Author Topic: Tone Bender II transistor bias issue  (Read 410 times)

newjackruby

Tone Bender II transistor bias issue
« on: August 24, 2019, 09:54:50 AM »
Hello,

Would like to get some opinions on how to proceed here.

I am building the Aion Deimos, and having problems getting the transistors to bias correctly.

Here's the build doc : https://aionelectronics.com/wp-content/uploads/2015/04/aion-deimos-tonebender-mkii-documentation.pdf

I am using a "Flying Dutchman" set of PNP Germanium transistors from Small Bear. They are marked as follows:

Q1 - Gain 64, Leakage 114
Q2 - Gain 75, Leakage 208
Q3 - Gain 116

Small Bear also included a set of resistors to use for R3, R4, and R6, which I did use instead of the values in the build doc, so my R3 is currently 27k instead of 47k and my R6 is currently 10k instead of 1k. (R4 remained at 100k)
This build also has a 50k trim pot on Q2 and a 10k trim pot on Q3.

The suggested readings in the build doc are -
Q1: C -8.5v B -0.05v E 0v
Q2: C -0.15v B -0.07v E 0v
Q3: C -8.5v B -0.15v E -0.1v

I cannot get to those values with the current setup. Here's the range I'm getting:

With both trims at 100% -
Q1: C -9.33v B -0.001v E 0v
Q2: C -0.55v B -0.03v E 0v
Q3: C -2.4v B -0.5v E -0.5v

With both trims at 0% -
Q1: C -9.33v B -0.001v E 0v
Q2: C -1.2v B -0.04v E 0v
Q3: C -1.1v B -1.2v E -1v

I'm not entirely sure what I need to do from here. I'm guessing I will have to switch out resistors until I find the right combo. Can anyone give me a suggestion as to where to start?

I should mention that I have been through the rest of the circuit and tested all components and connections. Everything is good. I get bypass signal through the pedal. When engaged, my LED lights, but no sound.

Thanks for reading!

Re: Tone Bender II transistor bias issue
« Reply #1 on: August 24, 2019, 10:23:43 AM »
You will need more resistance on Q2c. Try swapping R3 for a 47k. That will hopefully give you enough range on the trim pot.

With Q2 set correctly, Q3's collector voltage will be in the right ballpark with the stock 8k2 value. You will need to swap R6 for a 1k to put that within the range of your trim pot.

Did you use a 100k for R1?

newjackruby

Re: Tone Bender II transistor bias issue
« Reply #2 on: August 24, 2019, 10:41:04 AM »
You will need more resistance on Q2c. Try swapping R3 for a 47k. That will hopefully give you enough range on the trim pot.

With Q2 set correctly, Q3's collector voltage will be in the right ballpark with the stock 8k2 value. You will need to swap R6 for a 1k to put that within the range of your trim pot.

Did you use a 100k for R1?

No, I used 10k. Maybe I should switch that too?

Re: Tone Bender II transistor bias issue
« Reply #3 on: August 24, 2019, 10:55:36 AM »
Yes, a larger resistor would probably work better for these.

Sola Sound originally made these with a 10k on Q1b and a 47k  on Q2c with OC75 transistors. Those leak a lot.
When they started using OC81Ds with lower leakage they swapped both resistors for 100ks to bias them to the same voltages.

Maybe start with the resistor on Q2. If it turns out that you need 100k of total resistance on Q2c, chances are that it's a good ballpark for Q1b as well.

newjackruby

Re: Tone Bender II transistor bias issue
« Reply #4 on: August 24, 2019, 12:46:44 PM »
Cool. Will do. Thanks!

newjackruby

Re: Tone Bender II transistor bias issue
« Reply #5 on: August 24, 2019, 04:03:06 PM »
Okay,

I have switched R1 to 100k and R3 to 47k. Not there yet. Here's what I'm getting now:

Trims at 100%:

Q1 - C -9.34 B -.002 E 0
Q2 - C -0.9 B -0.04 E 0
Q3 - C -3.03 B -0.9 E -0.77

Trims at 0%:

Q1 - C -9.34 B -.002 E 0
Q2 - C -0.49 B -0.03 E 0
Q3 - C -3.13 B -0.49 E -0.38

 :icon_question:

Larger R3?


Re: Tone Bender II transistor bias issue
« Reply #6 on: August 24, 2019, 04:24:04 PM »
Funny how the 100k on Q1b made no difference at all  :o I have no idea what happened here.

With Q2 you're there. Back off its trimmer a little from 100% and it's good.
8k2 should be close to zero on Q3's trimmer...
« Last Edit: August 24, 2019, 04:30:31 PM by Electric Warrior »

newjackruby

Re: Tone Bender II transistor bias issue
« Reply #7 on: August 24, 2019, 05:18:14 PM »
So -

Quote
The suggested readings in the build doc are -
Q1: C -8.5v B -0.05v E 0v
Q2: C -0.15v B -0.07v E 0v
Q3: C -8.5v B -0.15v E -0.1v

...and currently -
Quote
Trims at 100%:

Q1 - C -9.34 B -.002 E 0
Q2 - C -0.9 B -0.04 E 0
Q3 - C -3.03 B -0.9 E -0.77

Trims at 0%:

Q1 - C -9.34 B -.002 E 0
Q2 - C -0.49 B -0.03 E 0
Q3 - C -3.13 B -0.49 E -0.38

The 0.9 is close enough to 0.15, and the rest of the numbers that are also close fall in line? I'm pretty green at this, and it's my first time attempting to bias germanium. Q1 Base is still way out. Will that cause a problem?

Re: Tone Bender II transistor bias issue
« Reply #8 on: August 25, 2019, 05:11:45 AM »
So -

Quote
The suggested readings in the build doc are -
Q1: C -8.5v B -0.05v E 0v
Q2: C -0.15v B -0.07v E 0v
Q3: C -8.5v B -0.15v E -0.1v

...and currently -
Quote
Trims at 100%:

Q1 - C -9.34 B -.002 E 0
Q2 - C -0.9 B -0.04 E 0
Q3 - C -3.03 B -0.9 E -0.77

Trims at 0%:

Q1 - C -9.34 B -.002 E 0
Q2 - C -0.49 B -0.03 E 0
Q3 - C -3.13 B -0.49 E -0.38

The 0.9 is close enough to 0.15, and the rest of the numbers that are also close fall in line? I'm pretty green at this, and it's my first time attempting to bias germanium. Q1 Base is still way out. Will that cause a problem?

close enough? yes. but it will probably behave better at 0.15, so just set it to that. It's within your trim pot's range.
Once that is set you can set Q3's trim pot.

Are you sure you really picked a 100k for Q1b?

newjackruby

Re: Tone Bender II transistor bias issue
« Reply #9 on: August 25, 2019, 02:56:37 PM »
Yep, it's 100k.

But...REALITY CHECK!

Just tested my Q1, and I have managed to FRY it.  :'( :'( :'( :'(

Oh well, back to the drawing board (or back to the internet for more transistors).

Thanks for all the great replies Electric Warrior!

I'll keep this thread updated until I get this damn thing to work.