Noob questions about biasing Germanium transistors... with trim pot/s

[Snufkinoob]

I was going to build a stock Ge fuzz face, but have noticed some have a trim pot or two installed for control over the bias to get the most out of the sound, but...

• Do you need one per transistor? I've seen diagrams with just one trim (at Q2), but the Bonamassa Dunlop has one before each transistor.


• What determines the trimpot value? 22k seems common, but again the JB has two different, a 10k and 25k. Might higher hfe equate to needing a bigger trimpot?


• I've only seen them on fuzz faces, and a tonebender, but can they be put into any Germanium fuzz circuit? If so, what's the general rule? Going by FF's it's between 9v and Out, going into the collector? And what would determine if it had one per transistor like the JB?
  

[LucifersTrip]

I was going to build a stock Ge fuzz face, but have noticed some have a trim pot or two installed for control over the bias to get the most out of the sound, but...

it's not always to get the most, but to be able to re-bias when the temperature changes

• Do you need one per transistor? I've seen diagrams with just one trim (at Q2), but the Bonamassa Dunlop has one before each transistor.

not for biasing...on either of the collectors is cool enough

• What determines the trimpot value? 22k seems common, but again the JB has two different, a 10k and 25k. Might higher hfe equate to needing a bigger trimpot?

depends on which transistor...Q1 around 50K and Q2 around 10-20K...but you don't want to alter too much from stcok values or you'll change the tone too much

• I've only seen them on fuzz faces, and a tonebender, but can they be put into any Germanium fuzz circuit? If so, what's the general rule? Going by FF's it's between 9v and Out, going into the collector? And what would determine if it had one per transistor like the JB?
  

trims can be used anywhere and totally depends on the specific circuit

[Snufkinoob]

Thanks.

trims can be used anywhere and totally depends on the specific circuit

So you might be able to have on that just connects to the collector of Q2, and that's it? Doesn't matter if the other two points of the trimmer just go to otherwise empty strip?

I've also noticed there are a couple of variations for the Ge Fuzz Face:

1k trim plus 8k2 - 1k trim is small enough to not replace/alter the 8k2
10k instead of 8k2 - 10k trim replaces 8k2

From that I'm guessing there's not hard and fast rule, but would using one method over the other offer the same, or different results?

So taking all that into account, could a general rule of thumb be to just put a bias trim at C of Q2, and if there's a resistor in the path, either replace with a similar value, or just add a 1k trim and keep the transistor?

[LucifersTrip]

So you might be able to have on that just connects to the collector of Q2, and that's it? Doesn't matter if the other two points of the trimmer just go to otherwise empty strip?

a trimmer is in place of a resistor, so you need to connect the middle lug (wiper) and either side, depending on whether you like to increase or decrease resistance

I've also noticed there are a couple of variations for the Ge Fuzz Face:

this is the original:

So taking all that into account, could a general rule of thumb be to just put a bias trim at C of Q2, and if there's a resistor in the path, either replace with a similar value, or just add a 1k trim and keep the transistor?

you need a small resistor in there so you don't turn to 0K on the trimmer and possibly blow the transistor. the stock value on Q2 is 8.2K, so it's reasonable to have a 3-5K resistor with a 10K trim or external pot giving you 3 - 13K or 5 - 15K. As I wrote before, if you alter the resistance too much, the tone will change. My preference is not go more than 3K in either direction, so ~ 5K - 11K.

leaving the 8.2k and altering the  33K on Q1 is many people's choice, since the tone will not change drastically...which is why many also trim that (Q2) on a TB MKII and leave the 8.2K (Q3) stock

[petemoore]

 A Q2 collector resistor value between 6k8 to 16k8 [a 6k8 fixed stop resistor + 10k pot], seemed to provide sufficient bias range for the transistor matches I wanted to bias in these 2Q feedback amplifier.