I've never seen BS170's in a can, have i missed something?
The tips on your jacks look like they are touching right there in the center. Are they?
If they're not touching in the photo what about when you insert the plugs?
dave
I've done that before. What you do is route them so that there's a quarter inch gap or more between input and output plugs, and turn them so that the long, or sleeve, side of the jacks are at a 120 degree or larger angle from each other. you can also work a stereo jack, used as the on/off switch, into the pedal, by making sure that shorter sleeve tab goes between the two lugs on the output jack. I have I think its a 1590A that can fit two jacks, perfboard circuit of a buffer with non-SMD components, and a battery, without any shorts. If the washers and nuts on the jacks come loose, they would easily swivel and short.
If your MOSFETs work (meaning the circuit passes audio through, whether it sounds good or not), then they probably haven't been zapped. You are right to be careful with them, as they are very sensitive, but my (amateur) understanding is that if that was your problem they probably wouldn't work at all.
However, there is a second, arguably more important, difficulty in dealing with MOSFETs: large variation in threshold voltage - that threshold voltage being what your meter is measuring when it tells you the "VSG". Those readings indicate that your two transistors which "sound good" simply have a lower threshold voltage than the ones that "sound garbled and gross", and your in-circuit voltages are telling the same story. This article on R.G.'s site explains quite-well why this is a problem; the fifth diagram (the one labeled "MOSFET Gain Stage - Bias Network") is closest to the SHO, and the next three diagrams with their associated text explain what you can do to have a good chance of making the "bad" MOSFETs sound good. Once biased properly, you might still hear subtle differences in sound versus the , and once you make that effort, you could even find that you prefer the sound of the MOSFETs that didn't bias right with the circuit as it currently stands.
Fantastic, thanks for the explanation and pointing me to R.G.'s article. I'll try biasing them differently and see how that affects their sound. I had been under the impression that while Ge transistors were temperamental and prone to varying hFe levels, Si transistors were more stable and thereafter would have a lower tolerance. So MOSFETS are fairly stable but canvary greatly from component to component?
From what I understand, Ge variations and low gains are based on the designs and fabrication while the industry was young. Si are better because quality control is better, advances in production, and so on. They still make new Ge transistors, you can even buy them from radio shack, so they're not really "bad" because of the material. The old ones have/had "mojo" due to being leaky, the type of circuit, and so on. An old FF that sounded amazing to someone would be produced alongside 100 others that sounded awful because of the biasing compared to the transistor's wide variances and leakage. Samples of how you'd rebias a circuit for specific gains of old Ge transistors can be seen on smallbearelec. I bought one that I believe is AC128s, and the 100 meg feedback resistor is somewhere around 150, with the biases being 22k and so on. Much different, but it was tweaked for the gains of those transistors so it was still nice and fuzzy. Messing around with fixed bias amp circuits for BJTs taught me how important the characteristics of individual transistors are, how they affect the bias due to
http://www.diystompboxes.com/smfforum/index.php?topic=68796.0;prev_next=prev
The big issue here is that Idss is different from JFET to JFET of the same type. So is Vp. So is the transconductance, the ratio of how much a change in the gate-source voltage changes the drain current. These are all interrelated by the device physics, which is nice, but in a way that makes it very complex and variable from device to device as to where a given bias network will make the currents and voltages settle.
Are mosfets as variable as Jfets? I don't know. I notice you're using a red LED in the circuit, is that in place of the 9.1v zener diode?
