Getting Metal Simplex Running, and Tuned up; Questions

[AeroSigma]

I got my first DIY stompbox running yesterday (yay!), Howie's Metal Simplex. I'm a decent electronics hobbyist (mechanical engineer by trade) so I know my way around a soldering iron and schematic, but I'm used to trying to get clean high-fidelity sound from my audio circuits, and using idealized components. So as I'm still learning the particularities of distortion circuits, (and in particular purposefully running components to their non-ideal range) I was hoping you all could help me understand what's going on with my circuit as I tune it up.

Last night I got the Metal Simplex definitely working, but it was sounding a bit underwhelming, and I'm looking for a little advice. I've read through the debugging guide, but that focuses on not working issues, and my pedal definitely works, just has a weak effect. Also, I admit that I haven't spent as much time as I'd like fiddling with it (after an hour it was 1a, too, so I had to go to sleep, and my gain knob is very hard to turn.)

Pots: Some of my parts came from the hand-me-down pile, and what was lying around, so they're of dubious quality, especially the pots. When I twizzle the tone pot (big black one in the pic, 'high precision') the sound (complete, not just effect) will cut out for a little then back in as I turn it more. Is it actually possible that the pot is damaged and dead spots could cause this, or is there something else going on in the circuit that I'm changing as I move (bump?) the board?

Eliminate Tone Pot: On that note, I've seen people talk about eliminating the tone pot, which I'm thinking of doing since my actual pot is (could be?) crap. How would I modify the schematic here though, just pick a resistance that I like (right in the middle?) and create the fixed voltage divider there with say, 2x 25k resistors, pulling the vol from the node in between?

2N2903 Transistor: Again, as a part on hand, I subbed the 2903 for the 2904 that the schematic calls for. I've got 2N2904s and 2N5098's on order, (the latter which is purported to have a more metal sound,) and I'll try them out when they arrive (you can see from the pic, I've socketed the trans and LED.) My research showed that the 2903 is a crappier version of the 2904, and I would assume that it would give maybe a less clear, more noisy, but possibly more clipped effect. Is my understanding of transistors off here? Do I just need to turn up the gain?

LED Diode: I socketed this, and can tell a noticeable, but subtle difference in LED, so I assume it's doing its job. However, none of the LEDs visibly light up, did I miss something, or is this just because I'm using high-brightness LEDs?

Double Diodes: I'll try using two diodes tonight, but I'm not quite sure how that should affect the sound. I read a great walk through of a pedal yesterday (by R.G. Keen?) where he went through each section, and described what the components all do, but I can't find it now, does that ring a bell for anyone?

Thanks! I know there's a lot of good information out there, but I want to make sure I get this pedal tuned up and sounding good (or sounding correct, at least :p) before I try out a more complicated one like the Insanity Box or Skyripper.

AeroSigma

[thermionix]

2N2903 Transistor: Again, as a part on hand, I subbed the 2903 for the 2904 that the schematic calls for. I've got 2N2904s and 2N5098's on order,

2N3904, 2N5089

Don't know what 2903 refers to.

[Kipper4]

Welcome Eric.
You are in the right spot.

R.G. Who?
Just kidding sir.

Your a mechanical engineer. I'm jealous already.

The search function top right of the page will be your friend too.

Enjoy your journey into stompboxs.

Rich

[AeroSigma]

Thanks!

Ya, I've searched around, mostly on this site and GEO, and I've learned a ton such that most of my questions have been answered. However, I haven't found specific answers to the ones I've posted, so I was hoping someone could pitch in. Sorry if I'm rusty with my search-fu, but I'm still unable to find good answers to how you remove the tone pot but leave everything else OK, so I would really love some help there. And I literally spent an hour  searching for that schematic walk-through which I think will answer the single vs double diode question.

Sorry, 2N3903. Searching around a few days ago, I found that the 2N3903 has "lower gain, and half the bandwidth", but I'm not sure exactly what that means when we're operating above it's normal capacity on purpose, and how would affect the pedal, so I'm wondering if you could help me reason that out. Now that you reminded me of the correct part number, some searching on this site shows that it "sounds fine" in this circuit though I'm still curious what exactly the difference in performance does for overdrive effects. Maybe that's an academic curiosity at this point, but I do want to really understand what's going on in these circuits, rather than just assembling schematics.

Thanks for the warm welcome!

[thermionix]

Can you post the whole schematic?  I see part (most?) of it in your picture.  Seems they specify particular hFEs, but it probably isn't super critical.  2N3903s would fall far below those numbers though.  Smaller bandwidth probably wouldn't be a concern for guitar audio purposes.  2N5089s would likely have hFEs above 500, and better signal-to-noise ratio than 3904s.

Also, is that a solder breadboard?  Are the black things sockets?

[AeroSigma]

Thanks! That helps to know re:bandwidth. Am I right in understanding that better S:N will clean up the sound, injecting less noise into the audio signal, while still providing the desired distortion to the waveform?

As to schematic, it's the Howies Heavy Metal Simplex from the schematics page linked on the top of the forum.
Yes those black things are sockets, clipped female headers, and that's the Adafruit perma-proto board which I LOVE, by the way. Solderable, but same pinout as breadboard so it's easy to transfer circuits over, without having to re-layout and worry you screwed up node [emoji14]

Sent from my SAMSUNG-SM-G920A using Tapatalk

[AeroSigma]

Finally, I got a chance to get back out to my shop this evening and tinker some more. Looks like I have the gain pot in backwards, and was turning it _down_ last night! I guess using a difficult to twizzle pot, and working when you're tired doesn't lead to good troubleshooting! With the gain up, it was sounding much better, and the LED was lighting up when played, so that answers that!

I did try out double diodes, a reverse direction LED across the same nodes, but the reverse led wasn't lighting up, nor making any difference in sound, so I still don't understand that effect :p

Sent from my SAMSUNG-SM-G920A using Tapatalk

[thermionix]

I see we have the same taste in batteries.  Two for $1 at Dollar Tree.

Let me be clear, I'm not any kind of engineer, and I'm still a relative beginner at pedals and solid state electronics.  The 2N5089s are marketed as low noise transistors, but I suppose that means a lower amount of noise (hiss) for a given amount of gain.  So they might offer more distortion in the circuit you're building, probably with no corresponding increase in noise.  But that's really just a guess.  Best way to know if you like the character of the distortion is to try them.  You said you had some ordered.

[allesz]

I woul do a dirty test: swap the gain pot with a plain 100k resistor, going from c to b of q1; then take off the led, leaving only the 100k resistor joining c and b of q2.
Now you should have a big, and probably already clipped, boost.
If not, then there is a mistake somewere. You can also try different trannies in this configuration.

Then, if the thing works, add the led and the gain pot... and the you may want to try the tone pot. I thin that a nice mod would be a resistor (10k, 22k?) in series with the gain pot, so when the pot is at 0, q1's c and b are not shorted.

[duck_arse]

also welcome. can we see a photo of your pot collection? from what I've seen, it must be a doosie.

leds as clippers are not indicators. they may light or not. if you want them to light, provide them with current. as for your blank spots when you turn the gain pot, if it still happens, this might be blocking or gulp distortion, or similar. when you turn the gain pot, it affects the transistor bias, which shifts [? both the base and ?] the collector voltage. this will disturb the cap voltages, which will take an R//C time constant to equalise, or like that.

clipping diodes - diodes all have a forward voltage, lower for Ge, Si in the middle, leds at the higher end. used one out, the lower Vf limits the clean signal before clipping, so you get more clean, and more volume, when you use a led. stringing diodes together adds their Vf's.

transistors - generally, in these type circuits, put em in, see if'n you like. your circuit shows a higher gain part at Q1 and a lower gain at Q2. you can always get a relative 'idea' of transistor gains with the transistor tester on your multimeter. your multimeter, right?

[AeroSigma]

Thanks for the warm welcomes!

Yes, that's the way to do 9Vs. I also live w/i walking distance of a Dollar Tree, so I probably get more things there than I should!

As requested, the re-use drawer at work:

I suspect that some of these pots are older than I am. And we have a full size 2 door cabinet full of old parts, not just pots! Needless to say I have another tab open where I'm ordering new Pots for the build.

I'm excited to try some spare diodes and LEDs that I have laying around as well, and those other transistors that are on the way. I found 2x 2N3904 as well that I'll swap in this weekend and see how that effects the sound.

I'll order a 150BB as well, since that seems to be the favorite enclosure, but I'll probably do another pedal next I could use it for anyway, and I have a plastic enclosure at home. Has anyone tried spray adhesive and aluminum foil for lining a plastic box to get the Faraday-cage shielding from RF?

[Kipper4]

Mojo pots.

[PRR]

> mechanical engineer by trade
> eliminating the tone pot, ...my actual pot is ..crap. How would I modify the schematic

You are engineering a bridge. The lowest-bid contractor delivered a section too bent-bad to use.



Cut off the crap part, nail the loose ends together, done.

Yes, in a real bridge you can't shove the banks together, why you designed and budgeted for a 2-section bridge. Electronics is different. If the traffic (audio) gets all the way to the end, job done.

[PRR]

> just pick a resistance that I like (right in the middle?)

The idea is to be adjustable. Apparently someone thought that was a good idea.

Now you are engineering a car-lift. The client wants it adjustable from low (for bodywork) to high (for muffler work). But the adjuster screws/racks/cylinders at hand are crap. Can you build it to a fixed mid-height? At least so some repairs can happen while hunting for proper adjusters?

Of course you "can", though you won't expect full payment until the specs are met better. The muffler guy will complain about squatting, the body guy will have to stand on a stool, so not complete happiness. Judicious choice of "middle" may help. The squatty muffler work suggests favoring the high side, and using platforms for body-work. It may be they don't need 48"-94" at infinite resolution, but can manage with 2 or 3 fixed heights (switch instead of pot).

[Tony Forestiere]

> just pick a resistance that I like (right in the middle?)

The idea is to be adjustable. Apparently someone thought that was a good idea.

Now you are engineering a car-lift. The client wants it adjustable from low (for bodywork) to high (for muffler work). But the adjuster screws/racks/cylinders at hand are crap. Can you build it to a fixed mid-height? At least so some repairs can happen while hunting for proper adjusters?

Of course you "can", though you won't expect full payment until the specs are met better. The muffler guy will complain about squatting, the body guy will have to stand on a stool, so not complete happiness. Judicious choice of "middle" may help. The squatty muffler work suggests favoring the high side, and using platforms for body-work. It may be they don't need 48"-94" at infinite resolution, but can manage with 2 or 3 fixed heights (switch instead of pot).

I don't understand how Paul's analogies constantly and consistently make perfect sense in my "Mind's Eye". I can visualize the physical concept he is describing in this post.
Backhoe hydraulic lines failing due to excessive "current", HVAC ventilation degraded by "resistance", and (a particularly insightful vision), I interpret "The Waste Water Drain" analogy to an obstructed path to ground. 

Thanks PRR.

[thermionix]

can we see a photo of your pot collection?

Well he's from Portland, maybe a sample in the mail would be better?

Has anyone tried spray adhesive and aluminum foil for lining a plastic box to get the Faraday-cage shielding from RF?

Some people here use plastic boxes sometimes, but with finished aluminum boxes available for $6 or less, I don't see the cost:benefit ratio working in that direction.  Then there's the old saying, "buy once, cry once," or you might save $3 using a plastic enclosure, only to move the circuit over to a metal one later, thus buying both and wasting money.

When people do use plastic or wood, I would guess that they usually shield the box with self-adhesive copper or aluminum tape.  That stuff's not cheap though, at least not the copper.  Personally I've never had a good experience with spray adhesive, but I haven't tried it for foil on plastic.

[AeroSigma]

Maybe that's my problem though, as a mechanical engineer (which is quite fun, btw, Rich), I CAN'T just drag the river bank over and expect the traffic to still get through! I like the analogy though, it helps me think more like an EE for this, thanks.

I ordered a 150bb, but  went ahead and dropped it in the plastic box i have lying around in the mean time. Mostly in just worried that the cord will drag it off my work bench while I'm fiddling with the diodes or something and I'll break it. I'll definitely put it in a metal enclosure as soon as I can though, thanks for the input.


As you can see, I decided to keep the tone pot too, for the adjustability, and, of course, extra mojo




Sent from my SAMSUNG-SM-G920A using Tapatalk

[thermionix]

Is the black pot a 10-turn?  That'll get old after a while.  "Hold on, I'm still adjusting my knob!"

[AeroSigma]

Yes, it sure is. I was told it has an extra one mojo per turn! 

I've got replacement pots coming in with the Al box, they'll get swapped out with a more homogeneous selection.

[duck_arse]

you know the replacement pots won't have the speed holes/weight savers drilled in any top plates, don't you, Aero?

are turns measured in mojos, or the other way around? "this MA510 diode rates 7 turns mojo, but only with the speedy pots."