Tone vendor bender mk2 bias

[Rimgrot]

Hi!
I'm currently building a Tone Vendor mk2. Bought a transistor set from Smallbear which came with the following components:
Q1 2n1305 - hfe: 67, leakage 40
Q2 OC75 - hfe: 70, leakage 270
Q3 2n1305 - hfe: 103, leakage 10

Smallbear suggest the following bias resistors:
R3 100k (R5 on the tone vendor pcb)
R4 91k
R6 6.2 k (R7 on the tone vendor pcb)

I have breadboarded the circuit and It works, but output volume is a bit low, about unity gain with the volume pot maxed. And the fuzz sounds undefined and gated/little sustain with attack pot on max.

The collector voltages with 9v power supply are:
Q1 c 8,29V
Q2 c 0.178V
Q3 c 8,7V

Is it possible to bias this circuit better? I have not tried any other Tone Bender circuits before so I am a bit unsure what I am aiming for and how it should behave.
Thanks!

 

[andare]

Personally, I prefer to use trimpots as biasing elements when breadboarding Ge dirt circuits. Once I find what "sounds good", (as opposed to what the measurements are), I jot down the resistance value and go from there to make whatever combo of resistors are needed to achieve the biasing resistance.

No that's CHEATING, they have to bias POIFECTLY with the stock values or you're disqualified!
Only a Cybercow would taint a sacred germanium circuit with trimpots...how dare you!

I have an MKII on a breadboard that sounds good and Q3c is at -8.17 V. No gating.

 

[Cybercow]

No that's CHEATING, they have to bias POIFECTLY with the stock values or you're disqualified!
Only a Cybercow would taint a sacred germanium circuit with trimpots...how dare you!

Mind you please, the trimpots are only used to acquire the proper bias values while on a breadboard. They seldom stay in a finished build. Taint no big thing.

 

[Rimgrot]

Hi!
I'm still struggling getting the bias right. Have tried with trimpots along the lines of the Aion build documents, but can't dial in sounds that really work.
Went back to using just the resistors on the breadboard and currently have the biasing resistors set up like this:
R1 100k
R4 88k
R5 33k
R7 47k

This gives the collector voltages:
Q1 8,23V
Q2 283 mV
Q3 7,63 V

The sound is no longer as gated and produces more volume. I can dial in some nice and very usable sounds with the attack pot on a low setting, but higher and full attack is just to gainy, mushy and too distorted.
I get a bit lost with so many variables and I am a bit uncertain about which components to change to move the circuit in the direction I want it.

Does anyone have some suggestions how to get a lower gain level?

 

[Big Monk]

Hi!
I'm still struggling getting the bias right. Have tried with trimpots along the lines of the Aion build documents, but can't dial in sounds that really work.
Went back to using just the resistors on the breadboard and currently have the biasing resistors set up like this:
R1 100k
R4 88k
R5 33k
R7 47k

This gives the collector voltages:
Q1 8,23V
Q2 283 mV
Q3 7,63 V

The sound is no longer as gated and produces more volume. I can dial in some nice and very usable sounds with the attack pot on a low setting, but higher and full attack is just to gainy, mushy and too distorted.
I get a bit lost with so many variables and I am a bit uncertain about which components to change to move the circuit in the direction I want it.

Does anyone have some suggestions how to get a lower gain level?

Can you post an annotated Aion schematic showing all the values, including capacitors, that you have installed? Also, what amp are you running this into? Amp settings?

 

[Rimgrot]

Can you post an annotated Aion schematic showing all the values, including capacitors, that you have installed? Also, what amp are you running this into? Amp settings?

I have used the attached schematic for what I now have on my breadboard. I have written the name and values I have changed in blue on the right hand side.

 

[Rimgrot]

Can you post an annotated Aion schematic showing all the values, including capacitors, that you have installed? Also, what amp are you running this into? Amp settings?

The Aion schematic was suggested earlier in the thread as an example of using trimpots to adjust the bias: https://aionfx.com/app/files/docs/deimos_legacy_documentation.pdf
I just used the trimpots + related resistors from Aion to the PedalPCB schematic.

I have tested the effect on a Fender Princeton (cleanish) and with my Vox MV50. I have tried settings from clean to overdrive.

 

[Big Monk]

I wonder if you leakage readings from SBE are erroneous.

One thing about Steve’s “Bare Bones” method was that I noticed over time procuring transistors from them that his method routinely underestimated leakage.

If they are still using the same method….

Might also be a good time to double check the breadboard build out.

 

[traumprinz]

Insanely late reply on this, but if you still have some version of this breadboarded @Rimgrot, I'm pretty sure that OC75 is meant to be your Q1. Swap it with Q1 and this should bias pretty well. As-is, it's gonna be difficult to get enough current flowing into the collector of Q3 without some pretty large tweaks.

Put OC75 in Q1 and put the Q1 2N1305 in Q2 and it should bias fairly well with the Smallbear values and won't be far off with stock Mk2 values either.

 

[andare]

Insanely late reply on this, but if you still have some version of this breadboarded @Rimgrot, I'm pretty sure that OC75 is meant to be your Q1. Swap it with Q1 and this should bias pretty well. As-is, it's gonna be difficult to get enough current flowing into the collector of Q3 without some pretty large tweaks.

Put OC75 in Q1 and put the Q1 2N1305 in Q2 and it should bias fairly well with the Smallbear values and won't be far off with stock Mk2 values either.

From my experimentation with MKII Tone Benders, Q1 doesn't need high gain and high leakage. A gain of 60-80 and >60 leakage seem to suffice. Too much leakage in Q1 creates a lot of noise. Q3 needs more leakage. The goal is to get Q3c close to -8V naturally, without messing with the bias. This will give a bright, raspy tone but still sound like a fuzz.

Raising the voltage that high from -4.5V in a Fuzz Face, conversely, can only be achieved by lowering resistance to the collector and that will make it sound more like an overdrive. It gets brighter but it loses the fuzz quality.

Of course original MKIIs all sound different but I think we can all agree they're supposed to be like a Fuzz Face but with more gain and more high end, and the Attack knob gives great tones across the whole sweep. Mine sounds best halfway up. It goes crazy if I max it out. Btw I need to box this thing up!

Correct me if anything I said is wrong!

 

[Big Monk]

The goal is to get Q3c close to -8V naturally, without messing with the bias. This will give a bright, raspy tone but still sound like a fuzz.

Raising the voltage that high from -4.5V in a Fuzz Face, conversely, can only be achieved by lowering resistance to the collector and that will make it sound more like an overdrive. It gets brighter but it loses the fuzz quality.

I don’t think the transistor cares how you get it the voltage. 8vDC is 8vDC no matter how you slice it.

Is the implication here that leakage biasing is different from changing the bias with resistance?

 

[traumprinz]

"From my experimentation with MKII Tone Benders, Q1 doesn't need high gain and high leakage."

It's less about what Q1 needs and more that the combination of Q2/3 here will not bias correctly. There are tons of ways to bias these, but OP's will bias poorly without that change or large resistance tweaks.

I do think 60-90 hfe, ~200-300uA of leakage is most accurate to what you're likely to measure for OC75s, and thus, what you'd find in original OC75 units.

You can get by better with lowish leakage in Q1 with the 100K resistor on Q1 base. That's what was in the OC81D MK2s and appears to be what SBE suggests. OC75 loaded ones had 10K there and you may have trouble getting low leakage types to bias with that.

"Raising the voltage that high from -4.5V in a Fuzz Face, conversely, can only be achieved by lowering resistance to the collector"

There are many ways to achieve that. Higher Q1 beta, more Q1 leakage, higher emitter resistance, less Q2 leakage, lower feedback resistance, will all also increase the Q2 voltage. You can achieve it in a FF with stock values.

 

[andare]

I don’t think the transistor cares how you get it the voltage. 8vDC is 8vDC no matter how you slice it.

Is the implication here that leakage biasing is different from changing the bias with resistance?

Just what I noticed on my breadboards but you guys know more than me. Or rather I know nothing and you guys actually have knowledge.

 

[Big Monk]

Just what I noticed on my breadboards but you guys know more than me. Or rather I know nothing and you guys actually have knowledge.

It’s not a knowledge thing, right?

Voltage is voltage. You were likely hearing the influence of something else.

Transistors don’t care if leakage or resistance gets them to a certain bias voltage.

 

[andare]

It’s not a knowledge thing, right?

Voltage is voltage. You were likely hearing the influence of something else.

Transistors don’t care if leakage or resistance gets them to a certain bias voltage.

I didn't know that. See That's what I'm talking about. I still don't know what transistors do and what biasing is, I just know they make fuzz.