Fetzer valve input overdriven by hot pickups... How can I fix this ???

[rosssurf]

I made a fetzer valve to use as a volume boost for an acoutic guitar. The guitar is a Gibson Chet Atkins model. The output of that guitar is overdriving the Fetzer causing distortion.
How can the Fetzer be modified to remedy this?

[PenPen]

Check out this: http://runoffgroove.com/fetzervalve.html

Try changing the input resistors to the pair of 68k's as suggested for a low gain input. That may help attenuate the signal enough.

[rosssurf]

Thanks PenPen. That is a help.
I must admit I do not fully understand this stuff yet but I am learning. Could I just change the 2nd R value( 1M) down to 68K to achieve the same thing without having to add another jack? If so I figure I would just socket that R until I have found the right input level.
I have done this build   http://aronnelson.com/gallery/album18/FETZER_VALVE_001

[ildar]

Actually, this problem was addressed to a point in another thread.
Most acoustic pickups have a preamp onboard, so what is happeneing is that you have two preamps in series, thus the distortion.
Which isn't to say that penpen's advice isn't applicable, it is. You may need to reduce the input resistor further.
Did you build it as original with the 1M resistor to ground? If so, just changing the 1M resistor to 68k will help.

[stm]

A J201-based Fetzer will add dirt to nearly any pickup, especially the hot ones.

If you have single coils, you'll have a cleaner sound with a J202 or 2N5457 JFET.

Now, if you have hot pickups (series humbuckers) and you just want a slight coloration to the sound (like 5% 2nd order harmonic distortion), I'd recommend using a 2N5458, 2N5459, 2N5952 or MPF102 JFET with a measured Vp between -2.0 and -3.0V.  In this case gain will be something between 1.5 times (3 dB) to 0.5 times (-6 dB), but you already have a hot pickup, don't you.  If you want to solve this low gain issue, then power your pedal with 18V instead, as this will allow turning up the gain. Use the Fetzer Valve Calculator to design your booster.  Enter some typical parameters for an MPF102: Vp=-2.5V and Idss=6mA and Vcc=18V and you'll see what I'm talking about.

[ildar]

stm, read the post. He doesn't have hot pickups, it's an acoustic pickup...piezo perhaps?

[Ben N]

Actually, this problem was addressed to a point in another thread.
Most acoustic pickups have a preamp onboard, so what is happeneing is that you have two preamps in series, thus the distortion.
Which isn't to say that penpen's advice isn't applicable, it is. You may need to reduce the input resistor further.
Did you build it as original with the 1M resistor to ground? If so, just changing the 1M resistor to 68k will help.

That should help, and I would think that the onboard pickup is the reason why it is doable at all, since drastically reducing the input Z right after a piezo would sound really crappy. 

But then the question is, why?  Why run two preamps in line, and why have one othem be a single-FET?  If you just need an uncolored solo boost with enough headroom to handle the guitar's preamp, the better thing would probly be a non-inverting opamp stage with a high enough supply voltage, something like a single channel version of Dave Barber's Launchpad--easy enough to DIY.  Or, better yet, if the guitar preamp has a high enough output, just stick a volume pedal after it.
JM2C-HIH
Ben

[PenPen]

Thanks PenPen. That is a help.
I must admit I do not fully understand this stuff yet but I am learning. Could I just change the 2nd R value( 1M) down to 68K to achieve the same thing without having to add another jack? If so I figure I would just socket that R until I have found the right input level.
I have done this build   http://aronnelson.com/gallery/album18/FETZER_VALVE_001

Yes, just lower the 1M to 68k and see if that clears it. To go even further (if that doesn't fully fix it) you could try going even lower with this value. Those two resistors are basically acting like a fixed volume pot, a voltage divider. Changing the ratio will affect how much signal gets into the jfet input.

After trying 68k, I'd next try 47k, then 33k. Let us know how it goes.

[rosssurf]

Thanks for all of your replies. I will lower  68K resistor. I will post my results later.
- Thanks again.

[JimRayden]

Okay, I've lost you somewhere. One of you is suggesting lowering the 1M resistor to ground, which will decrease the input impedance. The other one suggesting decreasing the 68k resistor which will increase the signal to hit the gate even further.

Following logics, it would be raising the resistance of the 68k to attentuate the input. Replacing the 68k with a 1M resistor (thus creating a 1M-1M voltage divider) will kick the input signal down by 50%. I suggest trying 100k, 220k or 470k in that position. That ought to kick the signal down a bit.

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Jimbo

[PenPen]

Okay, I've lost you somewhere. One of you is suggesting lowering the 1M resistor to ground, which will decrease the input impedance. The other one suggesting decreasing the 68k resistor which will increase the signal to hit the gate even further.

Following logics, it would be raising the resistance of the 68k to attentuate the input. Replacing the 68k with a 1M resistor (thus creating a 1M-1M voltage divider) will kick the input signal down by 50%. I suggest trying 100k, 220k or 470k in that position. That ought to kick the signal down a bit.

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Jimbo

ROG is suggesting lowering the 1M to ground. I was suggesting if that still doesn't do it to try going a bit lower with it. I almost also suggested swapping the 68k input resistor to a higher one as well, but I thought that was implied with the "voltage divider" comment.

[rosssurf]

Ok heres the scoop. I ran my Larrivee into a K&K pure western pickup, then into the fetzer. I ended up raising the first 68K resistor to just under 2 megs (1.975). This seemed to stop any input distortion. I think I may start making my Fetzers with this incorporated into a toggle switch. or maybe even a pot?...
I wonder if active electric guitars and basses would require this as well.

[PenPen]

You kept the 1M to ground then?

What is going on here is these two resistors are a voltage divider. Thus, the signal is reduced at a ratio of R2/R1. So if you have 2M for R1 (signal path), and 1M for R2 (to ground), then you have a ratio of 1/2. I'm not sure if you have a handy supply of 2M resistors (I know I don't) but you should be able to achieve the same thing by making R1 1M and R2 500k, which are more common values I think.

Just wanted to throw that out there in case anyone else asks.

[stm]

stm, read the post. He doesn't have hot pickups, it's an acoustic pickup...piezo perhaps?

A piezo or acoustic or active pickup is another form of hot pickup.  What I mentioned before is still valid.

Regarding the input attenuation, it is a simple matter of building the Fetzer Valve Deluxe which includes both input and output level control, as well as the presence capacitor.