Fuzz face bias problem

[luap77]

Hi,

I'm having a problem biasing a Arbiter germanium fuzz face per the GGG PNP positive ground project. Q2 collector voltage is 4.5V, but Q1 collector voltage is also around this value (not the same though, just within half a volt or so)!

I've replaced the 8.2k and 33k resistors with 20k and 50k trimmers and have tried changing these resistances to try and get Q2 collector at about 4.5V and Q1 collector at about 1V as per what I've read on this site. The problem is that no matter what I do, the collector voltages of Q1 and Q2 always stay within half a volt or so of each another! i.e. I can get Q2's collector to 4.5V, but cannot get Q1 collector below about 4V or so...no matter what.  The other components have the standard values (100k, 470ohm, 2.2uF, 20uF and are in the correct places and orientations).

Has anybody had this happen/seen this behaviour or can anybody suggest where to start fixing this please?

My trannies are PNP, Q1 leak = 90uA & hFE = 94, Q2 leak = 103uA & hFE = 125 using R.G.'s test rig. 

Thank you.

Paul E

[petemoore]

DEBUGGING - What to do when it doesn't work

[Dragonfly]

Check your ground connection off the emitter of Q1

[luap77]

I've printed the very clear debugging instructions (thanks R.G. and Aron) and will post again when I've measured the voltages etc

I was working under the assumption that I might get away without doing this as the circuit is so "simple", common (most are familiar with it) and that many have probably built it and that somebody may have noticed this biasing behaviour.

Cheers,

Paul E

[luap77]

OK, here goes. I'm really crossing my fingers that somebody will be able to help!

Project = Arbiter Fuzz Face from General guitar Gadgets. Positive ground PNP version. The link to the schematic is:

http://www.generalguitargadgets.com/diagrams/fuzzface_sc_pp.gif

The layout is:

http://www.generalguitargadgets.com/diagrams/fuzzface_lo_pp.gif

I followed the project to the letter, used the correct parts and made a pcb using the verified pattern:

http://www.generalguitargadgets.com/pdf/ggg_ffo_pcb.pdf

I put the 33k, 8.2k resistors and 2.2uF and 20uF caps in sockets so that I could bias with trimmers in place of the resistors and change caps at a later time if needed (eg reverse if I convert to NPN etc)

The problem is that no matter what I do, the collector voltages of Q1 and Q2 always stay within half a volt or so of each another! i.e. I can get Q2's collector to 4.5V, but cannot get Q1 collector below about 4V or so...no matter what (using 20k trimmer in place of 8.2k resistor and 50k trimmer for 33k resistor).  The other components have the standard values (100k, 470ohm, 2.2uF, 20uF and are in the correct places and orientations). The battery voltage is 8.9V.

Has anybody had this happen/seen this wierd bias behaviour or can anybody suggest where to start fixing this please?

My trannies are PNP, Q1 leak = 90uA & hFE = 94, Q2 leak = 103uA & hFE = 125 using R.G.'s test rig. I've tried numerous trannies, including with similar results.

The circuit is producing fuzz at decent sound levels, but it doesn't sound right. It does sound like a fuzz though, but the voltages are way off. With R2 and R5 being 33K and 8.2K respectively (1% metal film), I get the following voltages between ground and each of the transistor legs:

Q1
C= 0.43V
B= 0.09V
E= 0.00V

Q2
C= 0.31V
B= 0.43V
E= 6.51V

I then tried replacing all the capacitors (had them in sockets, so easy) and resistors and nothing changed. I verified that Q1's emitter is indeed connected to ground (as Dragonfly suggested) by checking with my multimeter.

Please help...I'm confused!

Thanks in advance.

Paul E

[luap77]

I solved it! Also found some very interesting stuff out on the way.

I had Q2 socket in the same orientation as Q1 socket, and hence Q2 was the wrong way around!  Reversed Q2 and now it sounds like an awesome germanium fuzz. REALLY happy with it!

What did I find out that's really interesting? Well, only the BEST brown sound I've ever heard through a solid state amp! When I had Q2 accidently reversed, I was fiddling with the trimmers that replaced the 8.2k and 33k resistors (see my posts above). At one particular point the thing doesn't sound like a fuzz (yes it still fuzzed even though Q2 was the wrong way 'round), but instead gets the brown sound down very very well. Very close to the AC/DC Angus Young sound. Try it and i'll think you'll be pleasantly surprised...

BTW, I don't have a sufficient technical background to explain why this works (I'm a trial an error type), but perhaps somebody here with more experience can provide an explanantion. I'll absolutely be building another one of these with Q2 reversed and sound optimised with trimmers for brown vs fuzz. A happy accident/mistake indeed!

Paul E

[zachomega]

Didn't Jack Orman just do an article on this? 

http://www.muzique.com/lab/reverse.htm



-Zach Omega

I solved it! Also found some very interesting stuff out on the way.

I had Q2 socket in the same orientation as Q1 socket, and hence Q2 was the wrong way around!  Reversed Q2 and now it sounds like an awesome germanium fuzz. REALLY happy with it!

What did I find out that's really interesting? Well, only the BEST brown sound I've ever heard through a solid state amp! When I had Q2 accidently reversed, I was fiddling with the trimmers that replaced the 8.2k and 33k resistors (see my posts above). At one particular point the thing doesn't sound like a fuzz (yes it still fuzzed even though Q2 was the wrong way 'round), but instead gets the brown sound down very very well. Very close to the AC/DC Angus Young sound. Try it and i'll think you'll be pleasantly surprised...

BTW, I don't have a sufficient technical background to explain why this works (I'm a trial an error type), but perhaps somebody here with more experience can provide an explanantion. I'll absolutely be building another one of these with Q2 reversed and sound optimised with trimmers for brown vs fuzz. A happy accident/mistake indeed!

Paul E

[luap77]

Well, there you go!  Thanks for the interesting link Zach.

My advice to anybody doing this "reverse Q2 mod" to a fuzz face would be:

1) Replace the 8.2k resistor with a 20k trimmer (muli-turn good)
2) Replace 33k with 50k or 100k trimmer
3) Don't bother measuring bias voltages; just fiddle with the trimmers (or you could use conventional pots) until you get the desired sound.

Or, you could always just try Jack's schematic.

Paul E