I've just about completed a 2-in-1 Ge Fuzz Face and Rangemaster build. Everything works first time (yay) but I can't quite get the bias point on the FF side within spec.
It's built using Black Horse Effects 1968 Fuzz PCB and the FF side uses an internal 10K bias trimmer plus an external 5KC bias pot (Sundial, as per the analogman Sunface)
(https://i.imgur.com/OZ8AIaq.jpg)
With the Sundial centred the Q2 collector voltage ranges from 8.7VDC - 5.5VDC when adjusted using the internal trimmer.
If I have the Sundial all the way to the left (max resistance) I can get to the recommended 4.5V on the Q2 collector (and it sounds freaking awesome!), but it doesn't leave me with any other adjustment. So I think I need to increase the value of one or more of the resistors or the trimmer. Looking for confirmation that it's as simple as that and that I'm not missing anything else.
Transistors are Mullard OC77 that came from Smallbear:
- Q1 hFE 113, leakage 159 uA
- Q2 hFE 126, leakage 149 uA
Here are the voltage measurements (Sundial centred)
Transistor | Q1 | Q2 |
Collector | 0.476 | 5.5 |
Base | 0.112 | 0.476 |
Emitter | 0.112 | 0.356 |
Any suggestions welcomed
Thanks,
Dougal
Try increasing the Bias Trimpot to a 20k, that should bring the voltage down enough to center the sundial at 4.5V
Quote from: dan.schumaker on December 13, 2019, 11:33:02 AM
Try increasing the Bias Trimpot to a 20k, that should bring the voltage down enough to center the sundial at 4.5V
Thanks you, that fixed it.
Much appreciated.
Quote from: dan.schumaker on December 13, 2019, 11:33:02 AM
Try increasing the Bias Trimpot to a 20k, that should bring the voltage down enough to center the sundial at 4.5V
This has a side effect. The voltage divider between the transistor output and the 9+V is now closer to +9V, resulting in less volume. This can be fixed by doubling the 470R as well.