Jfet Booster Clipping/Distortion Issue.. Help!

[ringoismydog]

I've recently built a fender preamp in pedal format. The circuit is almost exactly the same as a real blackface preamp with a few accommodations to work with Jfets.

My question is this, my Rhodes and my Single Coil guitars sound amazing through this. I have the transistors biased perfectly and this thing sounds Loud and creamy. The issue is when I plug in my SG, the humbuckers and clipping the sh!t out of this thing. I've tried every method that I could find online without any help.

Is this just the reality of using hot PAF style pickups with a J201? Is it just not possible to run a humbucker at full volume through a well biased J201 without distortion? With this said, how do pedal manufactures make sure that the input of their pedal can accommodate for low single coil levels and high humbucker levels?

[antonis]

Hi & Welcome..

Could you plz post a schematic of your build..??

how do pedal manufactures make sure that the input of their pedal can accommodate for low single coil levels and high humbucker levels?

Usually by Hi (for single-coil) and Low (for humbuckers) inputs..
(on later, signal is attenuated via a resistive voltage divider..)

[ringoismydog]

So the input gain stage is just the fetzer valve (revisited), without the source bypass capacitor. I understand that fender amplifiers and other sorts have hi and low inputs, but out of all of the Jfet based pedals that I've seen, none of them have had two inputs. I tried adding a voltage divider at the input, but my humbuckers still clipped the jfets. Curious if there is a workaround or if J201 isn't the best choice for a clean boost for humbuckers.

[antonis]

So the input gain stage is just the fetzer valve (revisited), without the source bypass capacitor.

I think you didn't understand my request..
(there is a huge difference between "post" and "describe" a circuit..)

So, could you plz post a schematic of your particular circuit..??
(as fas as it concerns the above mentioned booster, at least..)

P.S.
It should be much easier and more convenient for a bunch of well-intentioned persons to look at a presice diagram than for each one to search for it and imagine possible modifications, shouldn't it..??
(from web traffic point of view, at least..)

[GibsonGM]

I'm sensing that some kind of 'pre-gain' control is needed.    Waiting for schematic, as Antonis requested.

Could be that input attenuation is required, if the 1st stage clips...or a pot after 1st stage, or a pot in conjunction with a bypass cap (partial bypass gain control) for 1st stage...

Conjecture til we see the build. But have no fear - the problem CAN be solved   

[ringoismydog]

Here is the Fetzer Valve circuit that I used

[Steben]

J201's are very sensitive transistors. They became popular because they have a high amount of gain with usually a low "Vgs off". Very small voltages can drive a large output. The Vgs off is usually lower far lower than 1V.
Given the fact many humbuckers have more than 1V peak output (instead of 200 to 500mV of a single coil), they can easily cut off a J201.
jFETs with higher values of Vgs off are better for humbuckers. Starting with a 2N5457 or BF256A for example.

[Clint Eastwood]

I agree with Steben, except that the J201 really does not have a lot of gain. I think it became popular accidentally, just because it was used in Donald Tillman's preamp wich became very popular. He claims that it is a lower noise device than for example the 2n5457, but this is not true according to the FET databook I have.
If you are building a voltage gain stage for electric guitar the J201 is not the obvious choice, better try the J113 for example, has much more gain, a high enough cutoff voltage, and lower noise. And you can still get it as through hole part. Another matter is that different Jfets can sound differently. You will be the judge of that.

[antonis]

IMHO, if you want to use a JFET for a 1^st stage booster, you better bias its Gate via a voltage divider..
(or better, use a MosFet..)(*)

Of course, in the cost of an extra transistor and some resistors & caps, you can have a CC/CE combination who gets rid of all FETs inexpediencies..

(*) Time for Sir Mike to elaborate his Highness aspect..

[GibsonGM]

There is nothing to elaborate upon, Antonis - you are correct!  Mosfets are superior devices because they have a consistency that JFETs simply can't achieve. I can't recall seeing a post like this where people have to wonder if the Mosfet is the cause of the problems - but there are MANY regarding JFETs! 

[Steben]

I agree with Steben, except that the J201 really does not have a lot of gain.

true.
I think I'ld better describe it as being easily cut off (overdriven sounds).

[antonis]

I think I'ld better describe it as being easily cut off (overdriven sounds).

[ringoismydog]

J201's are very sensitive transistors. They became popular because they have a high amount of gain with usually a low "Vgs off". Very small voltages can drive a large output. The Vgs off is usually lower far lower than 1V.
Given the fact many humbuckers have more than 1V peak output (instead of 200 to 500mV of a single coil), they can easily cut off a J201.
jFETs with higher values of Vgs off are better for humbuckers. Starting with a 2N5457 or BF256A for example.

This is the exact answer that I was looking for. I was quite curious as to why the internet points builders (with little experience using JFETs) in the direction of using a J201. Thanks so much for the help. At the moment the J201 actually sounds great with my Rhodes and my Strat. I'll just dial back the volume on my bucker guitars until it doesn't clip the jfets for now.

[GibsonGM]

Look at an AMZ Mosfet Boost - I find it to be a very versatile boost, I use it all the time with SCs and HBs, the gain is adjustable.

Now Antonis will comment about my 'love' for the humble mosfet boost, ha ha!   But really, it IS versatile.   I consider the Tillman to be a good preamp input stage, just my feeling on it.  To be followed by more 'stuff' to shape your tone/drive.  The mosfet boost does it all; I use it for cranking up for leads, to enhance clean guitars, even on vocals while home recording! 

Just curious, what are you using for an amp? Could that be playing a role in your 'early breakup'?  (Tube vs solid state)

http://www.muzique.com/schem/mosfet.htm