Building A One Valve Radio Receiver

by Robert A Wilson

Building A One-Valve Radio Receiver

            This manuscript is 14 pages in length, has 4,244 words, and 30 images, mainly colour photographs.   It contains full instructions for building the receiver, including how to make the aluminium chassis, and where to get the components and valve.

This small receiver is built to a conventional design.   I have chosen all the components because, although it is a valve receiver, they are still very easy to obtain.

            It is built round an EF91 pentode valve.   This type of valve was very common in television sets of the 1950s, and they are still around in great numbers.   Prices vary, but they can usually be obtained from online suppliers such as Ebay, at very low prices, thus making them an ideal valve to make a start at valve radio construction.   Equivalents are the Z77 and 6AM6 valves.

            The EF91 uses a B7G valve holder, and these are also available on online.   But it is worth remembering that they are generally referred to (quite incorrectly) as “valve bases!”     So when searching, try either “B7G valve holder” or “B7G valve base”.    

            I used a small variable capacitor for tuning, as they are still in current production.   It was a double type, with one set of fixed vanes having a value of 141.6pF and the other set 59.1pF.   By connecting the fixed vanes together, this gave a maximum value of 200.7pF.     A normal air-spaced tuning capacitor would have a value between about 400 and 500pF.   In order to get a better medium wave coverage, I introduced a wave-change switch on the front panel.   This switch, when made, adds a further 220pF to the total value of VC1, giving a maximum of 420.7pF.     This splits the coverage into two separate wavebands, both adjacent to each other.   This proved very satisfactory and both bands brought in a number of stations at good headphone strength.

            In years gone by, it was normal to use high impedance headphones for small battery receivers.   These are now hard to find, and can be quite expensive, so I used a small mains transformer to match the valve to a pair of standard low impedance headphones that are commonly used for small radio, tape and CD players.

            Tuning coils for small receivers don’t seem to be manufactured anymore, and winding your own can be tedious.   I have used standard RF chokes that can be obtained for a few pence from online suppliers.

            A lot of radio construction enthusiasts will already have most of the parts to build this little set, but they are readily obtainable if you are starting from scratch.

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