EXTERNAL BIAS ORANGE SQUEEZER HARTMAN MOD, HOW TO TWEAK FETS FOR GOOD RESULTS?

[zindra]

This is my first post here, hi everyone.
I am waiting for GGG's Orange Squeezer and when I build it I want to add an external pot to adjust bias. I've read that the bias' sweet spot is so narrow that it's almost on-off and it's better to leave it as an internal trimpot. Some builders like Hartman do add the external pot and it seems they tweak the FETs to get a usable range:

"The other aspect of the design we tweak a bit are the FETs. Because we've externalized the Ratio control, we pay attention to the 'gain' of the FETs we install in the circuit.  The original OS had its Ratio set internally via trimpot at the factory, so FETs across the entire manufacturing spread could be accommodated.  Externalizing this control means limiting the FETs installed to a narrower range of values to ensure the external control provides a meaningful range of adjustment to the pedal's compression.  Otherwise, you could actually bias the thing clear out of its functioning range into silence!"

Anyone knows the FETs values Hartman uses for this mod and how to do it?

As for the external pot this is how I plan to do it after reading a post in another forum:
"According to the schematic at GGG the trim pot is a 10k ohm trimmer. If you wanted to convert that to an external control you'll need to replace the trimmer with a 10k pot. I'd recommend a linear taper rather than an audio taper so the adjustment is smooth across the range of the pot.
You'll need to remove the existing trim pot (R7) and install the 10k pot to the body. Then you'll need to wire the pot into the circuit where R7 was.
You'll need to bridge the middle lug of the pot to one of the outside lugs and wire this to the earth of the circuit board. The other lug should be wired to the point adjacent capacitor C3. "

It's going to be my second build and I know very little about electronics. Steep learning curve, I have enough trying to get on with the guitar!
Any suggestions?
Thank you very much.

[aziltz]

I've experimented with this circuit extensively.  I feel there are better ways to control compression, and having the Bias on an external knob isn't really useful.

If you search for Tangerine Peeler, Mark Hammer's Take on the OS, he has some interesting additions to the circuit.  The best of which in my opinion is the compression/sustain control by varying the gain of the op-amp circuit, but this only works if you've split the audio path and the envelope filter path.

Not to say a bias control won't give you good results, I've just been there and I wanted to share my thoughts.

[zindra]

I've been researching the Tangerine Peeler and I don't think I'd be able to do the mods required for that circuit. I was even surprised when my first pedal, BYOC's EA Tremolo kit, worked the first time. And that was yesterday! What I liked about Hartman's approach is that it seems really simple (just limiting the FETs values to improve the range). Doesn't sound very hard for a beguinner. If anyone could explain what FETs I should look for to replace the ones on GGG's OSQ kit (2N5457 JFET Transistor). Or maybe I am wrong and it's not that simple?

[zindra]

I found Hartman's mod in detail. He explains it himself in a GJD article here: http://www.guitarjamdaily.com/index.php/insiders/1932-pedal-insider-control-freak-part-2
Unfortunately I have no idea how to decipher the scheme and put it to work. He keeps the inside trimpot and adds an external pot. Hope it's useful for anyone who knows how to put it to practice. 

[aziltz]

I found Hartman's mod in detail. He explains it himself in a GJD article here: http://www.guitarjamdaily.com/index.php/insiders/1932-pedal-insider-control-freak-part-2
Unfortunately I have no idea how to decipher the scheme and put it to work. He keeps the inside trimpot and adds an external pot. Hope it's useful for anyone who knows how to put it to practice. 

Do that, it will be very easy to put onto the existing orange squeezer.

[bancika]

you have at least 30% less chance of somebody reading the thread if you write the title in ALL CAPS

[zindra]

I'm sorry! I don't usually post in forums and wasn't sure about the right way to do it. After posting I scrolled down the forums and noticed about the caps, but I couldn't find a way to edit the subject's field, is there a way to change it? Thank you.

[zindra]

I think I sorted out the mod looking at the scheme and it just consists of connecting the trimpot and the external pot's lugs...? Just to make sure I include a picture of the scheme with a sketch of the pots with the lugs numbered. Can anyone please tell me what lug should I connect to. I know this is like DIY for dummies, thank you.
I don't know how to paste an image in the message, the sketch can be seen here: http://picasaweb.google.es/ohwowmail/OSQMOD#5393211612385601586

[aziltz]

the wiper is obviously lug two (middle).

in many schematics, the arrow points from lug 1 to lug 2.  So the trim pot will turn up resistance as you turn it clockwise, and the external pot will turn down resistance as you turn it down.

i recommend using an internal trimpot like the article says.  those look a bit different from your standard pot.



Maybe someone with some insight will jump in here.  Mark Hammer has given more than enough useful information in the past and even more recently with all the OS Modding threads.

I'm wondering how different a control the External BIAS would be from a SUSTAIN control similar to what MH employed in the Tangerine Peeler.

It is worth it to have both in a Tangerine Peeler-like circuit?  I feel like External Bias is a quick and dirty/easy way to add control to the stock orange squeezer.

[zindra]

Yes, like the article says, I am plannig to use both: the standard 10k internal trimpot wich I'll set at the "sweet spot" and the external 2kpot to have more control over the bias range. For the external I'll use a small 9mm Alpha. What I don't know is how to connect the trimpot's lugs to the external pot's lugs. I'm not sure if it's a similar approach as the Tangerine Peel, but I've read it works well for Hartman's OS.

[aziltz]

are you building it on an existing OS board or are you making it from perf or something else?

you can see that both the trimpot and external pot are going to replace what was once the trimpot only.

from that picture,
Trimpot Lug 1 goes to Ground.
Lugs 2 & 3 are connected together and to Bias Lug 1
Bias Lugs 2 & 3 are connected together and to the on-board junction of the FET's Emitter and the Polarized Cap to ground.

Unfortunately, this will sort of "float" the trimpot off the board, so if you are using a perf or veroboard layout, you would want to secure the trimpot ON-BOARD.


I hope that helps.

Be sure to have the Bias set to minimum when you are adjusting the trimpot.

[zindra]

Done! It works great. Nice range of compression with the external knob, not a "set and forget" pedal anymore. But I wired it another way (or I just plain didn't understand Hartman's diagram and got to the same place). I am going to open a new topic with a very easy sketch, that way beguinners like me can understand it and save the two days of trial and error I've gone through. In case any of  you who know see something weird, please let me know. Thank you.

[oldrocker]

For jfets I used MPF102'S with great results.  I believe most jfets should work.

[zindra]

Here is where I posted the easy sketch:
http://www.diystompboxes.com/smfforum/index.php?topic=80001.msg660967#msg660967
http://picasaweb.google.es/ohwowmail/OSQMOD#

[1dallek1]

Here is where I posted the easy sketch:
http://www.diystompboxes.com/smfforum/index.php?topic=80001.msg660967#msg660967
http://picasaweb.google.es/ohwowmail/OSQMOD#

hi how does this
Trimpot Lug 1 goes to Ground.
Lugs 2 & 3 are connected together and to Bias Lug 1
Bias Lugs 2 & 3 are connected together and to the on-board junction of the FET's Emitter and the Polarized Cap to ground.
pertain to the picture you posted you dont have any lugs tied together?