FatPants question

[Mike B.]

Hi all,

Has anyone built the new FatPants that could maybe share their voltages on Q1 & Q2. Mine works, but all of the gain is bunched up in the last 10 degrees of the "fat" pot rotation (4:00-5:00). Cranked full out with the volume and fat maxed and the body switch on, I get just above unity gain. Is this normal?

Thanks,
Mike

[mgwhit]

Sorry, I haven't build the new version and my old version is on a friend's board right now, so I can't give you any voltages, but have you followed the instructions in the new build doc about "Setting up the FatPants" with the trimmer?

Leave the Body switch off, set the Fat control to max. Adjust T1 until you read approximately 9v on
the drain of Q2 with your multimeter. Tweak as desired for maximum gain.

Q1 in the new version is just a buffer, so it shouldn't really affect gain at all.  I can't remember how bunched the gain is on the Fat control, but I can tell you that with both knobs at Noon I was getting more than unity gain.  With the knobs past that it go pretty darned loud.

Are you getting ~18V at your extra 18V tap point?   That and the drain of Q2 (as above) would be a good place to start.  Good luck!

[Mike B.]

Thanks MG-

I suspected that wasn't normal, just wanted to confirm.

Yes, I have done the "biasing" with the trimpot. And yes, I have 9v everywhere that the schematic says I should have 9v and 18v everywhere that I am supposed to have 18v. Also tried another set of J201's and a set of 2n5457's just for grins - same result. Going to throw one more part at it and swap the FAT pot, then I'll start with an audio probe.

Mike

[mgwhit]

Cool.  I'd visually inspect the values of R5 and R6, too, and make sure they're the correct orders of magnitude.  If you'd like to post good photos of the board, I'd be happy to triple check everything.  Maybe someone who has built the latest version will chime in here soon and help out.

[madbean]

It does not have a huge amount of output but it should be over unity for sure as you can hear from the demo on the MBP youTube page. I'll check my build ASAP to make sure I did not put the wrong value somewhere in the build doc.

[Mike B.]

I noticed that R6 was 47K rather than 470K in the previous version. Maybe nothing but I thought I'd mention it.

[Mike B.]

Found my problem - had the fat and lvl pots swapped. For someone who spent the first 3 yrs of college as an EE major, I certainly make some embarrassing mistakes. Thanks guys for your time. MG you were on the right track.

[mgwhit]

Cool!  Glad you found it.  I did that with a Mudbunny once.  It took me forever to figure out what was going on.

[sgmezei]

Found my problem - had the fat and lvl pots swapped. For someone who spent the first 3 yrs of college as an EE major, I certainly make some embarrassing mistakes. Thanks guys for your time. MG you were on the right track.

I did the EXACT same thing on my first fatpants build!