Fuzz Face resistor calculator

[spoontex]

Hello,

anybody can explain how to use this tool?

http://www.muzique.com/lab/fuzz_face.php

I put my transistors values, and later? What resistor tweak? how much?

Thanks.

[mac]

I wrote a FF calculator which has similar symbols so I can assist you,

https://www.diystompboxes.com/biascalc/ffbias.html

pdf, and mac/win apps in my gallery here,

https://www.aronnelson.com/gallery/main.php/v/mac/


Note:
Do not expect quantum mechanics accuracy :-)
I wrote this pages and apps to help others understand how to calculate simple circuits bias, and how germs leakage mess things up!



You want to estimate the total collector resistor at Q2 for a given set of transistors to get around 4.5v at Q2 collector, after fixing R1, R3 and R4 at the original values, right?
In my page you have the standard resistors and a typical "good" germ preloaded.

The problematic germ is Q1 because Q2 "amplifies" Q1 leakage. In a real FF if you only heat Q1, Q2 will go out of scale :-)
A hybrid Silicon Q1 and Germ Q2 works fine.

Input Q1,2 hfe and leakage, fix 33k, 1k and 100k, and tweak R2 resistor until you find a value that shows Vc2 near 4.5v ( Q2 collector ).
For Silicons leakage is zero, nada, niente.
For Germs use milli amps. IIRC RG tester is in micro amps so remember to convert units, 1000ua=1ma.
For example, if you read 10ua input 0.01ma.

After you get familiar you can tweak R1, R3, R4 and R2 to estimate Vc2.

mac

[spoontex]

This is a pair of nkt275. I do it right? What do you think?

https://ibb.co/CQy7FPT


Thanks a lot!

[mac]

Check Vbe, I never read 0.3V, half that value instead.
In this case R2 near 10k might do it. R1,3,4 standard.

Remember that tomorrow you will read different values :-)

mac

[spoontex]

The vbe of a germanium transistor is near 0.30 no?

[mac]

The vbe of a germanium transistor is near 0.30 no?

In theory, my readings are lower, 0.15v and some ACxxx even lower. Maybe it is my cheap chinese DDM :-)

mac

[spoontex]

jaja, thanks a lot!!

I will try. Now I have an idea of how it's works.


Thanks again.

[PRR]

Small Germanium at low currents, 0.1 to 0.15 is much more typical than 0.3V.

[Electric Warrior]

I always assumed Vbe was around 0.1, because someone told me that the voltage on Q1b in a fuzz face equals Vbe.
On my Peak Atlas they usually measure around 0.3V.

These calculators are pretty good for learning the variables and dynamics in a fuzz face, but the voltages can be far off from actual measurements. Especially with germaniums..

[PRR]

The Peak Atlas may not be optimized for Ge.

There is wide variation; Ge processiing was slap-dash and you often only find reject-limits like "Vbe<1V". Also large parasitic resistance so in show-off datasheet conditions you find high Vbe at high current whereas many small-audio rigs run much lower current.

[mac]

Fixing Vbe simplifies calculations a lot.

I thought of using vbe = f(ib) but I had to write a program... too lazy :-)

If you know Vbe and hfe at working conditions estimations are acceptable for such a simple model.

mac

[PRR]

>> The vbe of a germanium transistor is near 0.30 no?
> my readings are lower, 0.15v

Some data from 1959 issues of Radiotronics.

2N109. Working at a large fraction of the rated Maximum current, we do indeed find around 0.3V. However in small audio we often work below 10%, even near 1%, of the max current. Near point Y, which shows 0.14V at 2mA.


Note the curve above 15mA is a straight line. In post-1970 Silicon devices we find an exponential curve all the way past half max current. Shockley's Law. But 1950s Ge technology was, in hindsight, crude, with large parasitic resistance. Here, about 3 Ohms added to the exponential. Below ~~10mA Shockley is valid, over 10mA we must add the parasitic resistance.

Here's some random snaps of the audio end of Ge radios. Depending on the part used and the current it is run at we find 0.060V to 0.200V.

[mac]

I have some old radio books too, same low values.

mac