a few Fezter Valve questions

[mordechai]

1. In the original FV configuration, am I correct that lowering the 68K to something like 56K or even 47K while still keeping the 1M to ground following it would increase how the signal hits the JFET? 

2. Would putting a small value cap to ground before the 68K resistor in the original FV have any adverse effect? 

3. In one of the revised FV versions, with the 1M to ground coming before the 33K...would upping the 33K to 47K make a noticable difference, or require a different pF cap afterwards to allow the JFET to function properly?

4.  Lastly...if one wanted to use an MPF102 instead of a JFET (for something with a touch less gain), what sort of modifications would be advisable?

[DDD]

First of all - FeTZer Valve...

[B Tremblay]

First of all - FeTZer Valve...

Maybe he is asking about the stylish version that you can wear as a hat, you never know...

Anyway, to answer the questions.

1. Yes, lowering the series resistance will provide more signal hitting the gate.
2. No adverse effect, but there will be a more pronounced effect if it is placed after the 68k
3. Probably not a worthwhile difference. While switching from 33k to 47k with a 220p cap the difference in cutoff frequency shifts down by 5kHz, it is still much higher than a guitar speaker's response.  Increasing the cap value will produce more noticeable results.
4. In the FVR test voltages near the end of the article you can see that the MPF102 produces nearly unity gain with a 9v supply.

Look at the Azabache gain stages to see what we've been doing with lower gain FET stages that avoid harsh clipping.

[mordechai]

Thanks so much for the answers, and for being forgiving of my dumb typo.  Of course, I am sure the Fetzer Valve would make for a very stylish cap.

I do see now that the MPF102 doesn't work so well in this circuit in terms of increasing gain.  I am curious, though, why it seems to work so nicely in the Professor Tweed...is it more of a tone shaper than a signal amplifier there?  If the J201 also works in that circuit, is it because it is being used less for gain (though it provides more of it) and more for the tone shaping?

[B Tremblay]

Keep in mind that the source bypass caps increase gain considerably, which make the MPF102 a viable gain stage.

[mordechai]

So it functions similar to how the bypass cap functions off of the emitter of a Q2 in a fuzz face, then?