Sola MKIII and guitar's volume knob

[alange5]

I just built an MKIII using the layout found on tagboardeffects.  I didn't deviate from the layout at all.  Overall, it sounds good, but I can't get that "cleaned up" tone bender sound by rolling back my guitar volume.  When the guitar volume knob is turned down more than just a hair, the sound becomes very dark, as if I had rolled my tone knob all the way back.  At full guitar volume, it has all the bite you'd expect from a bender.

I used older Russian PNP's for Q1 and Q2 - hfe about 70-80 for both, and an older unknown PNP for Q3 with about 120 hfe.  I settled on this trio after mixing and matching about a dozen transistors.  The volume knob reaction and resulting "muddy" sound was the same regardless of transistor placement.

I'm playing a tele through a Princeton Reverb, apartment-level volume (about 4), and I fingerpick exclusively.

Is this normal behavior for an MKIII? 

[alange5]

my voltages:

Q1 & Q2: C  -3.8v
               B -1.5v
               E -1.5v

Q3:         C -.15v
               B -.08v
               E 0v

Q3 seems a little off, no?

I forgot to mention one substitution...  a 1n34a GE diode in place of the 1N270 called for on the layout (Q3 base to emitter)

[Ben Lyman]

Is it this one?


and does this look like the right schematic?

[alange5]

Yes, that's the one.

I swapped in a different transistor for Q3 and got the collector voltage up to 2.5v

Still sounds pretty similar.  Not better or worse, just different.  Still gets muddy when I back off the guitar volume knob.  I guess I'm wondering if the symptoms described are a product of the setup (guitar used, amp volume, picking style), or if it's indicative of some internal fault.

[anotherjim]

None of those transistors are biased - going by the base-emitter voltage difference. PNP, the emitter should be more positive than the base by around 0.2V?

Do you have the power supply polarity right? This is positive ground. Easy & frequent mistake - rookie or not.

Or maybe that 220k bias resistor is too high value or missing - or the 47k bias is really 4.7k (might explain the darkness). Anyway, look thereabouts.

[alange5]

I'm using this offboard wiring:
http://tagboardeffects.blogspot.com/2012/02/offboard-wiring.html

reversed the LED and powered with a center-negative 1-spot through a center-positive adapter (no daisy chain) for a positive ground.

verified all resistors are correct value

Could you explain (or link to an article explaining) the biasing procedure?  Most info I've gathered shows the desired emitter voltage equal to or slightly less than the base voltage on Q1 and Q2 (which is right where my voltages are).

With the guitar volume up all the way, it sounds just about identical to most of the sound clips out there.... It only gets muddy when I back off the volume.  Nothing about the pedal sounds funny or out of character with the volume up.

I also tried it with a couple different guitars to make sure it wasn't just the taper on my tele.  Same results with any guitar.  For reference, with a strat, it gets muddy when the volume drops to about 8 (hardly turned down at all). 

[thermionix]

Am I correct to guess you don't have treble bleed kits on your guitars' volume controls?  I don't either, and my fuzz doesn't clean up well and gets dark when I roll the volume down some.

[alange5]

Am I correct to guess you don't have treble bleed kits on your guitars' volume controls?  I don't either, and my fuzz doesn't clean up well and gets dark when I roll the volume down some.

correct, I do not

[alange5]

Another quirk I just noticed while testing and tweaking...  When the guitar volume knob is rolled back, the guitar's tone knob does pretty much nothing.  The tone is consistent (muddy) whether the tone knob is at 0 or 10.  When the guitar volume is maxed, the tone knob works as you'd expect. 

Any more ideas?

[Ben Lyman]

I'm not the expert around here but i might be the guy with the most free time today 
I thought i'd make a vid showing you how my guitar volume reacts with my own Ge TB.
Probably not the perfect comparison to your own because mine is something i designed for myself.
It's a Ge MKII with a variable input cap and a treble bleed built into the fuzz pot (easy mod, btw). The Input cap is always set to the 10uF in this vid, my fuzz pot is 1k so not a lot of difference between high/low gain, however there's a big difference in the sensitivity, perhaps that's why they originally called it "Attack" doncha think?
It's a Strat on the bridge PU into a Fender Bassman 50 and 2x12" cab

[alange5]

how kind of you!

that's exactly how I'd expect a Tone Bender to react with the volume knob (sounds great, by the way).  I realize MKII's and MKIII's are slightly different beasts, but my MKIII sounds nothing like that with the volume turned down.  It literally sounds like a tone knob backed off all the way.

I'll see if I can put together a video of my own...

current voltages after some more transistor swapping and tweaking:

Q1 & Q2: C  -3.8v
               B -1.52v
               E -1.49v

Q3:         C -3.2v
               B -.06v
               E 0v

[robthequiet]

Hey Ben, sorry for a bit off topic but have you put a rangemaster in front of the tonebender at all? I think that the input impedance of the benders is kind of why the magic works between the guitar circuit and the box. Probably why people complain that buffers work against you on FFaces and similar.

[Ben Lyman]

Hellzya! Rob. i love my RM in front of my TB.
I'm currently in a conundrum of a quandary, as i try to keep my board down to 6 pedals at all times but keep on building new ones to see if i like them... and of course i do like them!

Back on topic (a little bit) does anyone else feel like a FuzzFace cleans up better than a TB? That's kinda been my experience when i compare the two.
alange5 maybe you could make a fuzzface, if you haven't already, they are super easy to make

[anotherjim]

As this is a Fuzz distortion, it can seem to be working as input signal itself may be strong enough to apply forward bias to a transistor - the low threshold of germanium can easily make this possible. It will sound distorted, but you are expecting distortion! It just isn't performing fully as it should.

Although test voltages all referenced to ground are useful info, the quickest way to check BJT bias conditions is to simply measure between base and emitter leads on the transistor.
The direction of the arrow symbol on the emitter tells you which should be the most positive -  just like a diode, and the other terminal of the "diode" is the base.

In circuits meant to carry an audio signal, the transistors can be expected to be biased so that the path from emitter to collector is partly conductive. To do that there must be some current flowing between the base and the emitter. To do that, the base-emitter must be "forward biased".

It is awkward to measure the base current in a connected circuit in order to prove the bias, but you can easily measure the barrier voltage across the base-emitter P_N junction. If this is about right, then the transistor is "probably" biased to work.
Germanium transistors have a lower barrier voltage than silicon's "700mv". 300mV say all the books, but you can measure a bit lower in practise, so at least 200mV "probably" means it's ok IF the polarity is correct according to the diode symbol.

Your Q1 & Q2 have base-emitter of -30mv. Wrong polarity and not enough to bias either.
Your Q3 has 60mv. Correct polarity, but not enough to bias.

[Electric Warrior]

I realize MKII's and MKIII's are slightly different beasts, but my MKIII sounds nothing like that with the volume turned down.  It literally sounds like a tone knob backed off all the way.

Slightly different beasts? The only thing they have in common is the Tone Bender name.

The vintage units I've had the pleasure to play through all cleaned up well, but I know not all vintage unit do that and some builders set them up to not clean up in order to have a wide variaty of fuzz sounds available to be dialed in with the guitar's volume knob. Not sure how to do that, though.

Between 2 and 3 volts on Q3's collector should be dead on. Not sure about the other voltages, though. I should make more measurements.
Not sure why your pedal gets so dull sounding with the volume turned down. Might be the wiring in your guitar or a mistake in your pedal's wiring or layout.

The Input cap is always set to the 10uF in this vid, my fuzz pot is 1k so not a lot of difference between high/low gain, however there's a big difference in the sensitivity, perhaps that's why they originally called it "Attack" doncha think?

The "ATTACK" label is something the MK1.5 and MKII inherited from the MKI/Maestro Fuzz Tone, in which it was a bias control.

[alange5]

I made a quick video:

[Ripdivot]

I have had several mkII and mkIII tonebenders. Mine have all worked pretty much just like your video. Treble bleed on the guitar volume helps a bit and single coils work better than humbuckers for clean up. If you really want the glassy clean up a fuzz face is the way to go imho. Generally speaking, tonebenders are not known for great clean up without tweaks.

[Electric Warrior]

It sounds all that much duller than the actual clean sound. 

[Bret608]

Hey, I just wanted to weigh in and say I've built two MkIIIs (one with Electric Warrior's '68 Vox-specs layout, and one with the '74 or so specs on a Madbean fabbed PCB), and neither of them clean up the same way as a Fuzz Face or even a MkII. I also have a Buzzaround built by Ghost Effects in the UK and it's about the same deal.

Your Q1 and 2 voltages are almost dead on to my '74 specs version (the one with the 220k/47k voltage divider up front), and both of the Q3 voltages you posted are in the ballpark as EW mentioned. That said, the '74 version I built has a slightly better clean-up, yielding some interesting semi-dirty sounds. I think the big difference is that I used slightly lower-gain transistors in Q1 and 2, like 45 and 49 hfe respectively and low leakage. The '68 had more like 45 and 65 for Q1 and 2. That may be one thing you could play with if you haven't already. But at the end of the day, the full-on fuzz sound is what this circuit is all about. I think your build sounds really good, by the way.

[alange5]

^thanks!

OK, an update....

I never quite solved the clean-up issue with the fuzz on its own, BUT, when its's on a pedalboard with a buffer BEFORE it, (TU-2 in my case), all the highs are retained, and it cleans up beautifully.  I experimented moving the pedal around in the chain, and I get the same results - when there's a buffer before it, it sounds great, and the guitar's volume and tone knobs function exactly as they should.  Remove the buffer, and it's woofy and muddy.

Also, when it's in the chain, it's a lot noisier than when it's by itself.  I hear a slightly audible pop when it's engaged, and theres a whoosh/hiss in the background.  I suspect the noise is a symptom of the 1spot, plus a non-isolated power supply powering the rest of the pedals.  I just ordered some IC's to make a voltage inverter so I can power it in the daisy chain.  Hopefully that'll take care of it.

What am I missing?  What's the buffer doing to clean it up so well?