Adding a bias pot to the Tone Vendor

J

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 :-)
 
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.
 
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.
 
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)
 
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.
 
Looking further at the sunface bias setup, Im really attracted to that flexibility. Could i use the below pictured trimmer in series with an external pot? Also I can see it in your schem above, but im not sure what to do with the third leg of a trimmer....


Screen Shot 2021-05-09 at 11.55.14 AM.jpg
 
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Solid. I'm gonna order some trimmers today. I'll go back and see which resistor they need to replace specifically. since im running both pots in series, I'm assuming thats just both outer lugs and both wipers to the same hole? Will I need to run the outer lug of my external pot to ground ?

I have some 100k linears for the external Bias knob. Should i wait to get readings before I order the trimmer (value?) or is there a ballpark you can reco?
 
@Big Monk
Looks like it's a 5k in both spots in the face version of this bias scheme. But i see a 100k in yours?
Just want to make sure i order the right pot and timmer combo.
I have a decent selection of pots on hand, but I'm not as well stocked on these trimmers.
 
I see it now..... What value are you using for Q2c trimmer? I now have wrapped my head around using a 100k dual gang to "fine tune" the trimmed bias on Q1 and Q2 together but I really don't think i have room for a dual gang (double boxing this one with a sunface). I had intended to only use the trimmer/100k external setup for Q2 alone. If you think this won't allow for enough adjustment I can maybe look for some 9mm dual gangs or think about drilling out a new case. I have two of the boards ready to go so I will def kick around the dual where i have room on my next build. If you think It's not worth it to just leverage Q3 by messing with Q2 alone, then maybe I just do an internal trimmer?

If you could only add in single gang external would you do it?

Here's where im at with it.
Sunface side is done and dialed in perfectly. (ignore the lug pots on the tonevender side. They were just placeholders so i could chose knobs that worked)

IMG_6131.jpeg IMG_6130.jpeg
 
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