Biasing RDV Tonebender.

[sopapo]

 I have made this silicon tonebender MKII circuit by RDV, I have used this  veroboard layout :http://tagboardeffects.blogspot.com/2013/07/npn-si-tonebender-mkii.html



I have used three 2n2369, with an hfe of (q1-q3):70/70/100
I too would like to know wich voltages to look for... I put a 10k trimmer instead of the 5,6k resistor and first biased at 4,5v, sound good, with certain tonebender character, later biased at 8v,  looking for what I have read are the usual voltages of the germanium tonebender, and the sound was equally good too, but with less tonebender character in my opinion, more overdrive like...

The voltages are (with an vcc:9,58)
Q1:  70 Hfe   C/ 7.51    B/ 1.10    E/ 0.45
Q2:  70 Hfe   C/ 1.78    B/ 0.67    E/ 0.01
Q3:  100 Hfe   C/ 8,03    B/ 1.78    E/ 1.09

I have adjusted the trimmer on q3 until 3,6v and the tone is better, with more grain and certain sputtery tone that I believe is the tonebender trademark...
My question is, why this voltage sound good-better? all the measures about I have read of the mk2  are in the 8v realm, the hfe of q3 is 100, I dont know why
Sound good becouse is a fuzzface?? q1 is doing nothing?
I think a trimmer for q2 is in the way....

Lots of questions... Can i look for the same voltages and Hfe of the the germanium tonebender?
(to sound good).. the others voltages are fine? are important?
what other voltages (B,E) must care for?
A second trimmer for q2? another transistors (Higher Hfe?)

Thanks for you time
Best regards

[bmsiddall]

I've played around with this circuit and I'm getting very close to what I want from it.

The last build:

 Q1 C-B = 1M (Q1C 8.3V).
 Q1E resistor- 3.3uF in parallel
 Q2C resistor 100k (could go higher- 220k??)
 Top 1K resistor back to stock 470 ohm
 Q3C resistor 5k trimpot (adjust to >8V)
 33nF cap in parallel with top 470R resistor
 Output capacitor 0.015uF
 Attack pot C1K, capacitor 4.7uF
 Q HFE = 39, 132, 400
 Volume pot 50k in series with 50k resistor to lower max volume
o Could go to 75/25k to match germanium version

Next build i think ill use a 500k trimpot on q2 collector to see how low i can get the voltage. Great cct and compares really well with the germanium version, probably more focussed i think.

[Mark Hammer]

Just a little note.  Ricky was an active and valued member here until his untimely passing at too young an age.

[sopapo]

Hello, yes Mark, I had already read about his passing, for all i know he was a very nice and enthusiastic individual, its very sad...

bmsiddall, I have changed the circuit to original MK2 values , in words of Rick, if you are using very low gain transistors, you can omit the mods..So i have:

-Omitted the emiter resistor of Q1, bridged to ground like the original
I have sub the 470k with 1 meg too, to bías the colector of Q1 to 8'2 v

-bridged the emiter of Q2 to ground, put a a 100k trimmer to try to bías the c to something in the realm of 0'4v -0'8v etcetc but I cant reach below 1'3 v, I have tried several things, changed the c resistor to several upper values, , even try 600k, try and hfe of 650, with non results, the lowest is 1'30v, why is this??? ...the upper resistor is he modded 1k of Rick design....should try the original 470r???

-q3 c is at 8'4v with a 10k trimmer, left the 330r resister, as i understand that has no efect on bías

So, in short, I have returnes to original schematic, but my problem is the c of Q2 cant low the voltajes to the intended values, the hfe is 78.
Has the leakage something to do with this?? (The lack of leakage) i dont understand  the efect of leakage in this circuit...
bmsiddall, which is your q2 c voltage??

Thanks to all

[antonis]

Although it's drawn for Fuzz Face, it might help you due to almost identical configuration..
(just re-number your particular BJTs..)



In your case, for Q1 Collector biased lower than 1.3V (VA) you need a Q1 Collector current (blue arrow) larger than 164μA..
That current consists of Q1 C-E current (light blue arrow) and Q2 Base current (red arrow)..
Q1 C-E current (light blue) is considered Q2 Emitter feedback current (black arrow) +  Q1 I_CEO (C-E leakage current)

You can make all necessary substitutions to find out if there is any room in calculations for I_CEO..

[sopapo]

Thanks one more time, Antonis,  :icon_wink:Im not sure if I am capanble to do this kind of calculations, is beyond my head, Its something relatee to change the feedback resistor? Or add an emitter resistor??

[antonis]

ΟΚ..Let's make things more simple..

You want Q2 Collector (with its Emitter grounded hence its Base at about 600mV), to sit at 0.4V to 0.8V (lets make it 0.6V for simplicity)..
Q2 Collector at 600mV means Q3 Emitter at 0V..!!
(no current through Q3 Emitter 220R + 1k trimpot hence no Q2 Base bias - very unhappy BJT.. )

To distract Q2 from its misery, we will need about 600mV at its Base PLUS additional 600mV at Q3 Base (we DO want both BJTs happy, don't we..??) PLUS some mV drop across 100k feedback resistor..

If you like to calculate those "some" mV across 100k resistor, you can easily do it as follows:
For Q2 Collector at 1.3V, Collector current should be about 160μA (ignoring Q3 Base current for simplicity)..
For h_FE = 78, Q2 needs a Base current of 2μA..
2μA across 100k result into 200mV drop..

What is the summation of 600mV + 600mV + 200mV..??

P.S.
exersise: If we replace "600mV" (Si BJT V_BE) with "300mV" (Ge BJT V_BE) how much lower Q2 could be biased..??

 

[antonis]

In case of simulateously dealing with both voltage and current puzzles you, let's talk about voltages only..

KVL loop between Q2 Collector & GND:



You can see that V1 can't be lower than 2 X 600mV plus (V_2-3 + V_3-4 + V₅)..
For Q2 Emitter grounded (no 100R resistor) and V_2-3 negligible, V₁ equals to 1.2V + V_3-4
(which V_3-4 can't be zero for a happy Q2..)

So, despite Q2 Collector resistor value, Q2 Collector can't be biased at a level lower than 1.2V plus something..

[sopapo]

Hello, sorry for not answering before, I get cought by  this covid s***..
Thank you for your great effort Antonis, so if I  have understood right, 1.4 v it would be the limit for a silicon transistor to be biased in this case, and for germanium...0.7v?? but the usual for this circuit with ge trannys is 0.3v, is for the ge leakage that is possible to bias it lower?
Thank you very much
Best regards

[antonis]

If we're talking about Ge Q2 Collector bias voltage, values as low as 100mV (with Base voltages as low as 60mV) can be obtained..

Values of 650mV for Si and 300mV for Ge p-n junctions are a general rule of thumb and assume particular working conditions (e.g. BJT in forward active mode, room temeprature, etc..)
Diode's forward voltage drop is logarithmically related with current, typically 70mV per decade..

Base - Collector junction is (or should be) reverse biased (Collector voltage higher than Base one) when transistor is in forward active mode..
By lowering that voltage difference, transistor enters into a "grey region" and finally results into reverse-active mode (Base voltage higher than Collector one or, more accurately, Collector voltage lower than Base one)..
Here comes leakage current, lowering Collector voltage while simulataneously raising Base voltage (you can find Ge n-p-n BJT circuits working with Emitter grounded and Base also grounded via a big value resistor..)

P.S.1
I might further confused you so, Yes..!!
(leakage current is responsible for such oddities..) 

P.S.2
I regard Electric Warrior (among other forum members) as Tonebender MK ii expert so it should be a wishful thinking to participate here..