Tone Bender II transistor bias issue

Forum dedicated to helping people debug and troubleshoot non-functional pedals or builds. Please use an clear and informative title, indicating circuit and basic problem. Don't forget to mark the issue as fixed if this is the case.
Post Reply
User avatar
robusto
Information
Posts: 4
Joined: 25 Jun 2019, 14:27
my favorite amplifier: Marshall Lead 12
Location: Sioux Falls, SD, USA
Has thanked: 2 times
Been thanked: 1 time
Contact:

Post by robusto »

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/ ... tation.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!

User avatar
crickyo
Information
Posts: 17
Joined: 08 Aug 2019, 15:57

Post by crickyo »

" One thing to keep in mind is that the Q2 bias trimmer actually adjusts the
bias of Q3 as well, while the Q3 pot only adjusts Q3. So, always bias Q2 first and then only use Q3’s bias
adjustment if you need to. The Q2 trimmer is going to be far more useful.

As a starting point, set the Q3 bias trimmer to around 75% rotation (approximately 8.2k total resistance
when combined with R6). Turn the Q2 bias trimmer all the way down. Then, with a multimeter, touch the black
(common) lead to ground and touch the red lead to the far-right leg of Q2. You’re looking for something near
these voltages.

Q1: Collector -8.5v, Base -0.05v, Emitter 0v
Q2: Collector -0.15v, Base -0.07v, Emitter 0v
Q3: Collector -8.5v, Base -0.15v, Emitter -0.1v


Since Q2’s voltages are so low, it’s best to test Q3’s collector as you adjust the Q2 trimmer and stop when
Q3 is where you want it. The Q3 bias trimmer should only be used if Q2’s voltage is way off once this is done.

this also might help you

Leakage (measured in microamps or μA) is important. Unlike most other transistor-based pedals, in this case
some leakage is required for it to function properly. You’re looking for somewhere between 100μA and 400μA
of leakage—any less than that and you might not get any signal past Q1. (Note that RX1 and RX2 have been
provided to simulate leakage on Q1 and Q3 if your transistors measure lower than the optimal range.)


The info above is from the build doc that you linked.
It looks like you need to have the right values before the signal goes through, it sounds like you have wired the pedal right just check the input and output too (i had these back to front before the bypass worked but no sound :) )

User avatar
Manfred
Tube Twister
Information
Posts: 1937
Joined: 04 Apr 2009, 23:42
Has thanked: 1671 times
Been thanked: 1344 times

Post by Manfred »

he suggested readings in the build doc are -
Q1: C -8.5v B -0.05v E 0v

With both trims at 100% -
Q1: C -9.33v B -0.001v E 0v
The base voltage of Q1 is determined by the collector base leakage current that flows through the 100 Kiloohms resistor between base and ground.

Your read base voltage ist too low, I suppose that either the leak current ist to low or Q1 is defektive.

User avatar
mozz
Breadboard Brother
Information
Posts: 93
Joined: 26 Jan 2016, 23:07
Location: Pennsylvania
Has thanked: 53 times
Been thanked: 22 times

Post by mozz »

Try swapping q3 over to q1. Also, be sure smallbear and the layout you are using are labeled the same (r1=r1, r4=r4)

User avatar
Electric Warrior
Diode Debunker
Information
Posts: 737
Joined: 23 Feb 2009, 03:37
Has thanked: 23 times
Been thanked: 104 times

Post by Electric Warrior »

Manfred wrote:
he suggested readings in the build doc are -
Q1: C -8.5v B -0.05v E 0v

With both trims at 100% -
Q1: C -9.33v B -0.001v E 0v
The base voltage of Q1 is determined by the collector base leakage current that flows through the 100 Kiloohms resistor between base and ground.

Your read base voltage ist too low, I suppose that either the leak current ist to low or Q1 is defektive.
Turned out it was both.
He used a 10k on Q1C, which certainly isn't enough for his low leakage transistors. Swapping it for a 100k made no difference, though, as the transistor is fried: https://www.diystompboxes.com/smfforum/ ... c=122972.0

Post Reply