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Post by JohnH on Nov 24, 2006 15:45:52 GMT -5
Last weekend I put a ‘Fetzer valve’ circuit into my Strat copy (with Tonemonster2 wiring). For this guitar, I have fairly low-output GFS vintage single coils, and I wanted to try a preamp with some gain. The Fetzer valve is a carefully designed single JFET stage which is described on Runoffgroove in great detail. It is set up to emulate the characteristics of a Fender tube input stage, using a solid state device. Too good to be true? I wanted to try it. Here’s the link to the design: runoffgroove.com/fetzervalve.htmlThe designers go into great detail of how exactly to bias and set up a JFET, to match the tube harmonic response, based on measuring the characteristics of the individual JFET. As an engineer, it appealed to my inner nerdishness. I was lucky to find a J201 with a relatively low cut off voltage which leads to higher gain and lower current draw in this case. Once I had worked out the components, I had a 27k drain resistor and a 1.8k source resistor, with a current draw of 0.12mA which is not bad. It gives me about x8 gain. I built it on a small scrap of veroboard and wired it in. The input was after the guitar volume, then it has its own 100k pot on the JFET output as a master volume. I was hoping that the pickups would start to drive the JFET into saturation – and it is just about getting there , with my all-in-series pup setting. A slight burr to the sound is added. Nice but I’d like a bit more. However it is very good at overdriving the amp. Turning up the output on the guitar gives me punchier, brighter overdrive of my DSL combo clean channel than does turning up the amp gain. It's interesting, and it seems to work well. I might try a switchable source bypass cap to get more gain. John
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Post by ccoleman on Nov 25, 2006 15:35:20 GMT -5
AWESOME John !! Could you post some mp3 examples ??
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