eastern pocket radios run from just 3V, which was actually quite an achievement at the time. They use two AA batteries in a small plastic holder. The holder is sometimes connected using the same type of press-stud connector as a PP3 9V battery, so again care needs to be taken. A good clue is to look at the voltage ratings of the electrolytic capacitors - 3V sets were generally fitted with capacitors rated at no more than 6V.

A few early sets ran from odd voltages. The Regency TR1 (the first transistor set released) for example uses a 22.5V battery.

PP9 and PP7 9V batteries can be obtained, but you will generally need to order them from a component supplier rather than buying then retail (although some branches of Halfords apparently stock one or both types). The intermediate sized 9V batteries are no longer made, but an alkaline PP3 will generally do the job, possibly with an adaptor lead and some foam to stop it rattling.

The 6V radio batteries are no longer available. The easiest solution here is to use four AA batteries in a suitable holder.

Car power supplies

Car radios operate from 12V, because there isn't any choice in the matter. Because plenty of current is available from the battery, the output stages are designed to deliver a respectable amount of power rather than to use minimum current.

This circuit shows the power connections and output stage of a typical car transistor radio. We will deal with the output stage later.

There are two complications with car power. Firstly it is quite noisy, with the ignition system pulling pulses of power and the alternator/dynamo applying a pulsed charging current. This is dealt with by filters in the set, in the leads to the set, and on the noise-producing items.

In this circuit the input power is filtered by L8, L7 and L6 together with C29 and C28. In addition, C27 is a large electrolytic capacitor connected across the power supply after the polarity selection.

Secondly, it could be either positive or negative earth. Modern cars all have the negative terminal of the battery connected to the chassis, so are negative earth. However, back in the 1960s and before, they could equally well have been positive earth. Indeed it is sometimes claimed that vehicles with positive earth arrangements are less prone to corrosion than negative-earth vehicles.

Since the metal case of the radio always connects to the vehicle earth, the manufacturers of the sets therefore have to allow for the power to be either polarity. This is normally achieved by means of a switch on a part of the case that cannot be disturbed when the set is installed, or by internal link wires that need to be unsoldered and resoldered in the correct positions. Some sets were made for one polarity only (especially those supplied already installed in vehicles). You will probably need the service data to sort this out.

In this circuit, two links are set depending on the polarity. If these links were connected incorrectly, considerable damage could be caused to the circuits. Hopefully a fuse would blow first, but this may not always happen particularly if the wrong size fuse has been fitted.

The dial lamp is connected before the links since it will work with either polarity and the holder is attached to the metalwork.

William White emailed me to remind me that many cars in the 50s/60s used 6V electrics so some sets might be made for 6V use or have some kind of sellectable input.

Mains supplies

A few sets run from mains power supplies. These are generally very straightforward, consisting of nothing more than a transformer, rectifier and smoothing capacitor. The two usual circuit arrangements are shown below. The occasional one added some regulation, but this is so rare that it isn't worth worrying about here.

The small transformers sometimes fail with the primary going open-circuit. Suitable replacements should be readily available from component suppliers, although the mountings may need to be adapted slightly.

The switching between mains and battery is normally done on the low voltage side. If the mains lead is detachable, the switch is normally part of the socket and is actuated by inserting the mains lead connector.

Some sets have a DC input jack for use with an external power supply unit. Again the switching is usually part of the socket. These sockets are an open invitation for people to connect power supplies of the wrong voltage and/or polarity, causing major damage to the set.

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No part of this website may be reproduced in any form without prior written permission from Paul Stenning.
All details are believed to be accurate, but no liability can be accepted for any errors.
The types of equipment discussed on this website may contain high voltages and/or operate at high temperatures.
Appropriate precautions must always be taken to minimise the risk of accidents.

Last updated 14th April 2006.