Wampler Curcuits Not Working

[Pavlos]

I currently have the boards for the Wampler Tweed '57, Black '65 and Plexi Drive all completed, however none of them are working at all except for the Plexi Drive which is almost silent. All componet values are correct, placed correctly and solder joints are good with no shorts. So my suspicions are leading towards biasing the J-Fets which are all closely matched well within spec SMD's though they are reading low on the DMM.

So the question is, rather than changing every individual biasing resistor for each J-FET is it possible to change just one for each circuit? I'm sure I read somewhere that for the Plexi Drive it was the 10 ohm resitor at the +9v but am a little cautious of consequences for the rest of the circuit
Many thanks in advance

[induction]

Most jfet-based Wampler pedals (including all the ones you mentioned) are designed to work with very specifically spec'd J201's. If you want accurate clones, you have to use the stock biasing resistors and swap J201's until you find ones that bias correctly. Generally any J201 that biases correctly in one of those circuits will not bias correctly in one of the other circuits. Being within datasheet spec is no guarantee that a specific J201 will work in a particular Wampler circuit. Datasheet specs for jfets have stunningly large tolerances, and the spec bins for J201's in your Wampler circuits are very small and different from one circuit to the next.

You can adjust the biasing resistors if you like, but that will change the gain, sometimes very dramatically, and your pedal will not sound like the original. If you choose to rebias, which I obviously don't recommend, you will probably want to adjust the drain resistors until the drain sits near 4.5 V. You can find some published voltages for an original unit, which may be higher or lower, but since you're changing the gain, your pedal won't sound like the original anyway, even if you get the bias perfect.

Your best bet would be to buy 20 or more J201's from 4 or 5 different suppliers (for a total of 80-100 J201's) and swap them out until you find the right ones for a particular circuit. That's how Wampler does it. Yes, it's a huge waste of components, but you can always use the extras for other, less picky circuits, or you can sell them.