Looking for a little help with Joe Davisson Shocktave.

[DavidS]

I feel a little embarrased asking for help with a circuit with such a low parts count, but this thing is driving me nuts!

I've got it put together on a breadboard, and I'm getting weird results. Very weird.

The sound is gated in the extreme. If I pluck REALLY hard, I get a little fart of a sound that lasts about an eighth of a second. And this is out of a dual EMG active humbucker bass running on 18 volts!

The audio gets through the emitter follower just fine, but after that comes the messed up gating. Q2 is outputting nothing, unless I whack the hell out of the strings. I've pentuple-checked the wiring, and measured all of the components, tried 5 different 2n5089s.

Oddly, I've hot this horrible contraption called a noisestick (from carrionsound.com) which has outrageously high output, and it works just fine through it. Also, if I put a Big Muff with the sustain and output cranked in front of the Shocktave, it sort of works, but there is still alot of gating.

Any ideas???

[Joe]

Q2 is probably misbiased, adjust the top 10k resistor until collector is at 4.5v. Also, check the orientation of the diode.

[DavidS]

If you mean 4.5v with respect to ground, that'd be one heck of an adjustment, it's barely below +V right now...

[Joe]

It's the biasing. It wouldn't be uncommon for the collector resistor to be 15k, 18k, or even 22k, whatever gets it in the ballpark of 4.5v.

[DavidS]

OK, I've been reading over some biasing info, and I'm rather confused about something. Isn't it the base that needs to be around 4.5v? I thought the collector resistor is just current -limiting in this circuit. How would I adjust the collector voltage without a voltage divider anyway???

Sorry if I'm being dense, never biased a transistor, but I'm trying to understand.

Thank you for your patience!

[Joe]

The collector resistor acts like the top half of a voltage divider, with the transistor and emitter resistor making up the bottom half.

[DavidS]

OK, somebody slap me. I looked over the circuit about ten times, and just now I realized "Hey... There's no connection between Q1's collector and V+...."

Yeah, I'd call that mis-biased... Ugh... That's why I was seeing such weird values on Q2, and confusing myself about what was going on.

Thanks for your help, Joe!

[toneman]

So......did U get it working??

INquiring minds etc etc etc....



Ahhh, now I C how to insert icons.......  !!   

[DavidS]

Oh, yeah, it works!

It's an awesome effect, and an easy build too, as long as you actually make all of the connections specified on the schematic...

Anyone out there using one of these with a bass? If so, how's the note tracking on the lower end of the register? the lowest fifth or so in particular (on a 4-string)? Mine seems a little hit-or-miss when you get down that low, but I've got a few funky component values, as I just threw it together on a whim with what I had on hand (5 .1uF caps in paralell to make a .47uF, for example). I like this circuit alot, though, I'll probably order all the correct components and box it up.

For a real thrill, try one of these in front of a Bass Balls! Pure evil! Very funky.

[DavidS]

Looking at the schematic, I'm getting an idea.

Could the tracking be improved by adding a buffer after Q2, followed by a lowpass filter, feeding the charge pump?

[Joe]

I don't know, but you'll still want to check the collector of Q2. It will work best & sound cleanest when it's at 4.5v. The .1uF caps on Q4/Q5 can be adjusted for different instruments. I'm guessing .22uF is right for bass.

[DavidS]

Yeah, I got it pretty close to 4.5v with a 12k resistor, sould be good enough. I'll try those capacitors, thatnks for the tip.

[DavidS]

Tried changing the caps (actually just added another pair of .1uFs) and it didn't help much. Seems to be more of a tracking issue, so I'm thinking a lowpass before the charge pump might do the trick.

[Quackzed]

i noticed that q2's bias has a big effect on the tracking...to little gain and the note dies too fast..(sounds like an atari sound effect comes in as the note dies/ more noticable on high notes that don't sustain as long). too much gain and the added harmonics confuse the tracking and it jumps from low octave to normal octave. i thought about some kind of lowpass filter to keep it tracking. i also noticed that mine tracks better with my sg(humbuckers) as opposed to my single coil equipped guitar(also i dont have tone controlls on the s.c. guit.
    - if  i find a spot that works well ill post it. but because i like the sythy high end buzz(description??!!) i might put the low pass on a switch so i can go back and forth...
  - i did alot of poking around in this circuit due to some shoddy wiring on my part but i like it alot!!!
  and because it took some tweaking for me, i'm maybee less reluctant to add/remove/adjust it due to having a (somewhat) clearer idea of what does what in there...i plugged my bass into it and the damn thing tracked the whole fingerboard!!! i thought the low e on my guitar was getting too low for it but NOW i'm pretty sure that it's the high frequency content/gain that causes tracking issues
so i figure a lowpass is the way to go...
still it tracks really well once you get in the "zone" soft/moderate pick attack. and i find myself hitting a new note when i hear the first one start to fade...
also the cleen side on mine has some nice fuzz... i may also tack on some resistors at the oct end of the blend pot as i can still hear a bit of oct down at mix 100%cleen... fun pedal, learned the mario brothers theme!!! zelda...star wars.. but this thing's not just for nintendo sounds!!! @50%blend sounds huge and evil!!! very heavy!!! like black sabbath 9000!!!

[jmusser]

I must have gotten lucky with my build, but I never had problems with biasing or tracking. Mine worked great right off. Mine tracks really well, and a whole lot better than say the MXR Blue Box. You can get some nice thick chording with it, if the blend is backed off like you said. As for the miswire, that stuff happens. If I get a problem like that, the first thing I do, is get a fresh schematic and a highlighter, and go through the circuit. About 9 times out of 10 that will tell me what I've left off or connected wrong. Usually about the only time I find misbiasing problems, are on the older circuits that used specific transistors that can't be found anymore, and the subs will not bias at all with the resistors for the original design. I am a big Joe Davisson fan, and he has designed some real nice circuits. So far for simplicity and huge tone, I have not found any to equal his Easy Drive. That one is a sweetie! Not only is it simple, but it's biased in such a way, that you can just about plug in any transistor of the same type in the circuit, and it will work and sound great. How many circuits do you know of that can be that universal?

[RDV]

Mine tracks really well with a fuzz driving it, and sustains forever that way.

Thanks again Joe for your contributions to DIYdom!

RDV

[Paul Marossy]

I  must have gotten lucky with my build, but I never had problems with biasing or tracking. Mine worked great right off.

Same here. 

[Quackzed]

oh yeah, it definately tracks great!! i just happened to notice the effect of messing with the bias... and noticed the weird effects it had on the tracking... tho i think i was trying to get more sustain and gain from the first stage...and ended up liking the slightly fuzzy clean sound(now the clean notes gate out with the octave down but not quite as soon)
i loved the mad scientist factor of this build, and had to get in there and see what did what.
at one point i had a led/resistor going from one(inverting?)input to ground that lit up when you played...wanted to make the enclosure "light up" electricity blue for added mad scientist vibe 
because this pedal just oozez robotic synthy future metal sounds to me!!

also RDV suggested trying the tonebender in front and that definately kicks this box into nice solid sustain buzz  

[cab42]

I think I have to look at the bias of Q2. I only measure 1,8V on the collector with a fresh battery.

Allthough it gives an interesting sound, like it is jumping between the octaves.

Regards

Carsten

[Quackzed]

yeah, if its sustaining good and the octave is juming back and forth that means theres too much gain going into the "octave section"(q3 4 +5) if its gating out fast and theres not much sustain, that means not enough gain going into "octave section"...try a 100k pot instead of the 10 k on q2 to try and dial it into a good range for your transistors...also remember that neck p.u./tone rolled off will give you the best tracking...
you can also add more resistance between the gain pot and the emitter, (i took out the gain pot and used a 4k7 in parallel to ground with the 10uf cap. from the q2 emitter. and my q2 collector is a 15k
i broke my v-ometer so im not sure where the voltages are but i think i read 4.5-5 volts on q2 works best.
also i used a 2n4401 as q2 which is a pretty low gain tranny (all i had) so my solution may not be right for you but...like i said too much gain from q2 and you get jumping due to the added harmonic content of too much gain( too little gain and it doesn't sustain well and gates out too soon... hope this helps
but im just a tinkerer and this is just my personal experience buildng this one, not a dfinative answer
still i hope this helps...