Fetzer Valve mods-higher gain & FAFG

[Noplasticrobots]

Just built the Fetzer Valve-what a great circuit! I want to hear what the higher gain FV sounds like but I'm a bit confused...

"It can be easily modified to reproduce the higher gain input by moving the 1M resistor to the input and changing the 68k to a 33k resistor."

If I move the 1M to the guitar input (replacing the 68k), doesn't that negate the changing of the 68k to a 33k? Obviously I'm reading this wrong. Such a simple mod but I just can't figure it out!

Also, they say to check out the Fender Amp Field Guide to customize the FV, but I dn't know what they mean by "Some Fenders used different cathode (mirrored by the source pin of the FET) resistors."

So I went and looked at some random schematics and don't know how to apply anything tube related to the Fetzer Valve. Anyone got any pointers?

[Ge_Whiz]

Move the 1M resistor to the left-hand side of the 68k, i.e. directly between IN and GROUND. THEN change the 68k to 33k, so that the 33k goes from junction of IN and 1M to the gate of Q1.

[B Tremblay]

Thanks for the clearer instructions, Phil.  I'll poach them for the page!

[Noplasticrobots]

Ok, makes sense but I hear no noticable difference. Just to clarify:

The guitar input is connected to the junction of the 1M (which goes to ground) and the 33k (which goes to the Gate of Q1)?

I wired the different inputs up to a SPDT with the input wired to common (center terminal?) and the two remaining terminals to the two different inputs. Is the switch wiring correct?

Well, I just realized that adding a switch (at least a SPDT) won't cut the 1M connection from Gate to ground for the cleaner version. Is there any way to add a gain control? Nothing that's gonna fuzz it out, just something to get a little grit into the signal. I tried a 1M pot bridged at the input with terminal 1 to input resistor and I get a slight boost, but nothing gritty. Any ideas? This is my first FET experience, and my instinct is to apply BJT functions to it but I don't wanna go messin anything up. Thanks for any help!

[wampcat1]

Just built the Fetzer Valve-what a great circuit! I want to hear what the higher gain FV sounds like but I'm a bit confused...

"It can be easily modified to reproduce the higher gain input by moving the 1M resistor to the input and changing the 68k to a 33k resistor."

If I move the 1M to the guitar input (replacing the 68k), doesn't that negate the changing of the 68k to a 33k? Obviously I'm reading this wrong. Such a simple mod but I just can't figure it out!

Also, they say to check out the Fender Amp Field Guide to customize the FV, but I dn't know what they mean by "Some Fenders used different cathode (mirrored by the source pin of the FET) resistors."

So I went and looked at some random schematics and don't know how to apply anything tube related to the Fetzer Valve. Anyone got any pointers?

Changing that resistor at the input WILL have somewhat of a difference, or you could delete if as well for the strongest *input* signal strength, resulting in a bit more clipping. HOWEVER, it isn't going to be a high gain circuit unless you add more clipping stages or run a clean boost into it (and thus having more signal to clip).

Luckily, I'm in the middle of writing a new book detailing how to modify everyday amps (tube and SS, http://www.indyguitarist.com/forum/viewtopic.php?t=233 ) so I happened to have this image on my harddrive. Looking at it, you can see a basic clipping stage on the top, the fetzer valve (JFET) clipping stage on the bottom. Look at it for a minute and notice how everything is nearly the same except the mechanism used to create the clipping is different (well, there is ALOT more than that different, but for this example...). Make that 1.5k resistor a 1k and you'll have a little bit more clipping. Remove the 22uF cap there and you'll have less clipping. Make that 1.5k resistor a larger resistor..say 10k or 22k, remove that 22uF cap and you'll have a clean boost circuit.

Hope that helps!

Thanks,
Brian

[Noplasticrobots]

Wow, that really is nearly identical on a basic level. I'll give those mods a shot. Thanks for your help!

And by the way, I use a Park G10R.....I need something new.

[Noplasticrobots]

My last problem with this circuit that I forgot to mention earlier is that it seems to introduce a lot of noise to the signal. I thought it might just be boosting my amp's normal noise, but when I bypass it's totally clean when compared. Could it be that it's still on the breadboard and therefore unshielded?

[wampcat1]

My last problem with this circuit that I forgot to mention earlier is that it seems to introduce a lot of noise to the signal. I thought it might just be boosting my amp's normal noise, but when I bypass it's totally clean when compared. Could it be that it's still on the breadboard and therefore unshielded?

In my experience, JFET's can be somewhat noisy compared to an opamp boosting/clipping type circuit. You can usually roll some low pass filters between each circuit and get rid of some of the noise but I've always noticed the jfets are just simply noisier. YMMV though!

Thanks,
Brian

[Ge_Whiz]

Thanks for the clearer instructions, Phil.  I'll poach them for the page!

By all means, Brian. But maybe a picture would be easier in the long run...

[Noplasticrobots]

So if I wanted to install a low pass filter in the FV, would I place a cap to ground on the input of the circuit after the 68k resistor? That would make a low pass, correct?