Zvex Distortron / Box of Rock

[Plexi]

Built one as request with low expectations, and finally I've get amazed of how great it sounds.
Nice compression and sustain.
I'll built one for me.

The only concern is about the C5K gain pot, I think I'll replace it for a B1K..where all the usable gain is.
Only tried a B5K there, and the clean to full distortion was almost at the last 30% of the sweep.

Any suggestions or experiences?

[antonis]

IMHO, it should be better to use a R13 by-pass capacitor for OD switch on max gain setting to avoid DC bias disturbance.. 

[Plexi]

IMHO, it should be better to use a R13 by-pass capacitor for OD switch on max gain setting to avoid DC bias disturbance.. 

Sure!
22uF maybe?

[antonis]

22uF maybe?

Why not..??

Any cap value greater than, 10μF say, would do the job.. 

You may form a frequency dependable gain by shunting R13 with cap's capacitive reactance..
( e.g. 330R // 194R at 82Hz for 10μF cap value..)

[Kennt82]

The C5K pot should even out the sweep on that 1st SHO boost stage.

[GibsonGM]

It's a great design, and only needs one more boost section at the end....   

[duck_arse]

IMHO, it should be better to use a R13 by-pass capacitor for OD switch on max gain setting to avoid DC bias disturbance.. 

at the very least, R13 shold be hard wired to the fet, and the switch wired open or short-to-ground. switching between ground and R13 will disturb antonis' biases very badly.

[Ripthorn]

I built one a year or two ago and really like it.  The C5k is chosen for a reason and I think it's as even of a sweep as you'll get out of this circuit.

[antonis]

IMHO, it should be better to use a R13 by-pass capacitor for OD switch on max gain setting to avoid DC bias disturbance.. 

at the very least, R13 shold be hard wired to the fet, and the switch wired open or short-to-ground. switching between ground and R13 will disturb antonis' biases very badly.

Maybe a 15% alter in V_GS bias shouldn't even be noticed, but for the sake of good design practice it shouldn't happen..

[Plexi]

Update!

- Q3 gain switch:
As many says, it bother some place. I hear an annoying squeal/oscillations, sometimes when turned the guitar volume down.
Even with bypass cap (10uF and 22uF) to ground.
Anyway, the amount of gain increase beautifully; I would like to keep it somehow.
I'll try later some small resistor (20R to 100R maybe?) and see if the squeal desapears.
Maybe it comes from my tester, or cables, and will dissapear as soon I box it.

- About the c5K/b1K/b5K Gain pot: only have/found b1k and b5k.
The first stage is basically a SHO, nothing more, nothing less.
I could use a fixed value here (maybe 180R or 330R), and place a 'master volume' pot at the end (after C2 maybe). But, it isn't the magic of the design I guess.
The b5k works worst than I've remember: almost the 90% of the sweep is clean, and the gain increase in the last 10% of sweep. (I know... I know, whit the C's sweep it wouldn't happen).
The b1K works great here, but at 0 it haven't a 100% clean. I'll keep this one.

[thermionix]

B1K with a 3K9 fixed resistor in series?

[Plexi]

B1K with a 3K9 fixed resistor in series?

That way it will be clean all the sweep.
The gain increase from ~2k to 0 resistance (emiter linked to ground).

[antonis]

- Q3 gain switch:
As many says, it bother some place. I hear an annoying squeal/oscillations, sometimes when turned the guitar volume down.
Even with bypass cap (10uF and 22uF) to ground.
Anyway, the amount of gain increase beautifully; I would like to keep it somehow.
I'll try later some small resistor (20R to 100R maybe?) and see if the squeal desapears.

Better try a 47pF (arbitrary start value..) feedback cap in parallel with R11.. 

[marcelomd]

Hi,

Maybe using an extra resistor with a linear pot you can get the sweep you want?

http://www.geofex.com/Article_Folders/potsecrets/potscret.htm

The box of rock makes some of my favourite tones. Only problem is it is so noisy, on breadboard at least.

[Plexi]

Found another schematic, and noticed that C7 is 100n instead 10n.
As I've readed on freestompboxes, 100n is the correct value.
I'll try, and now I think the tone switch will have more sense.

https://files2.soniccdn.com/old/1140704a981b9c69e93.gif

The only trouble that I can't get rid off, is the squealing when switch to high gain (jump 330R from S to ground).
I'm guessing is the test box (too much exposed). Maybe boxing and grounding the box this problem desapears?

[Plexi]

Another schematic:
http://www.freestompboxes.org/download/file.php?id=22948&mode=view