Tonebender biasing - the bain of my existence!

[gmr1]

Ok, maybe it's not that bad...

I can't get enough voltage to the collector of Q3 with the 500K trimpot in the GGG "Boutique Bender". With the pot open all the way, it's still only in the 2.5-3v range

If I lower the resistance, will this be a possible fix, or is more likely I just messed the circuit up elsewhere? It sounds ok, esp. on leads -but could be a lot better. The other transistors test pretty close to what I've seen posted in my search.

I've been using a DCA55 to test leakage/gain and I think I'm finding nice "sets" of transistors, so I'm fairly confident that's not the problem.

Other than Rangemasters (I can build those great all day...), I never dealt with germanium transistor circuits - guess I should have started with a Fuzz Face, and got a little more confidence before moving onto the Bender.

As always, I look to you, the people much smarter than I, to help me figure this all out.

Thanks!

- Greg

[John Lyons]

Q3 is biased with a 500K pot?
Should be closer to 10K maybe 50K trimmer, 500K is huge.

John

[alteredsounds]

I always use 20k trims that seems plenty enough.  But yes, 500k is HUGE!

[gmr1]

I know... It seemed odd to me, but GGG is usually very reliable, and I can't be the first guy trying this layout:

http://www.generalguitargadgets.com/diagrams/tb3bb_lo.gif?phpMyAdmin=4a28f86a515b7883e7bc35a68d4e7b6d

(not sure if that link will work, or if it's ok to post it - if it's not, somebody let me know)

I'm used to 10K trimpots... I even had to go get some 500K's. That being said, the layout may be right. I've went over the circuit several times, and can't find any issues with my board.

I guess I'll yank the trimmer when I get home, throw in a couple sockets, and see if I can get that collector voltage in the ideal range with a resistor or a smaller value trim. Still doesn't mean it's gonna be right, but it sounds so-so now with only 2-3v to Q3, so I'd imagine it'll help.

Thanks guys!

[Solidhex]

500K is what is specified in the schem and layout. How does the pedal sound? I thought I was having trouble biasing mine but I sort of gave up and did it by ear.

--Brad

[gmr1]

it's ok. the fuzz isn't real even, and it oscillates oddly. It farts out a little when you hit it hard. It could be better, esp. compared to some of the clips I've heard on the web. I'd like it to sound like this:

http://homepage.mac.com/geekmacdaddy/FileSharing32.html

[skiraly017]

GGG's Boutique Bender is a part for part match against the Soul Bender (at least the ones I've compared). With Smallbear's matched trio of transistors the Bender I built sounded great.

[gmr1]

I got it sounding pretty good... I had to add (quite a bit) of resistance to the 500K trimmer (1M resistor in series with the trim pot did the trick) to get the Q3 collector to hit the correct voltage. I can dial it in nicely now. Does the resistance allow a certain amount of juice to drain before hitting the collector, or is it because it's a positive ground circuit? My mind is blown right now... I have so much to learn!

Clearly I messed the circuit up somewhere along the way, and for some odd reason, it works pretty well with the above solution. It's gonna take me a lot more reading before I even can begin to understand exactly what's going on along the signal path of most pedals...

[slacker]

Have you wired the trim pot up correctly?
On the parts layout it could look like the bottom pin of the trim is connected to ground but according to the schematic it shouldn't be. If you've soldered that pin to ground that could be what's stopping it biasing properly.

[gmr1]

yep, I clipped the leg going to ground. I double checked the board against the schematic and saw that (I did at one point have the 3rd pin soldered in)

I threw some sockets in, starting messing with resistors, while checking the voltage at the Q3 collector... took 1.5M to get it in the right range. I'm still not really sure what going on with the pedal, but it's sounding pretty good through my little class A Oahu amp with it's 10" speaker. I'll bring it to a rehearsal tomorrow, put it through the paces at stage volume through a big amp, and see if it is indeed sounding "right". If it's not, I'll start over - I etched 4 of the board.

It seems to clean up quite nicely when I roll some volume off the strat. that's a pleasant surprise.

[John Lyons]

That layout is kind of sketchy. Not sure why they set up folks to fail with the trimpot (potentially) connected to ground like that...
I would think that the 18K/R8 resistor would have a better time adjusting the bias.
The schem:

The 500K trim will set the bias as a fine tune trimmer but if you can't get close then the 18K needs to be adjusted.

I'd put the 500k (not connected to ground) bak as stock and set it in the middle.
Temporarily solder in a 20K, 50K pot and twiddle until you get about 3 volts. Then twiddle the 500K pot to set the bias as a fine tune.
This way you have some latitude with the 500K up and down.
Replace the Temporary R8 pot with a resistor of the value that is measures when your done setting the bias.

With the 1M5 in there I would think that that stage is going to be pretty squishy and compressed.

John

[gmr1]

John -

Replace R8 with a trim pot? When doing so, is it the first two legs of the trimpot that take the place of the resistor leads? leave the third leg open.

Are you saying a 20K pot or a 50k pot - depending on which range gets the Q3 collector in the 3v ballpark?

Could the same thing be achieved with a couple of sockets, and swapping resistors until you reach the 3v range?

yeah, it's a little squishy now. Just played it a bit more - when I dig in, it's pretty noticeable.

Thanks for the help on this, it's really appreciated.

- Greg

[aron]

Wire the 50K as a variable resistor (use the middle and one outer lug), plug in to socket, tune to taste. Take out the pot and measure. Replace with closest resistor value.

[John Lyons]

You got it Aron.

The temporary pot is just to find out what resistance you need. Much easier then trying 30 resistors to find which one works.
Just twiddle the pot until you get the voltage you need. Follow the steps I mentioned above.
either a 25K of 50K pot will work as you just need some range or resistance to play with...whatever you have that's close.

John

[gmr1]

Excellent. Thanks guys! I'll give this a go later today and see what I end up with.

[Gus]

Look carefully at Q3s circuit fragment.  There are a few things going on.

First what is the battery, base and collector voltage on a real one that is operating correctly?  This will help a bit to understand it.  That will give Ic and help figure out the current in D1 to look at the transistor and diode curves at that voltage

I would guess it was designed with an Ic(collector current) and collector resistor(s) as being fixed.  You might not want to mess with R9 and R8 because of the designed output resistance of Q3 fragment and the tone control.

R14, R15 and D1 are the bias network for the transistor.  The curves of the Diode used and Q3 used might be the things to check.

Look at a BMP note the bias network The BMP has a resistor from base to ground.  The TB has a diode, Diodes conduction changes a bit with current.
Current in this TB D1 is via the C to B voltage drop divided by R14 plus R15 then you have leakage current from the transistor.

Not know what the DESIGNERS had in mind makes it hard to a study.
  I am guessing D1 is there as part of the design and maybe the circuit is designed to operate with the collector not at 1/2 the battery voltage

Also the gain of the Q3 stage is the open loop in combination with R14 and R15 along with the conductance of D1 and R7 setting resistance and the output resistance of Q1,2

[gmr1]

Gus -

I'm not experienced enough to understand 100% of your response, but I understand a lot more of it than I would have a week ago.

In regards to the diode. I just have a general purpose germanium diode in there (all I had - can't remember the model number, I'm not at home right now). Any chance this could be part of the issue?

I'm guessing I've made an error along the way somehow, but I can't find it. Is it an issue to use 2 caps back to back to reach the correct total value? I didn't have a 150pf cap, so I used a 100 and a 47 back to back - with the inside legs of the caps soldered together, and the outer legs going to the correct points on the circuit. Does this work like I thought it did? If not, that's gonna be an issue.

It seems that adjusting the R8 resistor is gonna be a better idea than the 1M5 i currently have at the 500K trim location, but I understand it's probably not gonna make everything OK, just potentially better.

Again - all this help is great. It's most helpful in helping me understand whats actually going on in the circuit. If it never works right, not the end of the world... I can always try again as I have 3 more of these boards etched anyway (it seems to take the same amount of time to etch 4 as it does one).

Thanks again!

- Greg

[johngreene]

Look carefully at Q3s circuit fragment.  There are a few things going on.

First what is the battery, base and collector voltage on a real one that is operating correctly?  This will help a bit to understand it.  That will give Ic and help figure out the current in D1 to look at the transistor and diode curves at that voltage

I would guess it was designed with an Ic(collector current) and collector resistor(s) as being fixed.  You might not want to mess with R9 and R8 because of the designed output resistance of Q3 fragment and the tone control.

R14, R15 and D1 are the bias network for the transistor.  The curves of the Diode used and Q3 used might be the things to check.

Look at a BMP note the bias network The BMP has a resistor from base to ground.  The TB has a diode, Diodes conduction changes a bit with current.
Current in this TB D1 is via the C to B voltage drop divided by R14 plus R15 then you have leakage current from the transistor.

Not know what the DESIGNERS had in mind makes it hard to a study.
  I am guessing D1 is there as part of the design and maybe the circuit is designed to operate with the collector not at 1/2 the battery voltage

Also the gain of the Q3 stage is the open loop in combination with R14 and R15 along with the conductance of D1 and R7 setting resistance and the output resistance of Q1,2

But Gus, this is a positive ground circuit. Isn't D1 only there to protect the input of Q3 from going below ground (actually 'more positive')? The diode is reversed biased and therefore has no current flowing other than leakage.

--john

[gmr1]

that was my understanding, and why it could be just any ol' germanium transistor. (why does it have to be germanium?)

[Gus]

John yes you are correct. my mistake about the diode part. 
I had not checked that circuit in detail before.

Maybe it is about what GFR has written at
http://www.geocities.com/sunsetstrip/studio/2987/britface.html
look at the end

I still would want to know what the operating points of a real TB like this one were designed at.  I would not change the collector resistors I would make R15 what value needed to bias Q3.

Now I have bought Ge diodes in packs in the past that some were bad.  I alway check them with a DMM set to diode test and the ohms both ways.  Use clip on probes at room temp if possible and don't hold the diode in your hands