Trying to BIAS RunOffGroove English Channel VOX pedal

[paulrm]

I'm trying to set the bias on my English Channel FET based VOX pedal, it has 5 J201 FET transistors and the bias on the drain on q1,2,3 and 5 should be 4.5v. The q1 bias fine, but q2 voltage slowly cycles between 0 and 9 volts whenever I touch any part of the pedal or change q4 or 5. It makes it almost impossible, it's driving me mad. Any thoughts?

[antonis]

A circuit schematic should be helpfull...

Are ALL Fets biased from the same voltage source by the same trimpot (and/or resistor..) ..

[Kipper4]

Here you go

http://www.runoffgroove.com/englishchannel.html

[GibsonGM]

"To obtain 1.6v at the source of Q4, you may need to try different J201 or adjust the 4k7 resistor. During testing and development, values as high as 12k were necessary for some J201. Try to get close to 1.6v, but slightly lower values will have no audible difference."

If this doesn't work, maybe you have a wiring error?   Is this on breadboard, or did you build it already?

[GGBB]

Does adjusting the volume (or other controls) make any difference?

[duck_arse]

sounds like you have a fauly pot, either the trimm, or the volume. seeing as that stage biases through the volume pot, you could try wiring a 1M resistor from Q2 gate to ground. if this fixes, your pot or its wiring is not 'good'.

[PRR]

> q2 voltage slowly cycles between 0 and 9 volts

Wiring error. Maybe a bad connection through the Volume pot to ground (lets Q2 Gate float to "any" voltage, which throws Drain high/low). Maybe something sneakier.

[paulrm]

Thanks for the replies. I've been right through all of the components, checked everything. If I remove q1 I can bias it fine, when I put q1 back in, it starts to oscillate. I've checked the forward current for each of the J201 transistors, and they are .24, .33, .35, .43 and .43 mA. It seems to me the problem is with the transistors and that they need to be matched in some way. I'll get some from China and try swopping them out. It's taking ages, not sure it's worth it but we'll see.

[PRR]

> need to be matched in some way

No.

> remove q1 I can bias it fine, when I put q1 back in, it starts to oscillate

You got it figured. It is oscillating. Ground the input connection so it can't pick-up sneak signal radiated from the output section. You may even need to ground point "A", a handy place to kill AC/Audio signals while you work on the DC conditions. But when totally DC_happy, it will also be very "happy" to amplify audio so much that output sneaks back to input and it oscillates like a PA mike turned to 10. It will need careful layout in the box, and good segregation of output and input cables.

[paulrm]

Hurray all sorted. I made an audio probe with a .1uf cap clipped to a plectrum for stability, and worked through the source on each transistor, great fun and a great eye-opener. The probe is a must with things like this. The transistor was measuring .24mA  in my tester but also as an NPN, so some current must have been leaking through the gate. Got some new J201 transistors on ebay. The pedal is amazing once it works, extremely bright and responsive like a real amp. Thanks again for all the replies, I was going to bin it...