AMZ Mosfet Boost: questions

[Gringo]

I'm having a hard time biasing this one, but i think i have it down (at least for the bs170 i'm currently using).

Huge boost at top settings, all merry and clean, but...(there always seems to be a but):

With full gain, it breaks up little at single notes(noticeable, but not annoying, very little considering my hot pickups), and with chords, the clipping gets a little more present.

I'd like to know if this is normal, or do i have to tweek/debug/bias a little harder to get there. I have a 100k trimpot at R2, actually set at ~51k, here are the pins voltage readings (~9.3v battery):

D: 5.45
G: 0.53
S: 3.86

I sweeped through the trimpot, and this is the best sounding setting.

One curious thing was when biased at D=5.53, there was more fuzz when hit hard, yet clean if strummed softly, but if i turned down my guitar volume pot, it would go all fuzzy, no matter how i picked  :?

Thanks in advance.

[R.G.]

For some in-depth insight into biasing single MOSFET boosters, read the MOSFET Boosters design article at GEO.

As an aside, I think you have the wrong pins noted for your voltages. An n-channel MOSFET like the BS170 with its gate at a lower voltage than its source shouldn't be conducting at all.

[Doug H]

Your gate voltage is too low. Looking at the bs170 datasheet (http://www.fairchildsemi.com/ds/BS/BS170.pdf), it looks like it needs a gate threshold voltage around 2V to turn it on. Check the bias network around the gate that sets the voltage. I think Jack may have suggested subbing a pot for one of those resistors to set up the proper bias, as it will vary from device to device. (Maybe he already has the pot in there, I don't remember...)

Doug

[petemoore]

We think they sound great !!!
 One has  J201 jfet instead...IIRC that's how I got a usable bias on that one.
 The other has had 2N7000s, I haven't gotten bias to 'sit right' and sound good, so I went for sounds great instead...I don't know that it is working 'right', but it certainly works the way I 'we' like it, very nice clean boost that makes 'that' tube amp sound more like 'itself'.
 I gave up trying to bias it and insted go for the great sound...lol
 The third one has a MPS102...I haven't been able to comprehend enough about biasing Mosfets to use the info to any degree beyond tweekin the Drain R's in existing circuits.

[bwanasonic]

My experience was that the trimpot I used for R2 was more of an on/off control for the BS170. Although I did detect some subtle differences within the *on* section of the sweep, it's not a real *tweaky* circuit. I just set it to the advised 4.5v. If you are getting breakup at full gain - turn down the gain! I never use higher than 2-3 o'clock on the gain knob. If turning down your guitar volume is causing a fuzzy sound, something is wrong. Not sure how the AMZ boost behaves with active pickups, are you using actives?

If it's any help, here are the voltages on mine:

(oops, had pins labeled wrong)

s 3.31
g 0.47
d 4.52

Kerry M

[ErikMiller]

For some in-depth insight into biasing single MOSFET boosters, read the MOSFET Boosters design article at GEO.

A typically great article, and I hope there is more of it someday!

[Gringo]

My experience was that the trimpot I used for R2 was more of an on/off control for the BS170. Although I did detect some subtle differences within the *on* section of the sweep, it's not a real *tweaky* circuit.

Exactly the same to me, but my mosfet gets its best at 5.4v

I just set it to the advised 4.5v. If you are getting breakup at full gain - turn down the gain! I never use higher than 2-3 o'clock on the gain knob. If turning down your guitar volume is causing a fuzzy sound, something is wrong. Not sure how the AMZ boost behaves with active pickups, are you using actives?

If it's any help, here are the voltages on mine:

(oops, had pins labeled wrong)

s 3.31
g 0.47
d 4.52

Kerry M

I'm using passive pu's (tone zone on bridge, full shred at neck). Correctly biased, it doesn't fuzz when i roll the vol pot, i said it used to do it when slightly off-bias.

By now i know better than to go against RG, Doug, and the rest of the veterans, but this is quite strange, as my unit works!. I remeasured the voltages at tthe three pins (black probe to ground, red to pin, datasheet by my side),  and the readings are the ones i posted before. And now bwanasonic's measures are similar to mine  :?

I'm using my own layout, so i'll transfer everything to the one in tonepad and see what happens.

Thanks for all the answers.

PD: I read RG's article (2 or 3 times, it'll take me some time to process it )and everything i could find using the forum's search tool before asking.

[Jered]

What manufactered BS170's are you using? There are two different pinouts between two different makers. My guess is both of you with the odd voltage readings are using BS170's from the same maker.
 Still doesn't exlain why its working though.?.?
 Jered

[Gringo]

Jered, i don't think it's the pinout, since i have tested it with 2n7000 from 2 different manufacturers(i have both datasheets also), as well as the bs170, and the result is the same, but a little more gain. If i reverse the fets, i doesn't work, either.

I also made another using tonepad layout, exactely the same results/readings. Maybe i'm entering the twilight zone...

I think i'll leave em as they are, since they both work.

[bwanasonic]

The only datasheets I could find when I built mine had the same pinout (DGS with *flat* side toward you). I didn't realize the voltages I got were *odd*! The effect certainly works as advertised ( I can barely live without it!).


Kerry M

[Jered]

LOL! The way I wrote that didn't make any sense did it? LOL! Hey, if it works for you, that is all that matters.
 Jered