Small fires in jam jars

By the time I got interested in radio and electronics – when I was about 10 years old, in the early 1970s, most new radios and TVs weren’t using valves anymore; you’d still find old stuff in junk stores (or in the homes of older relatives) that were stuffed with valves, and TV sets still had them, but except in specialist applications valves were becoming increasingly rare in domestic electronics.

I remember reading an article in  a magazine that referred to valves as ‘small fires in jam-jars’ – and to be honest, if you took a look at a powered up and working valve radio you would see the warm orange or yellow glow from some of the valves – indeed suggesting that in the bottom of the glass tube was a small fire.  The amount of heat generate also gave that impression as well!

Of course, this wasn’t the case – the orange glow came not from a small fire but from a heated wire – but the phrase stuck with me,  and still makes me smile whenever I recollect it.

So, what bought about this trip down memory lane? The other day I purchased a small electronic amplifier module for a project off of eBay – the sort of thing that I could easily build myself but when I could buy it ready made for a couple of quid it seemed churlish not to. As for the project, it’s a ‘watch this space’ thing!  When I powered it up to test it I hooked the output up to a loudspeaker that I had screwed to a piece of plywood, and that’s when the trip down memory lane kicked in.

Like many people I’m very responsive to smells, and the smell of the electronics and the plywood took me back over 40 years to the garden shed where my dad had allowed me to set up some miscellaneous electronic bits and pieces, including a massive 12″ loudspeaker mounted in a plywood cabinet that was almost as tall as me.

The particular memory invoked was one of the earliest I have around electronics, and is actually a very strong memory. I was building a small amplifier to allow me to use the big loudspeaker to hear the output from a crystal set I’d built, and the design I was using for the amplifier was from an electronics kit I’d been bought at Christmas. A peculiarity of the design (caused by the number of components in the kit being limited) was that a particular electrical resistance in the amplifier was provided by a small electric light bulb.  I remember wiring things up, connecting the loudspeaker, then the battery, and being greeted by a hum in the loudspeaker. I was doing this on a late autumn evening – I think it was a Sunday – and in the twilight in the shed I was delighted to see the filament of the light-bulb glowing – my own ‘small fire in a jam jar’!

What was interesting was that once I managed to tune in a radio station on the crystal set, and wound up the volume on the amplifier as far as it would go, the brightness of the bulb filament would vary in sympathy with the music or voice coming form the loudspeaker.

I remember staying in the darkening shed, the last sunlight of the day coming in through the window, until my mother called me in.  There was the smell of the plywood, the slight frying smell of electronic components being pushed to the limit (and usually with my enthusiasm exceeding my design ability somewhat beyond), and that tiny flickering glow from the bulb.

I have other memories of building kit – some very vivid as well – but this one was an almost a religious experience in terms of the way it’s stuck with me.

Valve gear seems to be coming back in to vogue – you can buy kits for radios that use valves, and I’m sorely tempted to try one out.  I doubt that it will have the impact of that first experience with my non-valve ‘fire in a jam jar’ but I think it might be fun.




Leave a Reply

Your email address will not be published. Required fields are marked *