A little MkII/Supa Fuzz help

[Bret608]

Hi all,

This summer I built a Marshall Supafuzz according to the layout at Tagboard Effects. See below:

http://tagboardeffects.blogspot.com/2012/02/marshall-supa-fuzz.html

It did work initially (quite rocking, actually), but I was having some scratchy sounds when I tweaked the volume knob. I hypothesized that I had a bad input cap (since I grabbed electros of unknown age where I work), so changed it out two or three times. Each time when I put it on my test rig (which I power with an Ibanez wall wart, so have to reverse the power and ground leads for this PNP circuit), it would sort of "charge up", come to life and then be fine other than the scratchiness coming back.

So this week, I swapped out all the electros for fresh ones that I got in my last Small Bear order to see what would happen. Same sound-fading-in behavior, only this time it didn't get up to full fuzz. On a whim I soldered in a battery clip--black lead to the board, red lead attached to the board and pot ground wires with an alligator clip. I only used the in and out leads on my test rig. Instant improvement! Less noise and no scratchiness. But the sound is basically clean, and the pots are acting weird. The volume pot has the same volume level, all the way down to zero. It just makes the tone brighter or darker. I can't tell the gain pot is doing anything, other than an increase in background hiss at max rotation. Also, the bias on the Q3 collector is about a volt lower than when I checked it this summer--about 5.7v rather than 6.6v.

So my question is, am I hooking this up wrong, now that it's not on wall wart power? Or have I broken a wire for one of the pots internally (can't see any visible breaks)?

Thanks a million,

Bret

[LucifersTrip]

there's multiple things going on there and you're changing too many variables without fixing one first.

1) it was working and you changed stuff, now the pots are "acting weird".  something's hooked up wrong, there's a bridged or missing connection somewhere, something burned out during soldering, etc

that's step one...get back to where you were before the cap switching. did you take all voltages when it was working so you can get back there? what are the voltages now?

2) wal-wart vs battery. in a very high % of MKII's I've built, they didn't sound good with a wal wart right off. the volume drops and it sounds wimpier, thinner....just not good in some way. a 100 ohm resistor coming in on the hot line of the power supply always solved the problem. ie, look at this current schematic jimi posted:
http://i605.photobucket.com/albums/tt137/pinkjimiphoton/FUSZOUND1schem_zps60727fe8.png

3) the voltage dropped with battery. it's very possible the wal wart was a higher voltage?....or, of course, the error you caused during cap switching is responsible for the change

4) even when you had 6.6V before, that's not high enough since originals are in the 7.5-8+ range
(though, if you like it where it is, keep it...this has nothing to do with the above problems)

[Bret608]

You are correct; I didn't use the best debugging methodology here! So I appreciate your response.

I had time to look at this a bit more this morning, just checking wiring, continuity, possible bridges, etc. I noticed something that I hope will shed some light.

This thing passes signal with no transistors installed, and doesn't sound a bit different (i.e. a loud, clean signal with basically non-functioning pots)! I don't know much about this circuit, but would that even be possible with a working build?

[LucifersTrip]

This thing passes signal with no transistors installed, and doesn't sound a bit different (i.e. a loud, clean signal with basically non-functioning pots)! I don't know much about this circuit, but would that even be possible with a working build?

always check out the schematic first:



orig
http://forum.musikding.de/cpg/albums/userpics/16602/MKIIOC75schem.png

[dwmorrin]

It is not possible for a correctly built tone bender to pass signal without the transistors installed.
You've definitely wired it incorrectly.  Other than starting anew, maybe you could post pics for help.

Check that your in/out jacks aren't wired backwards.  If the in/out cables have a common shield connection, then you'll get sound through by connecting both tips to any floating ground.  This could be what you've done.

[Bret608]

I bet I have hooked this up wrong to my test box somehow as I haven't changed the in/out and pot wiring from when this was originally working (and a continuity check reveals no broken wires/connections). The only difference is the battery clip usage. Let me describe how I'm hooking this up.

So, my test box (a bare metal 1590b) just has in, out, +9v, and ground alligator clips. On the Supa Fuzz board, I have the black battery lead going into the -9v connection. The red battery lead is dangling free--when I tested, I used an alligator clip to connect it to the positive ground wires coming off the board and the volume pot. The power and ground clips on my test box aren't connected to anything.

This is the first time I've used my test box with a battery at all, much less a positive ground circuit, so let me know if this is the problem.

[Bret608]

This thing passes signal with no transistors installed, and doesn't sound a bit different (i.e. a loud, clean signal with basically non-functioning pots)! I don't know much about this circuit, but would that even be possible with a working build?

always check out the schematic first:



orig
http://forum.musikding.de/cpg/albums/userpics/16602/MKIIOC75schem.png

I just want to make sure I've thought through what you offered here! If I follow your logic, with the transistors out of the path, the only way for the signal to reach the output is through the power path. So if that's also happening with them in the sockets, it follows that the audio path has somehow connected to the -9v path, right? And would the method I described of hooking this to my test box allow that?

I always feel like I learn a lot from a good debug! 

[pinkjimiphoton]

to ME. it sounds like what happens if you mistake the wiring on the input jack between audio in and ring to ground. if you reverse them, you'll get exactly what you describe.

don't ask me how i know about this. 

[Bret608]

Thanks Jimi! I think this is getting me somewhere. I think I can phrase a question now that will get me to the bottom of this, provided I haven't screwed something else up that I haven't discovered yet.

If I want to test a PNP fuzz circuit unboxed and with a battery, what should the red lead of the battery be connected to? Ground or something else?

'Cause right now, it's most definitely connected to the board's ground.

[dwmorrin]

Red lead of battery to ground, black lead to -9V.
Try it without your test box. New jacks to be sure of the wiring.

[pinkjimiphoton]

bret, if it's connected to ground, that's your problem.

ideally, the black battery lead should go to what would be +. the grounds should go to the ground lug of the input jack, and the red lead should go to the RING connection of the input jack. that way when you plug it in, it completes the circuit to ground and turns it "on".

i bet this is your problem, you describe EXACTLY what i encountered the other day!!

and i've already testified what the fix was... good luck, and keep us posted bro!!!

[Bret608]

Thanks to Jimi and LucifersTrip, I was able to get this taken care of. I just ditched the battery snap and went back to the ol' wall wart on the test rig. I did include the 100r resistor in series with the -9v input as you suggested. The circuit is still a temperamental beast (supposed to be I guess), but it did help somewhat. Especially if I turn my guitar's volume pot all the way down, I no longer hear a loud wall of hum that doesn't go away!

One funny thing I've noticed with this build is that every time I change a part, goof something up, or whatever, when I get it working again it almost has to recalibrate itself a bit. By this I mean weird stuff like the max fuzz will be happening at halfway on the guitar's volume pot rotation, the sound will drop out if I turn that all the way up, etc. But it always sorts out and gets back on track after the first few time or two I plug it in. It's almost like it needs some "burn in" time for the bias to get going right. No idea it if's just my build or the nature of the beast. I have learned from this one though.

[pinkjimiphoton]

hey bret,
do you by chance wire your footswitch so the input of the fuzz is shunted to ground during bypass? that can cause the phenomenon in fuzzfaces and tonebenders you describe... the weird biasing when you first fire up.
if you used germanium especially... when you shunt the input to ground i think it makes the fuzz take a moment to recover, almost seems to "swell" in.

temperature changes can do it too.. a hot iron being within 5-6 inches of a ge tranny will change it's gain.

i think it has something to do with  the caps charging too.. until they're fully charged the bias can be unstable.

i actually lummoxed across something i built... broke, more like it, but hey, it worked out.

i used a 1044 cp so i could run my face in series with other pedals on one power supply.

i had figured, well, i'm gonna use humongous caps on it. more caps=more filter=cleaner louder fuzzface, right?

well, i screwed something up cuz i still need to give it it's own wart, but found an interesting side effect to my f'up.

now, the fuzz won't work at all until the charge builds/temp goes up/something.

it's a trip. you power the board. everything lights up except the fuzzface. BUT somewhere between 5-15 minutes after it's juiced, the led comes up, and suddenly the thing works perfect.

it's the strangest thing. no idea what i did or how, hell, no idea why the FF i built even works cuz i am SURE something's wrong with it!!

but it does. one day i need to investigate and figure out what i did and why it works like that, it may save some other fuzzaholics some grief some day!

[Bret608]

Thanks for sharing, Jimi! Fuzz is definitely my main thing but it can be seriously weird sometimes. I'm sure my occasional bin-diving for parts of, shall we say, unknown vintage in the electronics lab where I work (a tech college) doesn't help. Glad to know I'm not the only one with the occasional swelling in thing.

This fuzz is not boxed at the moment; it's just hooked up to my test box with clips. I use the one from the Madbean forum:

http://www.madbeanpedals.com/forum/index.php?topic=1140.0

I don't know if technically the input is shunted to ground in this situation; and actually my box has no footswitch and is always "on" you could say. I bet it's more likely the caps charging up for the first time, as new caps is when I get the behavior. They are most definitely ge transistors--some nice, low leakage General Electrics 2n1305s. There was a handful at work and I'm about the only person here with any use for them. When I tested them a TB set pretty much jumped out at me. I've also got a low-gain one on my Juergulator, by the way!

[pinkjimiphoton]

awesome. always good to know more fuzzaholics. i am an unrepentant one, i just cracked another rare jewell with the help of some friends that i'm hoping is gonna be really cool.
definitely in new caps, and DISCHARGED caps. with no switch, i don't think that could be the issue, so i'd say you've got it all figured out.
good luck on the juerg!!
it's a cool box.. the extra knobs have this je ne se quoit or however ya spell it.
they don't seem to DO anything sonically as much as they do to the feel and response of it.
i put a switch on mine between a ge and a si that were similar in gain and tone... i do that a lot these days.
i call it the 7W1573D 7R@N-51S73R QU1<K <#ANG3 5OCK37

i assume you are fluent in 1337. i am not.