Jfet / Mosfet into Boss pedals

[Tinker]

Hi.  I have lately started to mod my own pedal and I was wondering if there is a way to use a Mosfet in a regular overdrive pedal?
I have a Boss sd1, ds 1 and od 3 that I have tested out different parts.  Can't figure out the mosfets though.  Like the mosfet in the Fulldrive so I thought maybe it could be inserted elsewhere as well.
Best from T.

[96ecss]

Hi and welcome aboard,

Here is some info for you to check out

http://www.diystompboxes.com/smfforum/index.php?topic=65996.0

http://www.diystompboxes.com/smfforum/index.php?topic=65996.0

I hope this is what you're looking for.

Dave

[Ben N]

Tinker, that information is very good, but I get the sense you may need a little background first. If I'm wrong, sorry for the misread. If you can read a schematic, look at the examples referred to below.

Many overdrives work by clipping either the output or the feedback of an amplifier. An example of an output clipper is the MXR Distortion + or the classic ProCo Rat. An example of a feedback loop clipper is the Ibanez Tube Screamer, in all its variations and mutations, or the EH Big Muff Pi.

Others do not use this technique, but instead generate distortion by saturating a (second) amplifier stage. Examples: Fuzz Face, Crowther Hotcake, Tech 21 XXL. Compare the runoffgroove Whisker Biscuit (no diode clippers) to the Big Muff Pi.

Actually, a lot of pedals that have clippers really use combinations of amplifier saturation and diode clipping.

FETs can be used in two ways in an overdrive: either as the amplifier(s), or as the clippers. Overdrives that use FETs as amplifiers are designed from the ground up for the purpose--FETs are not interchangeable with IC opamps, and usually not with bipolar junction transistors, or even with each other. Examples of FET amplifier overdrives abound in the DIY realm--check out AMZ's venerable mini-booster, various iterations of the Shaka Brada concept, the many amp-inspired designs at runoffgroove, such as the Supreaux. Usually the only fruitful substitutions you can try to make are substituting different values for strategic components, and substituting different JFETS or MOSFETS as indicated--either different types or different exemplars within a type. These WILL yield variations in tone, sustain, noise, etc. Because FETs have a great deal of variability even within a single batch of a single type and often require labor-intensive testing or circuit tweaking, commercial mass-manufacturers tend to avoid them except for switching, so don't even bother looking for mods there.

MOSFETs as clippers are a different story. As you note, Fulldrive offers this on some pedals, and it is supposed to have a distinctive tone. There are a lot of different possibilities for clippers that can offer tonal variation, such as LEDs (for more volume, less overdrive), germanium diodes, slicon diodes, assymetrical clippers, that you can explore. There are even a couple of different ways to use MOSFETs as clippers. If you are modding a commercial pedal, the place to look is for a pair of diodes on the circuit board connected back to back. You should confirm that these are in fact clippers, from the schematic, if available, or by tracing their connections. Then you can remove those diodes and explore other clipper options. If you want to do this more than once it may make sense to make yourself a clipper testing box with, say, a rotary switch connecting several different clipping configurations. Then you can just connect this box where the diodes used to be and try the variations out. I think Dano12 has something like this on his site.

All of these things have been discussed in various threads here at great length, so just SEARCH for more info, but start with the thread that Dave gave you for the hows and whys of MOSFET clippers.

[Tinker]

Thanks for an understandable answer.  I see infos about this alot on this site, but being fairly new to the inside of boxes it's a bit to musch info to digest at once.  The mosfet has intriqued me since I tried the Fulldrive, thought maybe it could be done to a regular overdrive too.  The fulldrive is a bit like the old routine of drives I understand?

Best T.

[Ben N]

Use the search function to find out about each particular pedal. Boss pedals are pretty good for this, because their boards are easy to remove from their enclosures, well-made and usually have enough space to get in and work on them. For the SD-1, besides searching for SD-1 look also for Tubescreamer mods and Fulldrive, as their circuits are nearly identical. (Main difference: asymmetrical clipping on the SD-1 uses 3 diodes instead of two.)

[Steben]

The mosfet has intriqued me since I tried the Fulldrive, thought maybe it could be done to a regular overdrive too.

The truth is most clipping overdrives are in fact the same as stated above.
SD-1, Tube Screamer, Fulldrive, ... all can be modded to sound the same in fact.
I think as a matter of fact I already saw SD-1 to TS and vice versa mods around...
it figures...