Adding a bias pot to the Tone Vendor

jcrews

Member
It's nearly impossible to get rando ebay transistors to bias right without swapping out resistors. I'd like to add a bias pot (I've seen trimmers that fit in a narrow resistor spot).

Looking at the scheme I'm suspecting the 8.2k, but i want to go about this the right way. I'm open to suggestions on this and a quality source for PNP germstuffs for this.

Would not be opposed to an external bias pot either if anyone knows how to pull that off :)
 

Big Monk

Member
It's nearly impossible to get rando ebay transistors to bias right without swapping out resistors. I'd like to add a bias pot (I've seen trimmers that fit in a narrow resistor spot).

Looking at the scheme I'm suspecting the 8.2k, but i want to go about this the right way. I'm open to suggestions on this and a quality source for PNP germstuffs for this.

Would not be opposed to an external bias pot either if anyone knows how to pull that off :)

You could install an external bias pot in place of the Q2 collector resistor, with a series resistor to set the adjustment range.

You could also use a dual gang pot to simultaneously adjust the Q1 base resistor as well. This is an idea I’ve breadboarded recently with good results. Seeing as the vintage units had these two parts as the turntable ones between OC75 and OC81 equipped units, this sees like the logical way to start.
 

Big Monk

Member
Here is the link to my wish list request for mods to the Tone Vendor based on my building of the MK II circuits over the last 8 months or so:


I think the bias pot should function less as a way to get the transistors in good bias range (this should be sorted out on the breadboard so limiting resistors can be chosen accordingly) and more of a way to externally balance changes in temperature as they directly attribute to changes in leakage and ultimately bias.

An external bias knob could be better descrfibed as a "leakage offset" control. I had terrible luck with the "plug and chug" method with my 7 OC75s, and even when I found base and collector resistors that biased them perfect, playing for a while cause the leakage to increase and threw off my voltage.

By having a control (likely a dual gang pot adjusting Q1b R and Q2C R) that can work in tandem with internally set limiting trimpots, you could have a control on the outside you can tune by ear for all temperatures.
 

jcrews

Member
This is EXACTLY what im looking for. I'll start drawing it out, but I'll likely have more questions as I get it in progress. Thanks so much.
I'm actually putting in a box with the sunface so it's particularly irritating to have all that control on one side and zero bias adjustment on the other.
 

Big Monk

Member
This is EXACTLY what im looking for. I'll start drawing it out, but I'll likely have more questions as I get it in progress. Thanks so much.
I'm actually putting in a box with the sunface so it's particularly irritating to have all that control on one side and zero bias adjustment on the other.

I have a schematic with these mods already drawn up. Let me see about getting the file and a picture up here. What are you using to draw? I use DIYLC for rough draft stuff and schematics.
 

Big Monk

Member
Here is the set of mods I'd love to see (;)) for the @PedalPCB Tone Vendor MKII:
Annotated Tonebender.png

Frankly, you COULD do most of this with the existing TV MKII board and a 4 Knob pre-drilled enclosure from Tayda. Implementation might go like this:

1.) "Pre-Gain" can be done with flying leads with the wiper connected to the input of the board and connection from switch going to the input of the pot. Although looking at it now I think i may have swapped the pot connections in this drawing, i.e. Input and Ground connections should be swapped for the "Pre-Gain" decrease when turned down.

2.) Vbias1 and Vbias2 are simply a 100k Dual Ganged Pot for adjusting the collector voltages of Q1 directly and Q3 indirectly by adjusting Q2. Vb1 and Vc2 would be trim pots like on the Sunface board for setting, internally of course, the range of the bias pots. The stock Tone Vendor would have trouble with this so you would have to:

a.) Subsitute trim pots into the fixed resistor pads or;
b.) Select fixed resistors that give you the adjustment range you require.

I think having a fixed resistor AND trim pot here would be superior because you could use the fixed resistor to get the proper biasing and tailoring the trim pot settings and dula ganged pot for temperature offset adjustment range.
 

Big Monk

Member
It's starting to resemble a Fuzz Factory!

It is certainly more than Attack and Level!

In reality, it’s really limiting the input signal (“Pre-Gain”) and exercising a greater degree of control over Q1 and Q3 bias which have a great affect on the overall tone.

I’m not a a fan of a “Bias” as a texture manipulator as much as trying to use it to maintain the right bias points.
 

Chuck D. Bones

Well-known member
A few thoughts...

My first choice for set & forget biasing of Germanium circuits is to select the right transistor. Q1's bias depends mainly on leakage. Q2's leakage & hFE determine Q3's collector voltage. It's not about part number, it's about getting a tranny with the right specs. If you're serious about building Germanium pedals, then you'll need a bunch of Germanium transistors from which to choose. Different pedals need different specs.

My plan B is to select one of the base bias resistors for Q1 and Rc3b for setting Q3's bias. Resistor tweaking only works if Q1 and Q2 are already in the ballpark. You can tweak Q2's collector resistor, but it has the least amount of leverage.

FYI, the ratio Rc3a/Rc3b sets the max volume available.

I usually do all of that on a breadboard because it's easier & cleaner than temp installing stuff on a PCB.

Try disconnecting the ground from pin 1 on the PRE-GAIN pot and use a C-taper.
 

Big Monk

Member
A few thoughts...

My first choice for set & forget biasing of Germanium circuits is to select the right transistor. Q1's bias depends mainly on leakage. Q2's leakage & hFE determine Q3's collector voltage. It's not about part number, it's about getting a tranny with the right specs. If you're serious about building Germanium pedals, then you'll need a bunch of Germanium transistors from which to choose. Different pedals need different specs.

My plan B is to select one of the base bias resistors for Q1 and Rc3b for setting Q3's bias. Resistor tweaking only works if Q1 and Q2 are already in the ballpark. You can tweak Q2's collector resistor, but it has the least amount of leverage.

FYI, the ratio Rc3a/Rc3b sets the max volume available.

I usually do all of that on a breadboard because it's easier & cleaner than temp installing stuff on a PCB.

Try disconnecting the ground from pin 1 on the PRE-GAIN pot and use a C-taper.

I have to agree with nearly everything @Chuck D. Bones says here.

For the sake of posterity and giving my anecdotal experience, i'll add just a few caveats:

1.) Set & Forget Using Transistors - This is viable option with the right transistors. However, even the right transistors, or more succinctly, not enough of them, won't allow you to get a sustainably biased circuit.

For instance, I took a chance on a lot of OC75s to use in my TB MKII/Supafuzz excursions. After much swapping, I was not able to get the stock circuit to a point where it was not excessively gated. Granted, I don't have a huge stash of them so I am at the mercy of the 10 I have in my possession.

2.) Set Bias by Changing Resistors - Combined with starting in the "Set & Forget" mindset, minor resistor tweaks can get you where you want to be but can't control for temperature induced leakage increases which may throw off the bias anyway. That's really the only reason I am in favor of a bias tweaking network composed of fixed resistors, trimmers, and an external "Bias Offset" control.

With that said, I agree wholeheartedly with what is written above. Just a few extra notes:

Q1b and Q2c Resistors - These are the values tweaked between the OC75 and OC81 equipped TB MK II/Supafuzz units. My personal preference is to tweak Q3c by altering Q2c and for the most part i have never had issues there.

Q3c Voltage Divider - I see many people tweak Rc3b and not lower Rc3a. In order to maintain the same bias characteristics when tuning Q3c with Q2c, youll need to lower Rc3a. If, however, you end up tweaking Rc3a directly with a trim pot, then this is less of a concern. In my experience, the stock 470/8.2k string gives plenty of output volume. YMMV.

"Pre-Gain" vs. "Smooth"/"Clean" Control - I've tried the 250kA "Pre-Gain" (think Gagan "Easy Face"), as well as the 100kA "Smooth" control (think Fuzz Central "Axis Face") and the Fulltone/Analog Man 50k "Clean" control. I'm not sure which I like best. My poor memory says I liked the 50k "Clean" but i need to try them all out again.

One thing I'm certain of: I love the MK II/Supafuzz circuit.
 

Big Monk

Member
Here is a revised schematic based on some of @Chuck D. Bones comments on the input gain control:

Annotated Tonebender.png
Please Note: Much of this is totally optional, even if not marked. These are simply suggestions I have from breadboarding the circuit many times.

1.) "Pre-Gain"/"Smooth" - This is completely optional. My experience has shown that I get a different dynamic, particularly with crunch tones if I have that control as opposed to simply turning the volume control down. As an electrical engineer i don;t know why this is and it evades me from a technical standpoint but my ears tell me it differs slightly in a favorable way.

2.) Rb2 - This is completely optional although may be helpful for a more stable bias of Q1. Also, if you build a Silicon version of the MK II/Supa, you'll need this.

3.) Vb1a, Vb1b, Vc2a, Vc2b - This is a completely optional bias network I devised to combat the effects of temperature induced leakage fluctuation. I've breadboarded this a few times so it's not experimental but takes some tweaking. In my mind the process would work like this:

- Choose suitable values for Rb1 and Rc2 as limiting resistors;
- Vb1a and Vc2a are set in such a way that they define the range of the "Bias Offset" control;
- Vb1b and Vc2b are a 100kL Dual Ganged pot. It has a range limited by the internal components to allow you to "Right the ship" so to speak if temperature induced gating occurs during play.

Let me shut up now, as I could talk about this circuit all day. We haven't even touched on the frequency shaping mods!
 

jcrews

Member
well yall went way deep on this :)

Looking at how much space i have for this particular project I'm limited to adding a SINGLE external pot.... (although i do want to go back and do it again with all this extra control, so i ordered another board)

so to go back to where we started: "You could install an external bias pot in place of the Q2 collector resistor, with a series resistor to set the adjustment range."


I'd like to start simple and attempt a working layout with this single pot variable Q2 resistor value (potentiometer)

I'm a little green still and didn't quite understand the "with a series resistor" part of your description.

I'm assuming (and likely incorrectly) i need to remove R4 or R5 (both 100k resistors) and wire one resistor hole to the left lug, the other resistor hole to the wiper, and the right lug to ground. on a 100k pot.

I know im missing something here. my apologies.

Also, If i could find a trimmer that fit where a resistor used to live, i could set the bias range that the external pot controls ala sunface? I might have room to do that, but i def can only fit in one external pot at the moment (going in a 1590bb with another fuzz circuit)
 

Big Monk

Member
@jcrews

The series resistor is a limiting resistor. It sets the lower limit of the bias pot arrangement.

Hypothetically speaking, let’s say a a 100k fixed resistor for Q2c gives us a desired 8.0 vDC on Q3c. In order to have a range between say 7.6 vDC 8.1 vDC, we may use a 50k ohm fixed resistor in series with a 100k ohm pot wired as a variable resistor.

As far as what bias point to go with that is a whole other discussion. There are a number of schools of thought on this and I won’t waste space here!
 

jcrews

Member
I think i get it... either way. I'll wait till I have the three transistors in and then I'll post voltages. Maybe you can help me pick my fixed resistor and pot value once I know specifically what im working with. Thanks for your patience.
 
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