RunOffGroove Fetzer Valve Revisited. This circuit aims to emulate 12AX7 triode stage in Fender amps. It uses a J201 N channel J-FET. Carefully biased, it works pretty well, you get a neat, loud and full of harmonics sound, with a lot of gain (depends on the J201, it's better to sort and measure them with the method described on RunOffGroove.com). I've added 2 resistors nets to avoid trimmers (I don't like trimmers). Each net let you use 1 resistors, 2 resistors in parallel or 2 serial resistors. You can get a wide range of values with only a small set of resistors.
What you get is an excellent boost, not as clean as a micro-amp for instance, but sharp, tidy and with full of harmonics you've probably never heard from your guitar or your amp. I really brings life out the amps, even the cheapest or worst. Mine is always on, I cannot imagine a gig without it.
The article is there : http://www.runoffgroove.com/fetzervalve.html A must read if you're involved in diy guitar effects.
How to use. Get one J201 transistor and measure it using this method : http://www.runoffgroove.com/fetzervalve.html#11
As an example, I measured Idss=0,34 mA and Vp=-0,621 V
Use ROG calculator ( http://www.runoffgroove.com/fetzervalve.html#12 ) and you'll get resulting values : Av = 16,6 dB, Rs= 1516 ohms and Rd = 20536 ohms.
We can expect a gain of 16,6 dB, the circuit will clip (nicely) at 0,621 V and we must have 1,516 K resistor at the source and 20,536 K resistor at the drain.
Looking in my stock, I found a 1513 ohms resistor for the source, almost perfect, I soldered it like this :
For the drain resistor, I was less lucky and didn't find 20536 ohms resistor.
This is why the PCB offers you 2 more chances. 20536 ohms can be 2 resistors in serie like 510 ohms + 20K. I found a super precise 20K in my stock and a 523 ohms resistor (marked 510) also. So I soldered them this way :
This is the way to use 2 resistors in serie on this PCB (appliable to source resistor also).
The other way to obtain 20536 ohms was to use 2 parallel resistors. For instance, 22K and 300K in parallel give you 20497 ohms, not bad at all.
In order to use resistors in parallel, you must solder them like this :
these methods apply to the source resistor also.
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