MXR blue box transistors

[shawsofhell]

I just finished populating the pcb for the blue box and before boxing it up and trying it out I thought I'd look here for any tweaks/mods. I noticed alot of people saying that the 3904 transistors are not that great and that they should be substituted. What difference will i notice in changing that trannies?

[Mark Hammer]

One of the things not discussed much is the matching (or putative need for matching the two transistors used for gating purposes.  On the Tonepad "Caja Azul" schematic, these are shown as Q2 and Q3.  The envelope voltage coming from D2 feeds the collectors via R13/R14.  I gather the idea is that any sputtery octave down would be defeated by making Q3 need a certain signal level to turn on, and Q2 would turn on with it to keep any hiss resulting from the compound gain in the fuzz signal hidden until a criterion/supra-threshold note was played.

I could not tell you what parameters these two trannies might need to be matched on or how closely, but my gut instinct is that if they are poorly chosen, then either some component adjustments are required elsewhere, or the unit will not function well.  I've built two of them, and they have yielded only bare minimum signal output (with all the signal properties being appropriate except for level), so I'm kind of hoping for a useful reply myself.

Those transistors ARE the noise gates of the BB, so proper identification of their properties will likely make a big difference to a great many builders.

[jimbob]

Im waiting for a great reply too. I have had the worst luck with this build. 2X on etched pcb and still no decent luck.

[MartyMart]

Me too !
A "rebuilt twice and de=bugged til I'm blue in the face" version has
yielded some decent fuzz but very poor octave tracking .... wierd !!

MM.

[KORGULL]

I built two Blue boxes; one perf, one PCB.
I tried out alot of transistors and found that the highest gain MPSA18's that I had yielded the best results.
The difference in sound and tracking was very noticeable to me when compared to 3904's, which were the type with the lowest gain that I tried.
I picked transistors with matching gain readings. I didn't know if it would matter, but figured it wouldn't hurt.

Also, I prefer the "feel" or range of a linear taper blend pot over the log pot that was listed for this circuit.

This pedal seems to perform best when I use it with my EMG active pickup equipped guitar -  and the pedal must be one of the first in the chain.
I'm running it like this: guitar > comp > wah > Blue Box > dist/OD > etc...
I sold one of them to a friend who said it was useless. I asked him where he had it in his pedal-chain, he said last, I told him to try it first and he said it is 100 times better and he likes it now.

I really, really like this pedal and think it actually tracks pretty well and sounds great. It probably just needs the right setup (pickup selection, transistors, placement, etc...)

[Mark Hammer]

The stock unit has a blend and a volume control.  The problems you describe seem to be due to differences in input signal level/quality.

You will note that the gain is set to 471 in IC1a and set to 100 in IC1b.  That's a whole lotta cumulative gain.  Placing that later in the chain, especially if there is both hum and gain accumulation along the way, places the unit in great jeopardy of false triggering to light finger noise and other nonmusical things.

What I would recommend highly is the insertion of a 5k or 10k lin or log pot as a variable resistor in series with R5, and replacing R5 with maybe 470-680R.  This should anticipate a wide range of possible input levels, so that you get triggering but not false triggering.  You can probably live with dropping C5 down to 2.2uf or even 1uf.

[KORGULL]

Mark,
I tried alot of different mods (mostly ones you had posted) when I breadboarded this thing...
I don't think this one you just explained was one of them though - I will definitely give it a shot when I feel like messing with this circuit again.
-sounds very useful.

The stock unit has a blend and a volume control.

Right, so do the ones I built. I just meant that I stuck with the stock log pot for volume control, but switched the blend pot to a linear.

[shawsofhell]

Ok I'm going to try firing it up today so hopefully it works stock. I'll see if I can get my hands on some MPSA18s to see the diffence.

[Mark Hammer]

I'm curious about where the idea of a log blend pot ever came up, since it would likely be an error in the info you got.  The midpoint of a linear pot would provide a 50/50 balance of the two signals.  A log pot would not.  Whenever people ask the inevitable log vs linear pot question (about twice a year, on average), my response always includes some variant of the phrase "If you need to know where 'middle' or 50/50 or 'flat' is, then use linear".

Ultimately, no big deal though, since you caught it and fixed it.

[KORGULL]

It's in the layout/parts list @ Tonepad http://www.tonepad.com/getFile.asp?id=73

[Rob Strand]

Francisco and I did a lot of  research and off-line yaking on this effect.  There were errors on the original schematics.  The stuff at tonepad captured all that - it's as close to being true to the original(s) that we could work our. 

I vaguely remember some base/base-emmiter resistors were wrong on one of the schematics and that would make the tone more dependent on the transistor gain.    Otherwise I wouldn't have expected it to make much difference.  I dont' have the schematic with me but I thought the transistors didn't have anything to do with the tracking ability.

[Paul Perry (Frostwave)]

Looking at the tonepad bluebox, I'm suprised there is any tracking ability at all! And so far as I can see, no way even to adjust input level (except from guitar volume). What IS a novel point, one I had missed before, is the use of a variable V+ to the output transistors, (generated by the voltage doubler diodes) presumeably to put some envelope back into the squared up and divided signal. Suprised nobody has put that elsewhere!

[KORGULL]

If the transistors don't directly effect the tracking, maybe the difference I noticed has something to do with how well those transistors operate the gating part of the circuit.
Perhaps the 3904's aren't turning the gate on/off as completely as the MPSA18's, and more glitches/false triggering is being passed through - creating a messier output that sounds like the note you hit is mis-tracking, but is really just extra sounds on top of it being triggered by string noise etc..

It seems like the hotter the signal you feed the pedal, the better it tracks/sounds.

...don't know if I'm adding anything useful here, just thinking out loud.

[Mark Hammer]

No, this is good out-loud thinking.  Tracking is always an issue for octave dividers, and anything that leads to identifying ways to make the tracking dependable, without adding dramatically to the size/cost of the circuit is useful.

My sense is that more dependable tracking in the BB will arise from suitable transistors for Q2/Q3 (and that may well simply mean using something that meets a modest minimum spec, not necessarily the degree of matching and hand-picking one does for phaser FETs), some sort of input gain stage adjustment, and perhaps some very modest low-end rolloff (i.e., cap selection) at one or two strategic points.

I don't know that it will provide the degree of dependability yielded by something like the compander in the PAiA Rocktave, but it may go much of the distance with less nuisance/cost and still fit in a 1590B.

[MartyMart]

You're on to something here, my Boss OC-3 say's that using a compressor with
boost infront of it, will "improve tracking performance"  !! 
Also, i think it mentions "HB" equiped gtr use also ....


MM.

[shawsofhell]

fired it up this arvo and was unable to get it to work first go  Lucky for me all I forgot to cut a few of the legs of some components soldered to the board and they were shorting in some spots, clipped these and then fired it up again. I was greeted by a deep synthy growl. 

The pedal had good adjustment and the fuzz sound on its own is fairly nice. The octave sound tracked very well above the 5 fret which suprised me since alot of people were saying the 3904s wouldnt work very well. I tried it out using a es335 with fairly high output passive humbuckers and as noted by others the neck offered the best results. The only part substitutions I made was a tlo72 for the 4558 but I will try out the 4558 when I get restock them.

I am going to add the 1 or 2 octave down switch mod.