Varying gate / bias effect.

[jamjuice]

Hi all -

This is my first post here.  I've been building and modding pedals for a short while and really really enjoying it.   I'm looking for a little advice with a mod on the Devi Ever Soda meiser; actually apart of the Rocket pedal.  I'd like to add a potentiometer to vary the (i think) gating effect on the pedal.  I've read it might be as simle as adding the pot and a resistor between the power and the first transistor. Is this the same as the bias on the transistor? Would really appreciate any tips and/or places to start.

Thanks!
Jam.     

[GibsonGM]

Hi Jam, glad you're enjoying pedal building!   What you might do is post a link to a schematic of the Devi pedal...people are familiar with many circuits, but not ALL of 'em!   With a schematic, we can talk the same language, y'know?

Varying the bias might be as simple as putting a trimmer in, yup, but it really depends on how the transistor is set up.   Without the schem, I'd take a guess that you may find a feedback and emitter resistor on the transistor, and that is where the bias is usually set, and where you'd put a trimmer....if you use simulation software like LT Spice, you can try it out first without risking your parts!!

[jamjuice]

Thanks for you reply ! I didn't know about LT Spice and it sounds great....

https://www.dropbox.com/s/yx06t4ezc3satsh/photo.GIF

[GibsonGM]

There's a bit of a learning curve, but you will use this software every day, for years and years, in your pedal-building obsession 
http://www.linear.com/designtools/software/   Get it, and get started!!    Tons of ppl on here use it, too, and can answer most questions you'd have, which is really handy too.

To mess around with some 'gating', or mis-biased sounds, I would alter the values of the 2.2 and 3.3 Meg resistors.   They feed voltage back tot he bases, and are involved in biasing the transistors.  Maybe play around between 470K and their original values, see what you get....there are a couple other ways to do this, too, but see what you get with that first.   The operating point should shift. 

Another way would be to put a pot in series with one of the 10k collector resistors (maybe another 10K) and move it from zero ohms up....that would throw things off, too.   Just don't go too LOW with that resistor, or you could fry the transistor ;o)     I'd leave the original in and ADD the pot....

[PRR]

In that circuit, the first two stages are good linear amplifiers. Q1 won't gate. Q@ won't until Q1 is really over-driving it.

But the last stage has NO bias, and won't pass any small signal just big signals. It is a strange idea. The gate threshold depends on the transistors.

[LucifersTrip]

gating is discussed

http://www.diystompboxes.com/smfforum/index.php?topic=96425.0

[GibsonGM]

In that circuit, the first two stages are good linear amplifiers. Q1 won't gate. Q@ won't until Q1 is really over-driving it.

But the last stage has NO bias, and won't pass any small signal just big signals. It is a strange idea. The gate threshold depends on the transistors.

Ya. Another look shows that. I simmed the first Q up and lo and behold, you're right on, Paul.

[jamjuice]

This is all great info and helping me understand a lot, so thank you !

Perhaps i should try to bias the last stage and/or try a different Q4...