freestompboxes.org

Still no joy on this. I ended up making a couple of these on veroboard, the first with my matched FETs and drain resistors, the second with trimmers. Both squeal (oscillate) when I use the boost setting and max volume/gain.
Its easy and interesting to get the trimmers set right, there is an audible sweet spot with good gain. With only three stages I can get a great clean sound that gets crunchy as the gain goes up. But getting that first stage to to sound good without driving everything into feedback, no go. I have taken both out of the boxes, separated input and output wires from pots etc without helping things.
Trying to "compromise" and dial the other stages away from their best spots sort of works,but then you are clipping and can't get a good clean sound.

And 4 volts probably is a good quiescent point: the sweet spot is not so narrow that it has to be 4.5v exactly. 4 or 5 volts gives plenty of room to swing.

It's the footswitch. All that tweaking and searching, and:
taking the out wire from the volume and the out jack wire and touching them together = no oscillation
touching the two wires to their terminals on the switch = oscillation. Just bringing them near the thing is enough.

I even tried an alternate wiring for the switch where the LED uses the middle pole, thinking this might eliminate the problem. No go. New switch, no help.
Better 3pdt?

I've never had that problem with any of the 3PDT I use, but you could always have a dodgy batch I suppose. Where did you get them from?

It looks like this one is out there on vero, so is it safe to call the schematic verified? Looks like it is, but there doesn't seem to be a lot of action on this one.

You need to keep in mind that we have here a fine example of 3 cascaded gain stages working at rail-tail outputs, as opposed to, say, standard clipping circuits where the amplitude is cut to ~0.5V by the diodes. Here the signal remains at ~8V peak to peak which introduces much more signal into the feedback path, even though we "feel" less of a distorted signal when playing through the thing (which might cause some disappointment since the box squeels but can't give you enough sustain). This thing is better left to do "its" thing.. rather that be used as a high gain pedal. When I built mine - I left out the optional 1st stage as I felt I already had enough drive pedals and what I wanted from this one was it's unique "color".
I guess the sorted J201 used in the original tend to have lower (rather than higher) gm which results in lower voltage gain in the circuit. If you don't want to take any of your FETs out of their sweet spot, I suggest you add some capacitor between the drain of the 1st FET and ground to control these oscillations. Maybe something in the range of 100pF to 1nF will do the work without really killing your sound.

Well the problem was not the foot switch, but the wire runs attached to it. Helpful hackers on another forum suggested shielded cable. I made some by removing the shield from Radio Shack dual conductor shielded cable and sticking my wires inside, then grounding the shield at one end only. This did it and the circuit works marvelously well.

I ended up with trimmers for the Drain resistors. I also reduced the values of the caps to ground before the output, sort of a speaker cab simulator? This made it clearer and more realistic to us. I also got rid of the big cap attached to the source of Q2: I liked the crazy sag/compression on the boost channel but its too much on the clean channel. For our tastes.

I tried this shielding on another copy made according to the posted vero design and could not get rid of the oscillation. My other was made on a larger vero design I cobbled together. I now have the real RG 174 stuff so I will use this on the next build.

PCB design/layout would seem important for these. Wampler's are famously super neat, ground planes, smart routing of signals, switch/pots attached to board... With all those off-board wires one is asking for trouble with this thing.

I also had the squealing problem in boost mode.
I could not figure it out, tried different jfets and shielding. all that helped was a buffer in front.
Then I bought a production unit second hand and found out that there are no 10pF's in the production unit, the 10pF's should be 100pF.
After I put the 100pF's in place of the 10pF's there are no more squealing.
The 100pF's are WIMA 100/100. I soldered on out to measure it.

Here is a shot of my production unit:

I wonder how different it would sound with jFet mu-amp gain stages?

Of course there are some Fender-ish distortion pedals with this topology, Notably from Catalinbread, (and I really like them..) but they are much higher gain, like a smaller, older Fender, maxed-out, and with a booster in front!
- and OKKO pedals, some very mild and others very high gain, but not trying to sound Fender-ish.

Jack O @ AMZ is now selling a board to build a cascaded mu-amp design with a Fender tonestack between. Has anybody built one?

Using mu-amps would save sorting-trimming trouble, but could they be made to sound the same?
(I imagine the gain would have to be dialed back a bit from standard minibooster design..)

Hi Lozboy,
Can you tell me where find out schematics ?

The schematic is on page 2 of this thread.

Ben N wrote:Thanks for the nice cap. 2x-sided board, though, so not much we can make of it. Except baseless speculation, which is always fun, so: Double Fetzer valve + tone stack + mosfet output stage, a la ROG? Nah, no trimpots. Then again: download/file.php?id=13073&mode=view.

Where is Q4 at schematics?