Black Horse Effects 1968 (FF + Rangemaster) Transistor Biasing Issue

[mshuptar]

Hey y'all,

I'm working on the Black Horse Effects "1968" (a Rangemaster + a Fuzz Face with an external bias knob similar to Analogman's Sun Face). I'm using some matched Ge transistors from Small Bear for the Fuzz Face side, and the circuit has an internal 50k trimpot, and uses a 5kC external pot for the bias control.

I'm having trouble getting the Q2 collector close to -4.5v. Even with the bias controls, my is -7.71v to -6.53v. Because the circuit uses those bias controls, I decided to use the stock resistor values on the build doc:

R2: 33k
R3: 470r
R4: 100k
R5: 2k7

Do I need to change any of these values to get the Q2 collector value closer to -4.5v? (I did socket R2-R5 in case I needed to make changes.)

Here's a link to the build doc

https://drive.google.com/file/d/0B8HPHm7M8U3zdGxMWEFEazdSNVk/view?usp=sharing

Other relevant info:
Q1: cv7007, 93hfe, L =220
Q2: oc42, 168hfe, L = 54

Thanks!

[mauman]

Yes, you'll need to increase R5 to lower the voltage to the collector of Q2.  I usually socket the same resistors you mentioned.   4.7k might be a good start.   Be sure to check the bias on Q1 before starting Q2.

Small Bear used to include a piece of paper stating the R values that they tested their transistors with, if so, try something closer to that value for your R5 (which probably has a different R number but you can recognize as the one feeding negative 9V to the collector of Q2).

Since you have the external bias knob (cool feature), you might want to set the knob at 12:00 noon while you are selecting your R5, so that your ideal bias point corresponds to noon on the knob.  Until you change the ambient temperature, that is....     Mike

[mshuptar]

Thanks for the response! I got busy and haven't had a chance to follow up since.

Putting a 5.1k resistor in R5 got me closer. The full sweep of the bias now gives me a range of -5.2 to -6.1v.

SmallBear did include a card with some resistor values, but they didn't include the resistors (which is weird, because they included resistors with the Rangemaster transistor I had in the same order, and also when I built a Woodstork a while ago). I don't have the values on hand because I build infrequently. I'll order those when I have some free time. I assume that should probably help.

[mauman]

You can probably hit it by increasing R5 again, sounds like you've moved it in the right direction.  If you don't have the right resistor values, you can use what you have on hand, combined in series or parallel, to test the correct value (6k = 2 x 3k in series or 2 x 12k in parallel, etc.)     Mike