Again a mosfet boost question........

[Eddie]

Hi!

I just finished my first perfboard project the AMZ Mosfet Booster (BS 170).

I get a light boost (I get some overdrive..). But if I turn the Pot the clockwise I get at the end a lot of distortion. But there is a strong distortion step between "9" and "10"( means at the the end of the Pot movement --hope you understand..horrible english).

Is this normal?I used a linear 5k POT.


Thanks for answering!

Eddie

[DaveTV]

It's generally considered better to use log pots for volume/boost controls. That being said, I used a 5k log pot for my mosfet boost and I have to turn the knob counter-clockwise to boost my signal. It doesn't really bother me too much, but a reverse log pot would probably have been a better choice.

[Eddie]

I had to turn the first counterclockwis too. Then I switched the wires.
I should try a log Pot.

Thanks !

Eddie

[dubs]

use a tapering resistor (1.5k??) across the linear pot, that should simulate a reverse log pot

[bwanasonic]

Hmm, light boost and overdrive don't sound like the Mosfet Boost I know. I get HELLA clean boost, at least 24db. Virtually no *overdrive* distortion from the pedal itself, even with humbuckers, but I can certainly pound the snot out of my amp. I used a trimpot for R2, and set the voltage marked at test point A (Jack's schematic: http://www.muzique.com/schem/mosfet.htm ) to about 5.5v. What do you get for a reading at that point? Maybe you need to tweak R2?

Kerry M

[Eddie]

Thanks for your help!

I measured first 6 volts at the testpoint A. I adjusted the R2 100k Trimpot to get 5.5 volts but without any improvement.

Could you give some other test points?
Would be easier for me to find the mistake.

I turned the preamp gain on the amp down to test whether the distortion comes from the amp or the Mosfet boost.I get a very light boost .Only at the end of the gain pot movement I get a distortion step from the Mosfet boost not from the amp.

Thanks!

Eddie

[Eddie]

Happy New year!

I searched for mistakes in my AMZ Mosfet boost.
I measured 5,5 volts on testpoint A, but nearly no voltage on G of the BS 170.
After that I changed the direction of the Mosfet and I got rid of the distortion but got a very light boost only.I could not adjust the voltage on point a to 5,5 volts (only 4 volts)So the positioning of the BS 170 had nearly no influence on the output.

Could somebody help me from that measurings finding the mistake??

Thanks!!!

Eddie :?:                  

[Eddie]

Anyone? :?:

[dan]

i'd check over the leads of the mosfet transistor.

i think this is what it should look like on the schematic:

Code Select Expand


                     |1
                     |
      3  ----->|=
                     |
                     |
                    2

   
     
  (___)
  |     |
  |___|
    |||
   / | \
  1 2 3
see if that helps. the code might come out a little screwy but use your imagination.

[bwanasonic]

The BS170 is static sensitive. I zapped one when I built my mosboost. Have you tried another one? Not sure if the symptoms you mentioned indicate a dead tranny, but it's worth a shot.

Kerry M

[Eddie]

I bought 2 new Mosfets today.I used a socket for the BS 170 too.
I still get a light overdrive at the end of the 5k pot`s turn(this distortion comes mainly  out of the Booster (from 200 Ohms to zero Ohms, I measured that too....).

I get these voltages:

Testpoint A ( D): 5,4 V----------I tried everything from 3 to 6 volts...
G:2,9 V
S: 3,2 V

I measured the resitors. Looked 100 times over the layout, had someone else have a look.


HELP!!!Please!!
:idea:

Eddie

[Joe Davisson]

The gate voltage has to be above the source. The BS170 does have a reversed pinout compared to a 2N7000 & most bipolars:

Flat side up & top: D G S

[danelectro]

im having the same trouble..

my voltages from top to bottom w/ the transistor like a D are:

5.48
3.3
3.95

Im using BS170s (two different ones, similar voltages, same results) and ive tried it on a little peavey SS amp (worst results) and a 18watt marshall tube amp which had better results but it still distorted a good bit and not in a great way.. and im using a trip pot for R2 to adjust the bias..

any more ideas on this one?

[Gringo]

I had the same problems. I tried many mosfets, and rechecked my layout  endlessly, but the problem was to correctly bias the bastard(s). I´d suggest to put a 33k res in series with a 50k pot instead of a 100k pot to find THE spot (IIRC, it was at about 5.6). At V <= 5.5 or >= 5.7 i had distortion, very small window.

Once correctly biased, it was awesome  

[danelectro]

hrmm so its al in teh bias eh? thanks, ill keep tweaking this sucker till it works

[danelectro]

hrmm is the sweet spot really so small that i might be missing it w/ a 100k trim pot?

my friend came over and he continuously played guitar through it as i sllllllowwwwwwly tweaked the trim pot and basically it all either sounded like crap or off.. not much variance in the crap either.  the pedal as it is is quite unusable.. pretty much w/ any amount of noticable gain it distorts.. ive tried two mosfets..

could anything else be to blame? anyone have any more reference voltages?

thanks

[Jered]

Hi guys, I believe there are two different manufacturer's of the BS170 that have two different pinouts. Find out which maker you have and get the datasheet from a google search to make sure you have the leads oriented correctly. Just go to  http://www.google.com   and type in  "bs170 data" and you should get what your looking for.
 Jered

[danelectro]

well ill be damned. if youre looking at the mosfet from the top and its positioned like a capital D,

Philip's go SGD from top to bottom

and OnSem's goes DGS (like the GGG layout) from top to bottom

i did try my mosfet flipped once and get very little output. however i dont think i tried messing w/ the bias too much.

i hope thats the problem!

[danelectro]

well damn. my mosfet was oriented correctly... i have no idea what its deal is. both mosfets behave exactly the same so i dont think theyre bad. ill try a 33ohm + 50k pot and see how that works out.. maybe im just missing the sweet spot

[danelectro]

anyone have any more voltage references i can check?

this thing is starting to piss me off