Sola Sound Yellow Hybrid Tone Bender

mdc

Active member
FUZZ - 87.5K
TONE - 18.3K
VOL - 12.5K

Thanks for the catch on the 1uF - things get a little twisty in that area.

[edit - just cleaning out the older versions of the schematic for clarity's sake]
 
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mdc

Active member
Voltages for the three transistors:

Q1
C - 4.18
B - 0.63
E - 0.04

Q2
C - 4.14
B - 0.65
E - 0.04

Q3
C - 2.59
B - 0.10
E - 0.00
 
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Mir9

Member
Thank you. I assembled this on a Muff board with a few little mods here and there.
I used 2n5088 & 2n5089 equivalents and Russian GT404 & MP38A. These are my voltages, trying different Zener diodes.

Q1
C - 4.01
B - 0.64
E - 0.04

Q2
C - 4.03
B - 0.64
E - 0.05

Q3 GT404
C - 2.44 (using 4.7v Zener) 2.70 (5.1v Zener)
B - 0.09
E - 0.00

Q3 MP38A
C - 2.57 (3.3v Zener) 2.74 (3.6 Zener)
B - 0.095
E – 0.00
 
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mdc

Active member
@Mir9 I'm going to guess that my multimeter freaked out and moved the decimal places on the emitter values—I was surprised by those on mine and yours look a lot more normal. Looks like the MP38A and the 3.3V Zener are the closest to stock—how does it sound? What pot values did you go with?
 

Mir9

Member
A100k, B10k, A100k. A couple people told me 10k for the tone makes sense to them considering the cap and resistor values, so I just went with that. It's still not very loud, It's about unity with volume cranked.

With an octave pedal, It sounds close to this bass demo to me:
 

mdc

Active member
Huh, curious - I get to unity around 1 or 2:00 on mine and there's plenty of volume on tap after that.
 

music6000

Well-known member
You listed before that the PCB appears to be stuck to the pedal, Did you try twisting a little Clockwise & Anti Clockwise to see if it would release?
It may need a A250K Volume Pot, Not A100K?
 

mdc

Active member
I did actually manage to get the board out - I was mistaken, it's not adhered to the case at all. The nuts are sort of recessed into the top and look like they're floating in the holes but they're actually very very slightly catching the edges. I managed to work them off and the board lifted. But there's no marking on the backside of the PCB and the pots are mounted through so there's still no way to see the markings.
Unfortunately I don't have any zeners in the correct voltage range at the moment, but I'll try to grab some in the next few days and see if I can just A/B some options with the stock pedal and see what's going on.
I do have a few OC140s at least.
 

Bret608

Member
The Q3 collector voltages reported by mdc do look like really typical MkIII territory. Now, it looks like for mir9, there were a variety of zeners that could get you in that neighborhood depending on the transistor. That tells me leakage is a factor, so anyone building this may need to audition a couple of different zeners to get in that 2.5v territory. On the other hand, historic MkIIIs could be anywhere from 1.5v to 3v--I've tweaked ones I've built within that range, and they pretty much always sound good. So maybe we just say 4.7v zener and leave it at that?

Thanks for everyone's work on this, by the way! I had more or less forgotten about this variant, so am excited to see it come together.
 

music6000

Well-known member
How's everyone feeling about this?

Do you mean Q1 -BC107 ?
VOLUME, TONE & FUZZ label's missing
 

mdc

Active member
@Mir9 still curious about the low output on yours—did you make any substitutions? I'm not sure if anyone's gone back and double checked the resistor values from the first page. Might be worth someone looking at it with fresh eyes in case I misread a band somewhere.
 

Mir9

Member
That W20k on tone changes things and gives what sounds like a big upper mid range boost towards the right. Is that your experience? It doesn't sound the same as other Muff tone stacks I've used.

For volume, removing both diodes raises the volume considerably, but doesn't sound as good of course. I'll keep experimenting with substituting both to find a good middle ground. It could just be me. Anyone can build this on a Muff board with some hacks. Even THEY used the same board for this as their SupaTB and Bass Fuzzes.

TB
Sola Sound Tone Bender TC 600 9xcbxgwe.jpg

Bass
Colorsound Bass Fuzz 600  ribzafgv.jpg
 

Chuck D. Bones

Well-known member
D1 looks ok to me. It does a few things.
1) It limits the reverse voltage on Q3's base to prevent overstress.
2) It provides a path for balancing the charge on Cxx (470nF). Can we get some ref designators on the passive components?
3) It is part of Q3's bias network. The reverse leakage in D1 more-or-less compensates for the collector-base leakage (Icbo) in Q3.
 

Julian A

New member
On the other hand, historic MkIIIs could be anywhere from 1.5v to 3v--I've tweaked ones I've built within that range, and they pretty much always sound good. So maybe we just say 4.7v zener and leave it at that?
Being a MKIII stage, it shouldn't be super sensitive to much. I wouldn't be surprised if factory units varied within a volt or so.

That Ge was meant to limit the voltage at the Base, I believe.
I've removed the diode from a MKIII and the output of the pedal pretty much dies unless you play extremely hard. It was originally used for bias and stability.
On the hybrid, though, they have that 220K from collector to base. I tried my hybrid build without the GE diode and I seem to remember it working just fine, but that was over a year ago. Never measured voltages or compared long enough to reach a consensus. It could be residual from the MKIII or maybe it does change the sound. I've got some testing to do this weekend.

I plugged the tone stack values into the tone stack calculator, and 10K and 20K don't seem to be massively different, although 20K cuts more bass when turned up. The point of the mid scoop is shifted a bit lower, and there is a sliver more mids with 20K (+1db between 500-2500hZ, when turned up ~90%). If I have a 20K, I'll definitely give it a shot. I can see it being a bit more tonally versatile.
 
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