t-bagger bass mod problem

[ibodog]

I built up the t-bagger w/the bass contour mod today.  All the controls are working (including the bass control) and as long as I have a strong guitar signal in it sounds good.  But, when the guitar vol has decayed or I'm not playing, there is a ticking/motorboating sound that starts in.  I used a 1uF film cap on the A50k pot and hooked it up per the pdf instructions.  I did some troubleshooting and got it down to a problem in the 1st opamp clipping stage.  I could hear the frequency of the ticking change as I changed the bass contour pot, so I pulled the bass contour mod out (lifted the wire from lug 1) and the problem is gone.  Anyone else try this board with this mod?  I didn't see this part on the schematic.  Probably I'll rig up a 2nd pot and cap and see if it works.

[madbean]

That is very strange. Actually, I'm not quite sure what might be causing that. How about if you eliminate the pot altogether and just try toggling between the two caps? You could do this on a breadboard using the wires that normally hook to the pot.

[ibodog]

I rigged up a 2nd pot and cap and had the same issue.

I think I see the problem.  On the Timmy these caps are connected to VREF and on the T-Bagger they're connected to ground.  Maybe I can unhook the caps from ground and attach to VREF somewhere else on the board?

[madbean]

Okay, I did a little research and now I understand a little better as to why this is happening. I will figure out the easiest solution I can to get the mod working. I really need to resist the urge of suggesting mods in projects when I haven't tested them first-hand. Otherwise I would have corrected this first!

Here's a little more: http://www.diystompboxes.com/smfforum/index.php?topic=63411.0

[ibodog]

I decided to git-r-done tonight by making R4/C3 and the bass control look exactly like the Timmy schematic.  Works like a charm, but it's an ugly hack since vref is on the other side of the pcb.  Madbean will probably come up with something more elegant, but here's what I did.

I swapped positions of C3 and R4 on the pcb and lifted the leg of C3 that would otherwise tie to ground.  That leg needs to go to vref.  Lug 1 of the bass pot can attach in one of the extra pads on the pcb at the junction of C3 and R4.  The 1uF cap off the other 2 lugs also needs to tie to vref. 

Getting to vref (VB on the schematic) was where it got ugly.  I tied the other end of the 1uF cap to the lifted leg of C3.  Then used a jumper wire to tie in to vref at the Vol 1 pad (stripped a little wire near the pcb and soldered the jumper there).

Now the bass control works and no thumping noise from the 1uF cap discharging.

[B_of_H]

fwiw I just built up one with a 100n for c3 and it's a little shy of unity bass perhaps but it's close.  47n is a pretty big bass cut IMO.  120n is definitely a fuller bass sound, maybe a tad too much for some people.

I'm not a fan of the B1m gain pot, I put an A500k in there and it's a lot better IMO.  The B1m makes all the range happen in before 9:30 or so.  In fact i'm thinking about going to A250k next as I don't really need nearly the amount of gain this thing produces.   

the circuit sounds fantastic though, big improvement in clipping over the stock diode IMHO. 

[madbean]

I've revised the documentation and posted the pdf to the website.
http://www.madbeanpedals.com/projects/T-Bagger/docs/T-Bagger.pdf

Simple illustration:



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