unresponsive bias control on a tonebender mk 2 circuit

[mordechai]

I just built a TB Mk2 and the gain was super, super saturated with all the different transistor choices I tried (OC75s, 2n1309s, even 2n404s) -- very nasal and, when the fuzz pot was maxed out, suddenly super muddy and oscillating.  I tried adjusting the 10k bias pot but there was no audible effect...I'd appreciate any suggestions about how to tame the circuit a bit to make it more usable, and especially why the bias pot is unresponsive and how I can fix it...

[LucifersTrip]

I just built a TB Mk2 and the gain was super, super saturated with all the different transistor choices I tried (OC75s, 2n1309s, even 2n404s) -- very nasal and, when the fuzz pot was maxed out, suddenly super muddy and oscillating.  I tried adjusting the 10k bias pot but there was no audible effect...I'd appreciate any suggestions about how to tame the circuit a bit to make it more usable, and especially why the bias pot is unresponsive and how I can fix it...

you're using this?


definitely need some voltage readings to troubleshoot.

but just like with a FF on Q1, oscillating can be stopped with small (100-200pf) cap across BC of Q2. though, I have never experienced muddiness at max fuzz.

if the bias pot doesn't work, then it's prob wired wrong or the transistors are pushed so far, adjusting it becomes insignificant.
what happens when you adjust the 8.2K?

[mordechai]

I just built a TB Mk2 and the gain was super, super saturated with all the different transistor choices I tried (OC75s, 2n1309s, even 2n404s) -- very nasal and, when the fuzz pot was maxed out, suddenly super muddy and oscillating.  I tried adjusting the 10k bias pot but there was no audible effect...I'd appreciate any suggestions about how to tame the circuit a bit to make it more usable, and especially why the bias pot is unresponsive and how I can fix it...

you're using this?


definitely need some voltage readings to troubleshoot.

but just like with a FF on Q1, oscillating can be stopped with small (100-200pf) cap across BC of Q2. though, I have never experienced muddiness at max fuzz.

if the bias pot doesn't work, then it's prob wired wrong or the transistors are pushed so far, adjusting it becomes insignificant.
what happens when you adjust the 8.2K?

I was using the schematic for the tonebender mk 2 project from tonepad.  I swapped out a few items several times to see what was going on -- the fuzz pot is now a 2k, the bias pot was either a 25k or a 10k (i've used both), and I've vried the feedback resistor between q2/q3 a few times as well.  I have also tried several different transistors, but they're all giving me a pretty scratchy, super-duper heavy tone to the point of virtual uselessness.

[mordechai]

Maybe the best way I can describe the overall sound is that it "motorboats"...after an initial decent saturated tone, it starts to get very sputtery.

I incorporated a 220pf cap between b and c of q2 and it's a little better, but still not terrific.  I am now using AC125s and the gains are 95, 101, and 137.

[petemoore]

  Transistor pin voltages allow workable equations which render useful suggestions.
  Audio probe can tellya stuff too, heavy bet on the transisitor misbias caused by...we'll probably be able to tell more after doing some calculating/if that coincided say with a signal block. 

[LucifersTrip]

I just built a TB Mk2 and the gain was super, super saturated with all the different transistor choices I tried (OC75s, 2n1309s, even 2n404s) -- very nasal and, when the fuzz pot was maxed out, suddenly super muddy and oscillating.  I tried adjusting the 10k bias pot but there was no audible effect...I'd appreciate any suggestions about how to tame the circuit a bit to make it more usable, and especially why the bias pot is unresponsive and how I can fix it...

definitely need some voltage readings to troubleshoot.

but just like with a FF on Q1, oscillating can be stopped with small (100-200pf) cap across BC of Q2. though, I have never experienced muddiness at max fuzz.

if the bias pot doesn't work, then it's prob wired wrong or the transistors are pushed so far, adjusting it becomes insignificant.
what happens when you adjust the 8.2K?

I was using the schematic for the tonebender mk 2 project from tonepad.

please link for reference

I swapped out a few items several times to see what was going on -- the fuzz pot is now a 2k, the bias pot was either a 25k or a 10k (i've used both), and I've vried the feedback resistor between q2/q3 a few times as well.  I have also tried several different transistors, but they're all giving me a pretty scratchy, super-duper heavy tone to the point of virtual uselessness.

still, "definitely need some voltage readings to troubleshoot", "Transistor pin voltages allow workable equations which render useful suggestions" & "what happens when you adjust the 8.2K*?"

*or whatever tonepad has there

[mordechai]

[/quote]


definitely need some voltage readings to troubleshoot.

but just like with a FF on Q1, oscillating can be stopped with small (100-200pf) cap across BC of Q2. though, I have never experienced muddiness at max fuzz.

if the bias pot doesn't work, then it's prob wired wrong or the transistors are pushed so far, adjusting it becomes insignificant.
what happens when you adjust the 8.2K?
[/quote]

Alright...I'll take some readings later this evening and get back to you.  In the interim, what factors would "push" the transistors so far that biasing becomes insignificant?

I was using the schematic for the tonebender mk 2 project from tonepad.
[/quote]

please link for reference

I swapped out a few items several times to see what was going on -- the fuzz pot is now a 2k, the bias pot was either a 25k or a 10k (i've used both), and I've vried the feedback resistor between q2/q3 a few times as well.  I have also tried several different transistors, but they're all giving me a pretty scratchy, super-duper heavy tone to the point of virtual uselessness.

still, "definitely need some voltage readings to troubleshoot", "Transistor pin voltages allow workable equations which render useful suggestions" & "what happens when you adjust the 8.2K*?"

*or whatever tonepad has there


[/quote]

Al

[mordechai]

Would the problem with unresponsive bias control and motorboating saturation arise from the collector resistor on Q1 being a 1K instead of a 10K??

[mordechai]

[/quote]


definitely need some voltage readings to troubleshoot.

[/quote]

Alright, here are the readings on each transistor:

q1: C: -9.15
      B:  -0.05
      E: -0.00

q2: C: -0.84
      B: -0.61
      E: -0.00

q3: C: -9.09
      B: -0.85
      E: -0.82

From these readings, can you get a sense of what's wrong with the circuit?  As a reminder: the sound is harsh, stuttery, WAY oversaturated and the bias adjustment does nothing...

[Electric Warrior]

doesn't look too far off. how many AC125s do you have? how much do they leak? I'd recommend socketing transistors and swapping them until you find a combination that works well.
If you don't have any more transistors just try swapping Q2 and Q3. I usually like Q2 to have the highest hfe..

[mordechai]

doesn't look too far off. how many AC125s do you have? how much do they leak? I'd recommend socketing transistors and swapping them until you find a combination that works well.
If you don't have any more transistors just try swapping Q2 and Q3. I usually like Q2 to have the highest hfe..

Well, I've swapped out a bunch of different transistors -- AC125s, 2n404, OC75s, combos of the three, and even a 2n4402 in the Q2 position...still get the same gruesome sound.  Maybe one of my resistors is fried, or the PCB I'm using is damaged...I don't know.

I will give the build another try, but if the voltages look good to you, then I'm stumped regarding this one.  Question -- what is the benefit of having Q2 be a higher gain than Q3?

[LucifersTrip]

definitely need some voltage readings to troubleshoot.

Alright, here are the readings on each transistor:

q1: C: -9.15
     B:  -0.05
     E: -0.00

q2: C: -0.84
     B: -0.61
     E: -0.00

q3: C: -9.09
     B: -0.85
     E: -0.82

From these readings, can you get a sense of what's wrong with the circuit?  As a reminder: the sound is harsh, stuttery, WAY oversaturated and the bias adjustment does nothing...

hey..

firstly, so you don't have any floating "[/quotes]", remember to remove the "/" from the first one.

secondly, still no link to the actual schematic you used...here it is:
http://www.tonepad.com/getFileInfo.asp?id=86

Q3 is way off...remember, you're trying to get ~ 4.5v for Q3C. home wrecker actually made a chart for his experiments with the mk ii pro
http://www.home-wrecker.com/tbmk2.html

have you tried adjusting R6 as I suggested above? you should have to increase it to decrease the 9.09. but remember, the closer you get to 8.2K when Q3C is 4.5v, the better chance you have of getting a good sound. too low and it'll be closer to an overdirve/distortion, and too high, it'll be softer, mushier & you'll lose gain.

if you can't get ~ 4 - 5v by adjusting R6, then you have to choose other transistors or you're wiring is off.

good luck

[Electric Warrior]

In vintage units Q3's collector voltage usually measures between -7.5 and -9 Volts. That's just where it ends up in that circuit.

[petemoore]

 1k is too small a value for Q1C bias resistor.

[mordechai]

I know that 1k is too small -- I accidentally put it in there, so I took it out and replaced with a 10K.  Still no help.

Now...I'm actually getthing the OPPOSITE problem with a (high gain) fuzz face circuit I wired up.  The signal comes through clearly (though there is a drop in volume when the effect is on) and there's a very small amount of light fuzz in the background.  It's a hybrid circuit, with a 2n4402 (Hfe 117) in Q1 and a high gain 2n404 (Hfe 265) in Q2.  I used a 1uf input cap, a .1uf output cap, a 2k fuzz pot and a 25k bias pot. The bias resistor on Q1 is 47k and I replaced the 470Ohm resistor with a 1.2k (as per the suggested mod on fuzz central). Here are the voltages:

Q1 C: - 0.80
      B: -0.59
      E: 0.00

Q2  C: -9.1
       B: -0.80
       E: -0.80

I'd appreciate any suggestions on what I'm doing wrong.  Two builds in a row can't be a coincidence!

[LucifersTrip]

In vintage units Q3's collector voltage usually measures between -7.5 and -9 Volts. That's just where it ends up in that circuit.

home-wrecker and small bear had it all wrong?
http://www.home-wrecker.com/tbmk2.html

[Electric Warrior]

Yup.

Stu provided some measurements of his pedals on the D*A*M forum:

OC75 MkII (10K on Q1 base, 47K on Q2 collector)
Battery -9.01
Q1 c -8.50 b -0.038 e 0
Q2 c -0.13 b -0.076 e 0
Q3 c -8.29 b -0.13 e -0.08

OC81D MkII #1 (100K on Q1 base, 100K on Q2 collector)
Battery -9.55
Q1 c -8.51 b -0.06 e 0
Q2 c -0.13 b -0.068 e 0
Q3 c -8.72 b -0.13 e -0.07

OC81D MkII #2 (100K on Q1 base, 100K on Q2 collector)
Battery -9.50
Q1 c -8.76 b -0.062 e 0
Q2 c -0.22 b -0.075 e 0
Q3 c -8.13 b -0.22 e -0.14

http://stompboxes.co.uk/forum/viewtopic.php?f=10&t=7185&start=20

Haven't seen any pedal with a 100k on Q1's base and a 47k on Q2's collector either. It's either 10k/47k (in units with OC75s or Impex S3-1Ts) or 100k/100k (in units with OC81Ds)

[mordechai]

Thanks for those readings and comments re: the 10K/47K.  I'll go back to that circuit and try those values.  IN the interim...any thoughts on my fuzz face dilemma?  I THINK it may have to do with the 1K (not a 1.2K) resistor I wanted to put in place of the 470Ohm...I mayhave read the color bars wrong and put in a 110Ohm (brown, brown, black, black, brown), which is the reverse of the 1K I wanted to use.  I'm a total noob, but -- could that be the problem??

[Electric Warrior]

give it a try and tell us.
with all these transistors you have you should be able to make it work well with the original component values..
Getting just the right amount of bass/treble and gating can be tricky, though. I keep experimenting with mine, but I guess I'm just being picky...

[LucifersTrip]

Yup.

Stu provided some measurements of his pedals on the D*A*M forum

Is it also -7.5 to -9 for the "professional" or is Fuzz Central also off?   ...and for the Supa Fuzz?
http://fuzzcentral.ssguitar.com/mkII.php