Sola Sound Yellow Hybrid Tone Bender

laky

New member
I want to thank the contributors on this thread. Really helped with my build (still on the breadboard).

A few things I found - Changing the resistor on Q3C helps with the biasing. The higher the resistor value, the lower the voltage (and vice versa). Sounds best around 2.8V (for my russian ge transistor). Any more than 3.2V, the fuzz loses its sparkle. Less than 2.8V, the fuzz gets compressed and more velcro-like (which is also interesting). I put in a 20k pot with a resistor in series. The resistor here is to start the bias at around 3.1V.

Fuzz knob is a bit useless below 11 o'clock and it does not do anything different from messing with my guitar volume knob. Hence I removed it (along with the 10nf cap) to have fuzz always at full. I then introduced a bass cut circuit (the bass part passive G&L PTB circuit) at the start. The bass cut circuit allows for control of amount of wooliness in the fuzz which might be more useful.
 

Chuck D. Bones

Well-known member
Interesting!

I breadboarded mine stock to begin with. It's definitely got the characteristic raspy Tonebender buzzsaw sound. I'm not liking the TONE control. Bass end (CW) is quite dark and the mid scoop make a dead spot with TONE at noon. I used an MP38A and a 4.7V zener, got 2.66V on Q3's collector. A 5.6V zener yields 3.08V on Q3. Also tried a 2N1308; the higher hFE required a larger base-collector resistor (330K vs 220K). I agree with your assessment of the FUZZ knob. I ended up changing mine to B100K and increased the 100Ω resistor at the bottom of the FUZZ control to 10K. Even with that change, the useful settings are above 10:00.

If you're not using the FUZZ control to vary the gain, it's easily repurposed as a variable bass cut. Remove the resistor at the bottom. Jumper the 10nF cap and install a 4.7nF cap from pin 2 to pin 3 of what used to be the FUZZ control. B100K or C100K will have a better feel than A100K. You could also try other cap values.

Next, I'll try retuning the TONE network. Dropping the 100nF to 33nF gives a flat response in the middle. 47nF gives a very slight dip at 1KHz. I don't believe that the TONE pot needs to be W20K. B20K or B25K should work well.
 

Chuck D. Bones

Well-known member
I changed the FUZZ control as described above, ended up with C250K as the BASS pot. 4.7nF in parallel seems to be a good value. I upped the 100nF cap between Q1-C and the pot to 220nF for more bottom when BASS is dimed. As Laky says, the guitar's volume control does the same thing the old FUZZ control did, but now we have a TIGHT/SLUDGE knob to go with the FUZZ knob on the guitar. I changed the 100nF tone cap to 47nF and that made the top half of the TONE controls range less dark.
 
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