Help me dial in this Tonebender?

[lincolnic]

Today I got my Tonebender Mk II up on the test rig - built on one of Stomptown's PCBs, with a transistor set from chromesphere. It's a stock build, but as per Paul's suggestion, I replaced the collector resistor of Q3 with a pot to have greater control over the biasing. (I believe I used a B25K, figuring that over twice the resistance of the 8.2k in the schematic would give me a fair variety of tones.)

It's sounding mostly good, but the low E and A strings develop a bit of...how to put this...fartiness...as the notes sustain. They start out okay, but you can hear the notes start to sound a bit pinched after a second or two. I can affect it a little bit with the bias pot, but it never seems to go away completely. Can anyone offer some pointers as to how I can get this thing a bit happier? I'm new to the Tonebender world, but I want to learn.

My transistor set, if it helps:
Q1 GT108V - 87.4 hfe - 51.6ua leakage
Q2 GT108V - 89 hfe - 76ua leakage
Q3 AC125 - 109.3 hfe - 178ua leakage

[dondougie]

The transistors look ok, but maybe not enough leakage for Q1/Q2. What you could try is to replace the Q2 collector resistor with a 100K trimpot. I've found that most of the time this works better than biasing just Q3C.

[Cortexturizer]

I've found that adjusting the base resistor of Q1 makes for a crazy good tone. Here are my findings in this thread
http://www.madbeanpedals.com/forum/index.php?topic=10844.30
A bunch of us posted our troubles over this build. Don't beat yourself if you're doing so, this is a sneaky muthafutzker, this MKII. They are hard to get going well.

[chromesphere]

hey Lincoln, that's an old set, haven't sold one like that for a while!  Wish I still had more of those gt108v's

You should read Jon's (midwayfairs) post in this thread.  Check out the voltages he mentions and see if you can bias yours the same. YOu may or may not like the sound better but Jon mentioned these are how the original TB MKii's were made. Also, your description to me sounds like it's misbaised (stuttery / farty decay).  You could try increase that pot to 50k or maybe mess around with the collector resistor on q2, might be were the problem lies.

Hope that helps!
Paul

[midwayfair]

Leakage on Q1 definitely looks low to me (remember, the leakage current is providing the positive bias for which there's no resistor!), and the gains are also a little higher than I would have gone with in all three positions. That doesn't mean that you need to swap transistors, but you might need to adjust the collector resistors.

Your bias voltages will help, too. Also, resign yourself to some fartyness. It's ... part of the sound.

[chromesphere]

Your bias voltages will help, too. Also, resign yourself to some fartyness. It's ... part of the sound.

^ I was going to say this too but forgot. Infact that's kind of part of what I like about the tonebender

[lincolnic]

Thanks, guys, this is all helpful. I read through the other thread, but I have to say, I listened to Cortexturizer's SoundCloud clip in the second post on that third page and my pedal actually sounds pretty similar to his. What I was trying to deal with was what happens when he sustains that low note at 0:38 - you can hear how the sound gets sort of "pinched" as the note sustains.

Maybe this is how it's supposed to sound?

Here are my voltages with the bias pot roughly where I had it sounding best last night (too late to plug in now and double check, unfortunately):

Q1: -8.56
Q2: -0.29
Q3: -5.65

These are mostly close to the voltages Scruffie posted from the D*A*M pedals, but I guess Q3 is pretty low, huh? I can easily get it to -7 or -8v with the bias knob, so maybe I just didn't have it set right last night. I'll do some more exploring tomorrow and see how it sounds up there.

[Blues Healer]

I like to bias Q3 to 4.5 - 5 V. That's where it sounds most musical to me. Much higher than 5, and you're giving up some sustain and harmonics. I'd suggest you at least start there and see how you like it.

You might check fuzz central's article: http://fuzzcentral.ssguitar.com/mkII.php

Smallbear also has a good article on the fuzz face circuit, a lot of which applies to a tonebender, as far as transistor gain. At the end of that article is a link to R.G. Keen's breakdown of a fuzz face, but that may be too much reading.

[Cortexturizer]

fuzzcentral is a great site, but the proposed value of 4.5V is just not right. Although, if it sounds right to some, why not use it, there's always that of course...cheers

[lincolnic]

Thanks for the link. I will probably swap out the 470 ohm resistor for a 1k at some point - I've got to crank the level all the way up to get above unity.

Anyway, as I mentioned, it sounded best (as it is now) with Q3 around -5, but I'll play with it some more and see what I like. I didn't get to do much besides work today, so hopefully I'll be turning knobs again soon.

If I was going to build another of these, I probably would make the bias pot a trimmer rather than a full external pot...I don't see myself changing it that much once I get it dialed in. Live and learn.

[Mich P]

I build the mindebender from Fuzz Central with low gain NPN Ge
and it sounded best (to me) with Q3 collecteur at 4,5V
as well.
Mich P.