Biasing transistors

Twistingcrow · Post by **Twistingcrow** » 26 Apr 2012, 18:41

Hi.
I'm new to this forum. Hi everyone!

I'm about to launch a fuzz face build (Joe Bonamassa fuzz replica) thanks to turretboard.org's layout and schematic. However I'm stumped by one "simple" thing. The layout provided by the website shows two resistors without a value but with a voltage indication: 0.5V on Q1's collector and 4.5V on Q2's collector. SO my question is simple: how in the hell do I manage to get these voltages there? Does it mean that I have to use trimpots there? How do I do, what do I need to buy to get the proper voltages at those spots?

Thanks a lot, if you give me that information, I'll be able to buy the parts and start the build!
Thanks!

Duckman · Post by **Duckman** » 26 Apr 2012, 19:10

Hi Twistingcrow and welcome to the maddest forum.

Those values are the recommended values and that's what you want to achieve.
With the suggested resistors, you can have a fairly near value, but if you don't, you can always use a trimpot.
Then, use your DDM, put on your pedal and take a voltage measure between ground and your reference points to check you values. That's it

Nocentelli · Post by **Nocentelli** » 26 Apr 2012, 19:11

You could use trim pots, or wire a pot of approximately the right value as a variable resistor (i.e. connect only lug 1 and 2), twiddle with it until you get the right voltage, then remove the pot and measure the resistance you ended up with: You can then just solder in a resistor of that value in its place. In a standard fuzzface, I think the Q2 collector resistor is usually around 8k2 so you could use a 10k pot with a 2k2 in series, and Q1 is around 50k so a 50k pot in series with a 10k resistor would give you some wiggle room.

With the trimmers, it won't fit the layout without some modification, but you can adjust the bias at a later date if you want a different sound: Check the schem here for the values used in the original unit (I'd advise reading the whole thread if you're planning to build it):

https://www.freestompboxes.org/viewtopic ... a&start=40

[edit]ah, duckman got there first![/edit]

Twistingcrow · Post by **Twistingcrow** » 26 Apr 2012, 19:54

Duckman wrote:Those values are the recommended values and that's what you want to achieve.
With the suggested resistors, you can have a fairly near value

Yes, the problem is that sinner doesn't give any resistor values on his layout, just the recommended voltage... Here's a link to it, I'm sure he won't mind my posting this as he is a member here...

http://www.turretboard.org/wp-content/u ... /JBF-3.pdf

Nocentelli · Post by **Nocentelli** » 26 Apr 2012, 20:14

According to the actual bonamassa thread that I linked to in the post above, the values are: Q1 collector = 20k + 25k trimmer, Q2 = 3.16k + 10k trimmer. Note that both trimmers have a ? next to them - They are SMD trimmers soldered on the track side of the pcb so presumably it's difficult/impossible to read values from the pictures. Again, I'd suggest a pot+fixed resistor combinations that will give you a range of values around 35-55k on Q1, and around 5-15k on Q2.

Twistingcrow · Post by **Twistingcrow** » 26 Apr 2012, 20:24

Nocentelli wrote:According to the actual bonamassa thread that I linked to in the post above, the values are: Q1 collector = 20k + 25k trimmer, Q2 = 3.16k + 10k trimmer. Note that both trimmers have a ? next to them - They are SMD trimmers soldered on the track side of the pcb so presumably it's difficult/impossible to read values from the pictures. Again, I'd suggest a pot+fixed resistor combinations that will give you a range of values around 35-55k on Q1, and around 5-15k on Q2.

OK I get it, thank you. Sinner's layout doesn't include the SMDs (what does SMD mean btw?)... So, for the 0.5V resistor I put a 20K in there, and for the 4.5V resistor I install a 3.16K. Would you mind telling me how the trimpots get soldered in there, where do I put their three legs and which leg do I connect to what???

Thanks so much!

Duckman · Post by **Duckman** » 26 Apr 2012, 22:12

Nocentelli wrote:You could use trim pots, or wire a pot of approximately the right value as a variable resistor (i.e. connect only lug 1 and 2)

You must read everything in your own thread

Twistingcrow · Post by **Twistingcrow** » 27 Apr 2012, 05:12

Oops... sorry...So I guess that lugs one and two are the two lugs opposite the lug that is on its own?

Nocentelli · Post by **Nocentelli** » 27 Apr 2012, 06:00

If it's a horizontal trimmer (i.e it lies flat against the board with the screwdriver slot facing up) lugs 1+3 are on one side and lug 2 is on its own on the opposite side. SMD stands for Surface Mount Device, tiny components that are soldered to the surface of a circuit board rather than "through hole" components which most people use for DIY builds where you shove the legs through the holes, solder them in place and snip off the excess lengths of wire. I wouldn't bother using a 3.16k resistor, which is precision value and might be harder to source: 3k3 is a much more common value, and the precise value is irrelevent since you will be setting the total resistance with a trimmer anyway,

Twistingcrow · Post by **Twistingcrow** » 27 Apr 2012, 09:11

Thanks a lot! I'm almost ready to buy the needed parts now!

So to make things clear: all I have to do is solder the trimpot at the same locations (holes) as the resistor that goes with it?

Nocentelli · Post by **Nocentelli** » 27 Apr 2012, 16:12

Twistingcrow wrote:So to make things clear: all I have to do is solder the trimpot at the same locations (holes) as the resistor that goes with it?

Nocentelli wrote:According to the actual bonamassa thread that I linked to in the post above, the values are: Q1 collector = 20k + 25k trimmer, Q2 = 3.16k + 10k trimmer....

Again, I'd suggest a pot+fixed resistor combinations that will give you a range of values around 35-55k on Q1, and around 5-15k on Q2.

Just to be clear, you'll want to have a trimmer AND fixed resistor so you don't risk having zero resistance: These will need to be wired in series in place of the bias resistors in the turretboard.org pdf. However -

Nocentelli wrote:With the trimmers [edit - plus resistors], it won't fit the layout without some modification

Shankar017 · Post by **Shankar017** » 22 Jun 2012, 12:28

sorry for thread jacking but i didn't think it was necessary to start a new thread regarding the same topic for just one more noobie question. anyway, when building a fuzz with a different transistors in different gain ranges than specified in the schematic, how big a hfe difference can be tolerated using the same resistance to bias the trannies. as in how big a difference in gain before i have to use a trim pot to find the optimum bias resistance instead of just dropping in the new trannies using the bias resistors specified in the schematic?