Chop Shop Sag knob

[mdc]

Quick q for anyone who's built a chop shop—I just boxed one up and the sag knob doesn't seem to be working properly; I'm wondering if anyone else has encountered this issue...

My understanding is that it's supposed to *increase* voltage as it goes from fully ccw to fully cw and thus get cleaner/brighter sounding.

On this one, the cleanest signal is at fully ccw and then it gets progressively more and more sputtery from full ccw to about 10:00, then the volume drops off sharply and it's silent by about 12:00 or so onwards.

Thanks in advance...

 

[Nostradoomus]

The way it’s wired in the schematic reflects what you are experiencing. Nothing wrong with it! Maybe try a 5K pot for the sag.

 

[mdc]

Wild—it doesn't seem like a particularly useful control as it is, but I'm glad the circuit is at least working as it should be.

 

[scheffehcs]

I’ve built two, and the voltage decreases as you go CW. However, I never lose signal. It starts getting sputtery around.. 2 or 3 o’clock I think? I can check for you later if you’d like.

 

[Nostradoomus]

I’m sure it has plenty to do with the J201’s Vgs off point. If I built this I would replace the 8.2k and 9.1k resistors with trimmers to really get it set.

 

[mdc]

Interesting, I’ve heard the through-hole J201s are really inconsistent (which is what I used). They’re socketed, so I’ll try a couple of different ones and see if it makes a difference. I also have some smds and boards around too so maybe they’re a bit more reliable.

 

[mdc]

Swapped out the J201s for a different pair (still through-hole), and no change. Sag knob works from fully ccw to about 11:00 or so and then no output. I'll give the smd a go tomorrow and see if that makes a difference. Here's hoping I can get it stuck onto the connector board...

 

[Nostradoomus]

Try lowering the pot value first and see if it makes a difference, can’t hurt!

 

[mdc]

Good call - I think I have some B5Ks here, though I'm not sure if desoldering board mounted pots or dealing with SMD parts is the lesser evil.

 

[phi1]

If it were me I’d try the smd j201s. Apparently mouser has good through hole j201s, but most stuff from eBay is garbage. Since switching to using smd j201s and 2n5457s, I’ve gotten very consistent results when measuring IDSS and Vp, so to me it’s really worth it. Let us know if you go that route if that solves it.

 

[mdc]

B5K did the trick - thanks for the suggestion! Sag knob works from ccw to cw.

 

[SYLV9ST9R]

I’m sure it has plenty to do with the J201’s Vgs off point. If I built this I would replace the 8.2k and 9.1k resistors with trimmers to really get it set.

That's what I did on my build. I used already soldered smd J201 from pedalpcb.
The trimpots are really nice to help you change the character of the distortion (and to bias the JFet, which is beyond this noob, but I went by ear comparing it to my Modèle B).
I also had a small dead spot on the Sag knob, but was still pretty usable. Didn't have time to play that much with it though, as it was for a friend, and I dropped it in his city while I was there last week.

 

[mdc]

If it were me I’d try the smd j201s. Apparently mouser has good through hole j201s, but most stuff from eBay is garbage. Since switching to using smd j201s and 2n5457s, I’ve gotten very consistent results when measuring IDSS and Vp, so to me it’s really worth it. Let us know if you go that route if that solves it.

I socketed the JFETs so if I can get smd J201s onto the pads without destroying them i'll see if switching them out does anything further. FWIW, these through-holes came from a surplus place and are (allegedly) NOS, not from eBay. But from what I've read the through-hole JFETs tended to be wonky even at the best of times.

That's what I did on my build. I used already soldered smd J201 from pedalpcb.
The trimpots are really nice to help you change the character of the distortion (and to bias the JFet, which is beyond this noob, but I went by ear comparing it to my Modèle B).
I also had a small dead spot on the Sag knob, but was still pretty usable. Didn't have time to play that much with it though, as it was for a friend, and I dropped it in his city while I was there last week.

Did you replace the 9.1K and 8.2K resistors with 10K trimmers? Any trouble getting everything to fit on the board?

 

[SYLV9ST9R]

Did you replace the 9.1K and 8.2K resistors with 10K trimmers? Any trouble getting everything to fit on the board?

I replaced both resistors with 20K trimmer because I had those available, and it gives you an additional range of values to play with.
Didn't have trouble fitting all on the board, I used the Trimmit! adapter and pin headers and fitted really well (gutshot).

 

[mdc]

nice! those adapters look so handy

 

[SYLV9ST9R]

nice! those adapters look so handy

They are. This was only my second build, and it went smoothly.

 

[Einzeln]

Just finished one. Last 25% of the SAG is dead. Lowering the pot will only keep the bias high enough right? SAG at zero won't change?