The CHAV EL84 Valve Amplifier was born out of a desire to re-live my early experience in electronics – starting in 1958 when I was 11 years old. Transistors were the latest ‘thing’ and first experiences with these proved disappointing due to both high cost and low reliability. (I could only afford ‘green spot’ and ‘red spot’ surplus types, which turned out to be fairly useless.) There were, however, hundreds of old (mostly very old) radios to be had for at absolutely no cost at all. Let me tell you. Where I lived in Gateshead, close by was an area of housing which was being slowly demolished in ’slum clearance’. The current inhabitants were being re-housed in new council-built homes out-of-town – with indoor plumbing and all (for the time) modern conveniences. Naturally, when they moved home, they left behind, amongst all the other unwanted rubbish, old radios, gramophones, etc., that either needed repair – or were deemed to be too old-fashioned for their new homes. As youngsters, my friends and I would hover – much like vultures – when we saw a removal van in the area, and hardly had it been filled and turned the corner on-route to the new home, we were into the vacated property to see what spoils awaited. There was always at least one old radio, which I would grab and take home, the others in our group would go for items that they could take down to the local scrapyard and exchange for cash.
Most of the radios dated from the 40’s, so that the valves were mostly octal-based and very sturdily built. Those radios I could fix – I did, good pocket money was to be had selling such items to old folk who still appreciated the beautifully-finished wooden cabinets and gloriously-mellow tones of these period pieces. When I couldn’t fix an item, it was duly reduced to it’s component parts, and my bedroom took on all the appearance of a scrapheap in miniature. My Mum refused to enter my bedroom which she regarded as downright hazardous, and to be honest that suited me as I was left to my own devices most of the time, and my parents had no idea of what I got up to.
My first guitar amplifier was made from a particularly monstrous example from the 40’s – a radio resembling a tombstone, which I converted and into which I plugged my home-made electric guitar. This abomination was made from an old spanish guitar furnished with a magnetic pick-up made from ‘Eclipse’ button magnets wound with cotton-covered wire liberated from an old radio coil. It looked – and sounded – terrible, but it was mine, cost me nothing, and I loved it.
Despite a long career in software development, I still enjoy switching on the soldering iron and getting stuck in to a hardware project. These days it’s mostly micro-controllers, but occasionally I hanker after the old days of chassis-bashing, followed by construction and being rewarded by the warm glow of a couple of valves. I especially miss the intense satisfaction of making something out of junk, and having it work – at hardly any cost at all. Sadly, today sees more and more use of Surface Mount Technology, the re-usability of which is questionable at best, and nearly impossible in practise. The ‘Chav’ amp represents an attempt to turn back the clock, using as far as possible, traditional late-fifties – early-sixties brick out-house technology with as minimal as possible outlay on new parts. However, although this Amp was built with some bits from my junk-pile – all components are still available. If you are nostalgic, or just want to build a little 3-watt amp that has sufficient gain to overdrive the output bottle, then have a go at building something like this!
In the circuit above, the first half of the ECC83 (V2A) is used as a voltage amplifier for the guitar signal which is then passed, via the volume control, to the grid of the output pentode EL84. The output transformer connections should be made such that the ideal output impedance of the EL84 is matched (i.e. around 5KΩ - Ignore the connection numbers on the schematic above, and refer to the datasheet of the transformer you are fitting). Note that the cathodes of the two halves of the ECC83 share a common self-bias resistor and by-pass capacitor. The second half of the ECC83 (V2B) is configured as a low-frequency phase-shift oscillator, whose speed can be varied using R9. Because this oscillator shares bias resistor with the amplifier stage, the gain of V2A is varied slightly at the speed of the oscillator – giving amplitude-modulation of the amplifiers output – or ‘tremelo’ as it was known. Note that there is no negative feedback – a large part of the ’sound’ of the sixties is the non-linear amplification used in the guitar amps of the period.
I won’t give construction notes other than this warning. If you are used to low-voltage projects only BEWARE that the voltages present in valve-based equipment ARE LETHAL – you may DIE, or at least get a nasty shock and/or burn if you are careless during testing. Be especially aware of residual charges on HT decoupling capacitors. (such as C6 and C9) These capacitors may retain high voltage charges long after you switch off the supply. Discharge the capacitors by applying a 100K resistor from the HT terminal to GND – DON’T short out the capacitor with a screwdriver!!! Oh, and ALWAYS disconnect the equipment from the mains supply before working on it. Please forgive the red text here & I’m sorry if I’m being a bit of a bore – but believe me, my boring you to death will be a lot less nasty, and take a lot longer, than a serious electric shock will.
Note that I’ve enclosed both the output and mains transformers in aluminium covers, mainly because the units I used had exposed terminals, but also I believe it improves both on interference, hum and noise. You can tell from the picture that the ‘tremelo’ was an added extra, making use of the spare half of the ECC83, so I added on the small sub-chassis with the tremelo socket and controls after the main chassis was built. The grille above the valves both deters the curious from getting their fingers burned, and protects the valves in the event of dropping a book etc., on them. The grille supports are made from re-cycled nylon pole. The front ‘panels’ are simply printed card which has been laminated and stuck to the chassis. Oh yes, the chassis and Tx covers etc is aluminium, bent to shape using my metal-bending gear. The holes for the valves were cut out using a venerable Q-max cutter. (another blast from the past) Note I brought out the tremelo switch as a separate foot-controlled item, connected via socket and the red cable on the right.
Although I’m wary of using ‘old’ capacitors, a vintage WIMA capacitor (New ‘Old’ Stock) can be seen underneath the volume control, and a 60’s 5watt 100K resistor at the top of the picture. The tag-strips are also from the sixties. Despite the apparent ‘untidiness’, both hum and noise levels are very low, and the amplifier is very stable.
The styling of the cabinet is a bit of an anachronistic joke. The ’sunrise’ 30’s-type grille I made from an old piece of thin plywood, cut out using a 1/4” router, then finished with gold paint. The ’sun’ in the bottom-left corner is a riveted-on ¼ circle of aluminium similarly painted. The cabinet is covered in a ‘Burberry’-type cloth I bought as a remnant from John Lewis for £1.20 – the cloth is favoured in a certain style of dress known as ‘Charver’ here in the North-East of England and as ‘Chav’ in the South.
The plastic corners, handles and feet used on the cabinet are all available from ESR
|You will need two transformers, two B9A valve-holders, and both an ECC83 & EL84|
Links to Valve (Tube) & Transformer data: