Q2 collector resistor value for PNP fuzz face

[ogandou]

Hello,

I'm new to this forum, I apologize if this has been discussed before or if I posted in the wrong thread. I just finished building an SEL tube amp from ax84.com, and while looking for a new project to work on I stumbled across this site, which looks most awesome! 

I started with a fuzz face, which looks simple enough. I actually initially found the schematics here. My question is: with the listed 8.2k resistor at Q2's collector, I find that the collector is pretty much at the same voltage than the emitter, which is not normal. And no sound comes out. If I lower the resistor to 3.3k, then the collector is roughly 2V below the emitter and the thing works (well, sorta, I'm not sure it's working quite like it should, but my point is that with the listed resistor value it just doesn't work at all). So clearly it looks like there's a kink there. Did it happen to anyone else?

Thanks for any help or light you can shed on this!

[midwayfair]

Welcome to the wonderful world of biasing! Yes, that resistor needs to be set such that you get 4.5v on Q2's collector. You're best off using a trim pot so it can be set perfectly.

Best resource available on this circuit:
http://www.geofex.com/article_folders/fuzzface/fffram.htm

[ogandou]

Thanks for your answer, and thanks for the link, it's very interesting!

What puzzles me is that to get the voltage you mentionned that 8.2k resistor has to be changed to 560 ohms. That's wildly different... It just doesn't sound right, does it?

[LucifersTrip]

if not wired incorrectly, most likely you simply haven't chosen the right pair.  you should be able to hit 4.5 - 5v with a collector resistor ~ 4 - 15K. If you go too far out of that range, you prob won't get the best sound.

here're the voltages from one of my germanium FF's:

e   b    c
Q1 -   0, .1, .6
Q2 - .45, .6, 4.5

...on the other forum, someone was cool enough to save all the voltages of good sounding ones that builders reported and then averaged them:

q1e: 0
q1b: 0.1     
q1c: 0.578   

q2e: 0.447
q2b: 0.578   
q2c: 4.51

[ogandou]

When I test them on my MM they report an hfe of about 16. That sounds kind of low? They're basic transistors from RadioShack... maybe not the best source... I'll try different ones.

[LucifersTrip]

When I test them on my MM they report an hfe of about 16. That sounds kind of low?

They're basic transistors from RadioShack... maybe not the best source... I'll try different ones.

that is low, but are you sure you're measuring correctly...Most RS ones have higher hfe's. what transistor #'s are you using?

the schematic you linked to is for PNP germanium.

[ogandou]

Oh, you're right about the germanium, sorry  . I found another one for silicon NPN transistors. Which is good, I think my NPN transistors are better than the PNP I have anyway. I'm going to redo the whole thing...

[ogandou]

OK so I'm using the silicon npn found here. I changed the 2.2microF input cap by 2 1 microF cap in parallel and the 8.2k resistor of Q2 is a combination of three resistors in series that produce 8.2k (actually, 8.1k, but that should be OK).

The voltages at Q1 and Q2 seem OK but I don't get any sound now:

Q1:
E=0V
B=0.47V
C=1.38V.


Q2:
E=0.74V
B=1.38V
C=4.68V

[LucifersTrip]

OK so I'm using the silicon npn found here. I changed the 2.2microF input cap by 2 1 microF cap in parallel and the 8.2k resistor of Q2 is a combination of three resistors in series that produce 8.2k (actually, 8.1k, but that should be OK).

The voltages at Q1 and Q2 seem OK but I don't get any sound now:

Q1:
E=0V
B=0.47V
C=1.38V.


Q2:
E=0.74V
B=1.38V
C=4.68V

excellent...almost there!

here're the voltages from GGG si NPN
http://www.generalguitargadgets.com/pdf/ggg_ff5_instruct.pdf

Q1 Collector 1.4v
Base 0.6v
Emitter 0.0v

Q2 Collector 4.5v
Base 1.4v
Emitter 0.8v

right on...so, it's time to doublecheck your off board wiring...jacks and switch, if it's hooked up

[ogandou]

Mmmm, I always get 0.47-0.49V at the base of Q1, is that bad? Too low to bias Q1 properly?

[LucifersTrip]

i would say alll your voltages are close enough to sound really good. just work on the offboard wiring (most likely the problem?) and deal with any tweaking afterwards

[ogandou]

Mph, OK. I am NOT going to go into the details  but it works.  All I'll say is: 1) I'm a dips**t, 2) thank you so much Lucifer for all your help, that was extremely nice of you and 3) I learned something today...

Now I'm going to play around with the components. I'm liking that stompbox thingy a lot!

[LucifersTrip]

Mph, OK. I am NOT going to go into the details 

sounds great, but you still may want to give details to help prevent the next newbie from making the same error.

but it works.  All I'll say is: 1) I'm a dips**t, 2) thank you so much Lucifer for all your help, that was extremely nice of you and 3) I learned something today...

Now I'm going to play around with the components. I'm liking that stompbox thingy a lot!

excellent...and another fuzz enters the world

[ogandou]

Well, you're right, but it's so dumb I doubt anyone would repeat it. Anyway... I had started using PNP transistors (and keep in mind I had never used transistors before!  ), which pins are, from left to right if you put the flat face up: E, B, C. Right? With an NPN (of course, now I know that...), they are in that order if the flat face is down... so basically when I switched to using NPNs, I didn't bother to look up how the pins are located, and I had my transistors reversed... doh...

[LucifersTrip]

that's actually nothing to be embarrassed about. seasoned veterans will still occasionally make that error...

but note that the switch from pnp to npn had nothing to do with the ebc change. that is specific to each transistor, type, case, etc regardless of the p or n. just check the datasheet(s) before you use any:

http://english.electronica-pt.com/db/cross-reference.php?ref=
http://transistor-spravochnik.ru/