(Biasing a transistor) Sola Sound Tonebender MKII clone

[powerplayj]

I need a bit of help with my MKII Tonebender clone. The collector of Q3 should be 4.5-5.5 DcV but, mine is closer to 8.0V.

http://fuzzcentral.tripod.com/benderII.html

I have tried a 25K trim pot in place of the 8.2K resistor with no luck. I have also constructed an audio probe and traced over the circuit. The only significant finding was a significant increase in gain at the Q3 collector. The pedal is a kit and the trannies are all matched.

What else can I do for Q3 short of trying a replacement?

[R.G.]

There is really very little that we can tell without more information. It's unlikely that 8V is where it's supposed to be, but there are many, many things that it could be.

Can you give us a link to the actual schematic, any subs or mods you made, actual transistors used, and voltages on all transistor pins and diode pins?

[phillip]

He was trying to link directly to the image from Fuzz Central, but Tripod doesn't allow remote access to the images, which is probably good since it would kill my bandwidth!  

Here's the schematic from Fuzz Central:



Phillip

[powerplayj]

He was trying to link directly to the image from Fuzz Central, but Tripod doesn't allow remote access to the images, which is probably good since it would kill my bandwidth!  

Phillip

Yep, yours is the best Fuzz site bar none IMO.  


The kit I'm using is a positive ground MKII (Ge) with http://www.bigtonemusic.com/mk2populate1.htm

Q1 e: 0.0
b: 0.0
c: 8.88

Q2 e: 0.09
b: 0.06
c: 0.09

Q3 e: 0.05
b: 0.09
c: 8.74

battery measures 8.8V

[R.G.]

I think that the transistors you're using may be suffering from leakage or high gain.

Try putting a resistor between base of Q2 and ground, of 1M or so. If that does not help, lower the resistor in steps down to 100K. Report back.

[Ge_Whiz]

Leaky transistor, or incorrect value of collector resistor or gain pot. Have you checked the latter carefully?

[powerplayj]

One additional note before I tinker a little more or just start all over again............  I took the pedal to work to have a friend check my circuit and we noticed the battery to be extremely hot.  What problems would a hot battery be the symptom of?

[R.G.]

What problems would a hot battery be the symptom of?

Very high battery current. You have either a short or a very low resistance path through the effect.

[fuzzymuff]

Well, its been about 8 years since this thread.  Is the consensus still 4.5v for he Q3 collector, and the gains still Q1 70, Q2 80, and Q3 110 for the Tonebender MKII?

[Electric Warrior]

No. powerplayj's voltages look alright. Here's some measurements from my vintage unit:

Battery: -9.67V

Q1 C -9.02 B -0.03 E 0
Q2 C -0.17 B -0.08 E 0
Q3 C -8.44 B -0.17 E -0.11


Gains: Whatever works. Guess there's more than one way to set it up. Plenty of leakage usually works rather well.
Note that there were different bias setups for OC75s/Impex S3-ITs and OC81Ds. Most of fuzzcentral's schematics have errors.

[fuzzymuff]

No. powerplayj's voltages look alright. Here's some measurements from my vintage unit:

Battery: -9.67V

Q1 C -9.02 B -0.03 E 0
Q2 C -0.17 B -0.08 E 0
Q3 C -8.44 B -0.17 E -0.11


Gains: Whatever works. Guess there's more than one way to set it up. Plenty of leakage usually works rather well.
Note that there were different bias setups for OC75s/Impex S3-ITs and OC81Ds. Most of fuzzcentral's schematics have errors.

I guess you are right.  I've heard so many tone benders, and they all seem to sound different, some sound very compressed and spitty while others are very open, noisey and dynamic, very sensitive to the guitar's volume adjustment.  

I noticed that using transistor with lower leakage , under 100uA, regardless of hfe, it sounds compressed with very long sustain and transistors with higher leakage sounds more open and nosier.  Correct observation?

[Electric Warrior]

Not quite. I found it easy to make it sound very open, trebly and hissy with too little leakage. With higher leakage transistors the hiss becomes very managable (200µA to 400µA should work well). Sounds much thicker and fuzzier, too. And gating works much better. I audition several transistors to get the amount of clean up or gated-/spittyness I desire. And I try to find something that works well in a wide range of temperatures to make gigging life easier.

Overly compressed (with very little range on the attack pot) usually happens when you use a 100k at Q1's base with OC75s (like JMI and others do) or other transistors that would work better with the OC75 setup.

[fuzzymuff]

Hmmm....thanks Warrior for your helpful tips.  Yes, I do have 100K at the Q1's base, and I'm using AC125s, which is suppose to be similar to the OC81.

[Electric Warrior]

AC125s vary a lot. I have a couple of Valvos that turned out to be an excellent replacement for OC75s. Tried to get more, but I couldn't find any in the same hfe and leakage range. 

[fuzzymuff]

AC125s vary a lot. I have a couple of Valvos that turned out to be an excellent replacement for OC75s. Tried to get more, but I couldn't find any in the same hfe and leakage range. 

I've got four bags of AC125s, two bags ranging from 50hfe-100hfe and two bags from 75hfe to 150hfe.  These are 150 counts per bag and Tungsgram branded.  What are your thoughts on these AC125s?

[Electric Warrior]

Never tried any Tungsrams, sorry.