Bias Control For Bumblebee

[BrianS]

I received my AC-125 transistor set from Paul (Chromesphere) today for my Bumblebee build. He wrote a note on the invoice, suggested adding a bias control to the pedal and to ask on here for advice on how to do it. Did a forum search and if there is something I am not finding it.  Anyone have an idea on how to go about doing this? Thanks, Brian

[chromesphere]

Hey Brian,

I could swear i had a thread somewhere on here but cant seem to find it...anyway, it was a long time again since i was messing about with this circuit, but briefly looking at the schematic would suggest balance is a bias control for q3.  You'd have to experiment but i would guess that you could relabel balance as bias and add a 4th pot "volume".  or use a trimpot for balance and your 3rd pot would be the added volume control.  Just add the pot like any other volume control on a guitar pedal circuit:
https://www.youtube.com/watch?v=dyfFHO2XeKY

Totally optional, but without a bias control different transistors will make this pedal sound totally different, whereas with a bias control you can make it sound however you like (in my experience).

Hope that helps!
Paul

[midwayfair]

That's what the balance control is. It's a voltage divider that varies the Q3 collector voltage between the positive voltage point provided by R9 and ground. Turned counterclockwise, the voltage on the collector drops and the transistor is closer to saturation. All the way CCW and the collector gets no voltage so the pedal turns off. Turned clockwise, it goes up a few volts. The Bias control is also a kind of lousy volume control already, but your volume will be way above unity when the Balance is turned up.

Madbean's build document describes how to wire a volume control for this:

"If you wish to add a true volume control, simply wire the "OUT" pad to lug3 of either a 100kA or 500kA pot. Lug1 should be connected to ground and lug2 is the new output that gets wired to your switch. Note that if you are adding a volume control you should not use a PCB mounted pot for the Sustain control. This will allow you to place the Sustain and Volume pots side­ by ­side."

[Bret608]

On my Buzzaround, I don't find that the balance control yields different enough sounds along its travel (other than the expected volume increase) to warrant using it as a bias control with a separate, "proper" volume control. Honestly, the sustain gets me sounds more like what I would picture a bias control doing, even if that's not technically what it is. When the sustain is turned low, it sounds like the box is dying (in a good way).