Substituting pot values

[ham_phist]

Question for you all:

I built the jhs mini bomb boost over the weekend and had to make some substitutions, but it was more just for experimentation. Bear with me, hopefully no one face palms too hard.

Didn't have any 22u electrolytics, so I wired two 10u in parallel for 20.

Didn't have any 5.1k resistors, tried a 4.7k.

This is the best (or maybe worst). Highest pot value I had was 100k, when it calls for 1m.

I wasn't shocked that it was less than perfect... lol. It did definitely work as a boost, but it is VERY staticky and noisy. Would that be directly caused by any of the above? I have four j201s from my friend who bought them a couple years back. Maybe a bad jfet?

Thank you all. I'm having a lot of fun with this. It's actually a bit addictive.

[Frank_NH]

Experimentation is why vero building is so fun.  I've done a lot of modding, and it is indeed addictive!

Your first two substitutions are just fine.  Remember that you can wire two resistors in series to get a value you need (4.7K + 390 ~ 5.1K).

For the pot, you'll eventually need to get a 1M value but in the interim you can solder a 680K to leg 1 of your pot so that you get the upper end of the boost, just for testing.

You have a lot of flexibility in part values as long as you don't go too far away from the nominal values (+/- 20% or so).  In some cases, depending on it's use, you can go way off.  Like pulldown resistors - don't have a 1M?  Use 1.5M or 4.7M.   Power filter caps are normally 100 uF, but you can substitute a 47 uF or 220 uF usually with no problem.

[ham_phist]

awesome! Can you think of any reason why it would be all staticky? And just to verify, by wiring them in parallel I just basically put two capacitor's leads together in each hole where I would just put one, right?

Thanks again man! I got an order from tayda coming, so I'll post when complete.

[ham_phist]

just an update. I took a hacksaw and sawed the tracks. didn't have a 680 K resistor, but took two 1mohms and put them in parallel with lug 1 of the 100k pot, so closer than before.

this time all of the static was gone. It still functions like a boost, but there's a ton of noise in front of it and the overall output is low. I will be interested to see how it goes with the proper pot and other stuff in there.

Thanks again!

[Frank_NH]

This effect may be noisy as it's a weirdly biased jfet amp circuit (unlike the convention common source jfet amp).

If you want a simple build that provides a very nice crunchy boost, try the eqd speaker cranker:

[Beaker]

If you are testing it before boxing it - in other words with no enclosure, you should expect noise on most builds, especially boosters or anything high gain. Weirdly I get German radio stations coming out of some of my pedals when testing - and I'm in the middle of the UK!

Remember the metal enclosure will act as a Faraday's cage, shielding the circuit from EMF radiation, so when you box it up, ground it correctly and screw the base plate on, you can expect most of the noise to go away.

If it's still noisy as hell, then you safely say that you have a problem - either with parts - not usually likely, or with your build - much more likely.

[ham_phist]

Definitely my thoughts as well. Thanks everyone for the help. Once I slap it in an enclosure with the right pot I'll update :)

[rocket88]

Haha. I get German and russian radio with a few effects unboxed, once I got polish radio. It was interesting cause the effect worked, but the radio was in the background. I almost wanted to keep it like that cause if was just neat.

[ham_phist]

Hi all, thanks again for all the help. Just an update on the mini bomb boost- got a 1M pot and replaced the two electros in parallel with the single one of the correct value and it seems to work just fine. Going to try to put it in a 1590A, it will be my first boxing actually. Gonna be REAL careful.