AMZ Mosfet booster- What's up with the Pot? Survey??

[boogietube]

So, I completed the AMZ Mosfet booster and the volume is bunched up at the last 20% of the pot.

Survey: does anyone else have this symptom?

Is there a solution? I have scoured the posts and have found one reference that  said to put a 25k linear in parallel with a 10k resistor. That did not work. I have added a trim pot,  biased and rebiased, switched the fet around, changed fets (I have an antistatic bag of BS170'S)
and tried the aforementioned pot trick to no avail.

Please help. This is my first successful build and I am frustrated.

Sean

[KerryF]

are you using a log or a linear pot?  I know it says to use linear, but are you?

[boogietube]

Yes, I'm using a linear pot like in the tonepad layout (millenium version)
Sean

[boogietube]

It's not "like" the tonepad layout. It IS  the tonepad layout. No substitutions except for the 100k trim pot. All resistors 1% metal film all caps poly or mica except for the electros.
Sean

[gaussmarkov]

i have seen several others, besides me, refer to this behavior.  and joe kramer made a suggestion to deal with it:  put a smaller pot in series with a fixed resistor with a total resistance of the 5K specified.  so if it's the last 20%, then put in a 4K fixed resistor in series with a 1K pot.  you see this sometimes in fuzz circuits where a similar thing can happen.

i have not learned enough to understand this sort of gain setup, but i believe that there is something generic about this kind of control.

all the best, gm

[markm]

Mine does the same thing.
I've lived with it but the series resistor idea is a good one 

[boogietube]

I tried another pot. The A and B pot thing is inconsistent between manufacturers. Anyway, I think that I had a log pot in there. I now have a linear and it is not as bunched up. Maybe 25% now instead of 20%.  Still not what I would want though. I will try the 4k resistor and 1K pot tomorrow and report back.
Sean

[Pushtone]

Mine does the same thing.
So does the Strat-o-blaster's pot.

Yes, it is disapointing. I too tried the recommended remedies without luck.
With all the hype the MosFET gets here I was expecting it to work like a normal booster.
I wondered if the secret lies in ordering Jack's CD-ROM. But that seems like too sleazy
a marketing move for someone with Jack's reputation. I decided that it must be the way it is and moved on.

Some good news,
Both the MosFET and Strat-o-blaster work great as unity gain buffers
to combat tone suck from too many TB pedals in a chain.
I removed the pot, put in a resitor and now I use it as a  buffer at the head of a long chain of TB pedals.

Try a Mini Booster or Fetzer Valve. Their pots work the way they should.

One of my favorite , and usefull, builds is a buffered booster that uses the
Strat-o-blaster (or MosFET if you like) as a buffered bypass and
a Fetzer Valve as the effect when stomped on.

[Dragonfly]

....then put in a 4K fixed resistor in series with a 1K pot.  you see this sometimes in fuzz circuits where a similar thing can happen.

or...you could use a 10k trim pot in series with a 5k pot...that way you could adjust the trim pot to where the 5k pot has the *most useable* range...

AC

[Xavier]

I had the same problem until I changed to a REVERSE log pot. End of problem.

[smallbearelec]

Jack has asked me to get Alpha to make a 16mm 5K reverse audio pot, and I will do so. Parts will be available shortly after New Year's.

Regards
SD

[boogietube]

Well, Steve, It looks like problem solved.Thanks for piping in. Also thanks to Jack at AMZ.  I'm very glad for all of us.
Thanks for the responses guys!
Sean

[ryanscissorhands]

I would actually recommend adding a 100k volume pot to ground at the output. This allows you to have full control of the volume of the boost, so you can get a little bit of the distortion without the level being too high if you like. It was a nice addition to mine when I built it.

[amz-fx]

Jack has asked me to get Alpha to make a 16mm 5K reverse audio pot, and I will do so. Parts will be available shortly after New Year's.

Thanks Steve, and thanks for stocking the part for my circuit.

I will post a link to the pot on my web site when it becomes available.

regards, Jack

[Gilles C]

The rev log pot is a good idea. I'll buy one (or 2...) when it's available.

I used a lin 5K pot on mine with a 100K log pot for the volume. Never changed anything since then, and still use it from time to time.



I always wanted to add a "Vol" and "Gain" and "Mosfet Boost" somewhere, but never decided on how to do it on leather.

Gilles

[soggybag]

Mark: I just built a MosFET booster and was thinking about exactly the same thing. What you might call a boost range pot to pan between to cap values to set the range of boost.

Does this work? That is placing a cap at either end of a cap, will this allow you to sort of mix the or pan between the values? I remember Joe Gagan had done something similar on one of his Fuzz Face varients that mixed be tween two input caps.

Here's a picture of what I have so far:

[amz-fx]

Here is one way to make a versatile gain control:

http://www.muzique.com/tech/mosfet5.gif

Turn it to the right and it adds boost as usual, turn it to the left and it adds a treble boost. In the middle = unity gain.  I've used variations of this method numerous times...

regards, Jack

[make1circle]

Newbie question ... Re: Jack's last post, is the 10K pot linear?  Also, is there a way to incorporate the pot with a DPDT switch?  Thanks a bunch!

[Mark Hammer]

Here is one way to make a versatile gain control:

http://www.muzique.com/tech/mosfet5.gif

Turn it to the right and it adds boost as usual, turn it to the left and it adds a treble boost. In the middle = unity gain.  I've used variations of this method numerous times...

regards, Jack

Bingo!  This may not distribute the gain adjustment across a broader arc of rotation, but it will make most parts of the pot do something useful.

As for pot taper, generally when "middle" is something you want to know about, a linear pot is called for.