Monday, 14 January 2013

ValveCaster Guitar Pedal

Recently, I saw a video on YouTube that shows various cool sound effects created by floppy disk drives. Seeing as I had many old drives lying around I decided to give it a go. However having got through 6 floppy drives with only a fairly poor effect to show for it I gave up. I had originally said I'd make it into a guitar pedal for one of my friends if it was any good, but seeing as it wasn't I asked if there were any other pedals they would like while I was in a pedal-making mood. They asked if I could do an 'old tube style' distortion, which I thought I probably could, so I started researching. I was going to make a solid state system that replicates the tones of a valve amplifier using the resistive properties of a light bulb, but then I realised it would much easier just to use a valve, and seeing as I wasn't paying for parts, buying a new one for £8 wasn't a problem.

It was the schematic above that made me decide on using valves ( found here ) as it ran off low voltages and was reportedly very good. Having read through most of the 155 forum pages that went with it, I tried it out on an old valve I found in the attic. The fact the part number had rubbed off made this harder, but it looked roughly like a double triode valve, and being made of glass it wasn't to hard to work out the pinout. So I wired it all up, and amazingly it worked first time! It sounded really good, with just a little of retro overdrive when pushed. So I ordered all the necessary parts and set about making the real thing. The pinout of the new valve (I ordered a 12AU7 as used in the diagram) wasn't the same as the old one, so had to re-route it all, but on firing this one up it also worked first time, although required a few tweaks to get it sounding nice.

Despite the retro sound, I still felt it needed a bit more of a kick. I set about working on a low gain, solid state preamp to drive the valves a bit harder to bring out the overdrive. The forums said that a Tillman preamp worked really well, so I ordered a few JFETs. Irritatingly though, RS no longer stocked them, so I started rooting round the attic again for more parts (it's like a retro electronics shop up there). I found a box containing a whole load of components from the 60's and 70's, including some germanium transistors. Having Googled the part numbers, I found that some of them were really quite rare, an one's like the Mullard OC81D sell for £10 each these days! I also found that they were very sought after for use in guitar pedals, so I thought I had to use one. Sadly, the OC81D is a PNP transistor which was not what I wanted, but a little more rooting bought up some old 60's aircraft logic boards using glass OC44s which were NPNs (and also quite sought after). Now as sacrilegious as it was nicking components off an old board like that, a few components had already been nicked off it many years ago by my father, and besides it wasn't the nicest one up there so I didn't feel too bad. The first one I tried worked, but it was producing far too much noise when no signal was going through it. So I switched it for another one (having carefully soldered the dodgy one back onto the board), and it all worked fine, with some very nice retro distortion of its own available if required. This is where I've got to so far, and I'm yet to do any proper testing on the overdrive with a guitar. Once its all checked, I'm planning to put it in an oak and brass case (pretentious? me?) with two pedals; one to activate the valve circuit, and one to patch in the 'Germanium Drive' :) .

Below is a video of an early test of the 12AU7 circuit using a fairly cheap guitar that another friend lent me. The 'overdrive' in the video is using a very high gain preamp I'd made for the floppy disk project, and it was far too sensitive. I only put it in to give a rough idea of overdrive.

And here's what it looks like at present:

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