yet another biasing question

[m7b52000]

I have built a PNP pos ground Fuzz Face with some old Mullard OC75 /45 with hfe 80 and 120. It is was slightly modified in line with the Fuzzcentral Axis Fuzz with:
R1 33k
R2 1.2k
R3 bias pot
R4 100k

I have also also added the pre-gain smooth control. It sounds great and cleans up nicely and I am happy with it but the biasing of Q2 is a bit odd. The 10k bias pot only allows a voltage of between -1 and -6 volts i.e with no resistance at R3 the bias voltage is -6V. It's really just an academic question as I am happy with the sound but what are the possible causes? Does it imply excessive leakage at Q2? Should I even worry?

thanks

[antonis]

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

Just reverse polarity or add minus sign on results..

[m7b52000]

I will have to go and measure the leakages again but I do see from the calculator that increasing the Q2 leakage does decrease the bias voltage.

[antonis]

I do see from the calculator that increasing the Q2 leakage does decrease the bias voltage.

You're free to put the blame on Jack Orman..

[GibsonGM]

While you are at AMZ, take a look at the Mosfet Boost, you may wish to build yourself one 

[antonis]

take a look at the Mosfet Boost

Sience should be interested in your manic obsessions, Sir..

[Electric Warrior]

I will have to go and measure the leakages again but I do see from the calculator that increasing the Q2 leakage does decrease the bias voltage.

Not really. Gain and leakage on Q2 hardly make any difference. The voltages depend mostly on Q1 and how it's biased.

The range you're getting fromt the pot does appear to be odd. Could you post the voltages of all other transistor leads as well?

[m7b52000]

will do. It will have to wait until I get home from work tonight.

[m7b52000]

As requested:

Battery 8.66 V

Q1 C -3.22V
     B  -76.3 mV
     E  -.1 mV

Q2 C -4.65
     B -3.36
     E -3.13

[Electric Warrior]

Ok, that looks all wrong. A germanium Fuzz Face would usually have Q1C somewhere between -0.3 and -0.7V, Q1B around -0.1V. Q1E should be 0 as it goes to ground, but I guess -0.1mV is probably close enough?

You better do some trouble shooting and find out where you went wrong.

Also, it may be time for a new battery.

[m7b52000]

OK, have just checked again after fixing a potential issue due to rotation of a pot against another pot leg..

Q1 C .85 V
     B 77mV
     E 0

Q2. C 4.4 V
      B  0.79V
      E  0.67 V

Is that better?
     

[Electric Warrior]

That does look much better. How is the range on the bias pot now?

Q1C measures really high. I guess its leakage measures close to zero. That must be the OC45. In a stock circuit, this would result in a very low voltage on Q2C.

[m7b52000]

the 10k bias pot now goes from -1.3 to -6.6V. It sounds better than it did an hour ago...

[Electric Warrior]

It does seem low according to the calculator, but as your supply voltage is very low and the fuzz pot has quite a bit of tolerance, maxing out at 6.6V is probably not all that far off.

[m7b52000]

Thanks, I'll check again with a new battery tonight.

Jobs for the weekend:
  -check hfe and leakage of all my Ge transistors
  -audition all of them in current build and document all voltages
  -build another version with a trimmer in place of 33k
  -stay up late fiddling....