converting a fx from jfet to mosfet

[copachino]

hi all, i have really bad luck with jfets most particular with j201, i want to build a plexidrive, but  i always have bad fets, but maybe would be cool tro convert taht or maybe a smaller circuit into a mosfet drive,

i know i cant just stick the mosfet wgere the j201 are, but what can i change in generals??

[midwayfair]

hi all, i have really bad luck with jfets most particular with j201, i want to build a plexidrive, but  i always have bad fets, but maybe would be cool tro convert taht or maybe a smaller circuit into a mosfet drive,

i know i cant just stick the mosfet wgere the j201 are, but what can i change in generals??

You need a bias point. 4.5V is fine. Reference all the gates to that.

The gain of the MOSFET might not be ideal. So for each FET with a fixed gain at a particular bias point, you probably need to use a bypass cap with a trim pot.

The input of a FET distorts with a signal of a size equal to its |-Vp|. For reference, this is inversely proportional to the gain, so FETs with high Vp can take a large signal but can't amplify as much. A J201 is particularly unusual in this regard, as its gain is very high and its Vp is very low, under a volt and averaging about .8V. This means it will distort with a relatively small signal. When you cascade them, you get a lot of distortion even though the gain of each JFET may be only 13-20x. A MOSFET does not exhibit this behavior, and will distort only when the voltage gain at the drain exceeds the supply voltage of the circuit. This means you need each MOSFET gain stage to amplify much more to distort the next MOSFET in line. The circuit will be much louder before the same amount of saturation is reached.

Also, a MOSFET will not have the same distortion profile as a MOSFET. The harmonics will be different. So while you may very well end up with a good sounding device, you will not get it to act or sound the same.

Depending on how much of that you understood, you may want to give FETs another shot or search for a similar sounding pedal that uses less finicky devices.

[jubal81]

There are several circuits you could try that use the BS170 mosfet like the Box of Rock and the RAH.


You can buy PCBs for those here:
http://shop.pedalparts.co.uk/OverdriveDistortion/cat847125_2465591.aspx

[copachino]

There are several circuits you could try that use the BS170 mosfet like the Box of Rock and the RAH.


You can buy PCBs for those here:
http://shop.pedalparts.co.uk/OverdriveDistortion/cat847125_2465591.aspx

that why im talking about, a have a BOR, and sound pretty good,and it uses mosfets, and circuit its similar al BIASB

[copachino]

hi all, i have really bad luck with jfets most particular with j201, i want to build a plexidrive, but  i always have bad fets, but maybe would be cool tro convert taht or maybe a smaller circuit into a mosfet drive,

i know i cant just stick the mosfet wgere the j201 are, but what can i change in generals??

You need a bias point. 4.5V is fine. Reference all the gates to that.

The gain of the MOSFET might not be ideal. So for each FET with a fixed gain at a particular bias point, you probably need to use a bypass cap with a trim pot.

The input of a FET distorts with a signal of a size equal to its |-Vp|. For reference, this is inversely proportional to the gain, so FETs with high Vp can take a large signal but can't amplify as much. A J201 is particularly unusual in this regard, as its gain is very high and its Vp is very low, under a volt and averaging about .8V. This means it will distort with a relatively small signal. When you cascade them, you get a lot of distortion even though the gain of each JFET may be only 13-20x. A MOSFET does not exhibit this behavior, and will distort only when the voltage gain at the drain exceeds the supply voltage of the circuit. This means you need each MOSFET gain stage to amplify much more to distort the next MOSFET in line. The circuit will be much louder before the same amount of saturation is reached.

Also, a MOSFET will not have the same distortion profile as a MOSFET. The harmonics will be different. So while you may very well end up with a good sounding device, you will not get it to act or sound the same.

Depending on how much of that you understood, you may want to give FETs another shot or search for a similar sounding pedal that uses less finicky devices.

i undertand most of it, but the BOR, has a great cascode drive, and its based on jfet drive, so... maybe they wont be exact as the jfet pedal, but i think they could have a different taste