Fuzz Face build - help swapping out silicone for germanium

[lmorse]

Hi All,

I am trying to build a straight Fuzz Face clone. I started by bread-boarding, and then moving the circuit to a PCB. There are a few mods: an IN2004 diode to protect polarity, a 1M resistor to ground before the 2.2uf cap (as shown in red on this image), and a 100uf capacitor to prevent popping. I initially used 2N2907 PNP transistors to test the build. The completed circuit worked fine.



I then swapped out the 2N2907 for a pair of NKT275 - Q1 is 71 hfe 139uA leakage, Q2 is 117 hfe 242uA (I literally un-soldered the 2N2907's and soldered in the germaniums). The output was very low (barely audible) and with no discernible fuzz. I traced everything back thinking maybe I had a short, or something had come loose, and the only thing I could find was that the voltage into Q1 base was tiny (0.036v). So I soldered the 2N2907's back in - the circuit worked fine....

With 2N2907's
Q1 (2N2907)
B 0.586
C 1.3
E 0.001

Q2 (2N2907)
B 1.306
C 3.235
E 0.683

So, in an attempt to fault find I swapped back in just the germanium Q1, and the output once again is very low with no discernible fuzz....
Q1 (NKT275)
B 0.059
C 1.682
E 0.001

Q2 (2N2907
B 1.680
C 1.018
E 1.008

Can anyone help me to understand what might be going on here?

Thanks

Lee

[LucifersTrip]

if nothing at all changed after the swap, then the guess would be the pinouts are wrong

here's ge Q1 voltages from a working one:

q1e: 0
q1b: 0.1    
q1c: 0.6


edit:
btw, 3.2 is too low for Q2C w/ 2907

[lmorse]

Hi,

Thanks for the reply, this is useful.

I don't suppose you could give me readings for Q1 and Q2 in both germanium and 2N2907 versions? It might help me track down the issue,

Thanks

Lee

[LucifersTrip]

common working voltages for both

ge

q1e: 0
q1b: 0.1     
q1c: 0.578   

q2e: 0.447
q2b: 0.578   
q2c: 4.51

========

si

q1e: 0
q1b: 0.6 
q1c: 1.4

q2e: 0.8
q2b: 1.4
q2c: 4.5

[lmorse]

I've been trying to track down why the voltage at Q2C is low. No success yet.

The current I am measuring is 0.02mA. The battery is a bit low now: 8.42V. What currents would you be expecting at Q2C? (Is this even a relevant question?). Before the 8.2k resistor (which I measure as closer to 8k), the voltage is around 8.42V, after the resistor I am seeing 3.2V.

I'll keep plodding on with my investigations, can't see anything untoward so far.

Thanks

Lee

[LucifersTrip]

I've been trying to track down why the voltage at Q2C is low. No success yet.

what is it with the two ge's now

[lmorse]

It's late here now. I'll have a look tomorrow. I'll solder the ge's then and post the results.

[tonyharker]

Have you remembered that the Germaniums are PNP and the Silicon transistors are NPN?  Did you reverse the polarity of the supply?

[LucifersTrip]

It's late here now. I'll have a look tomorrow. I'll solder the ge's then and post the results.

2N2907 is  pnp, but yes that schematic is for npn, so must swap all + & - for pnp's (electros & battery)

obviously, breadboarding would've been the cool way to go first (try that if you have one), because you don't really want to keep soldering & un-soldering ge's, especially ones that may have cost you a bit.

if you have alligator clips, you can easily connect the Q's off-board.

also, did you measure the hfe/leakage yourself or were u given those. it's always good to check for yourself

[lmorse]

Hi,

Yes I was aware that the ge's are PNP, that is why I chose the 2N2907's to prototype with; I figured I was less likely to make a clumsy mistake if I kept them both PNP. I have reversed the battery and caps.

I measured the ge's myself. They were similar to what it said on the tin. 

Lee

[lmorse]

Right, just swapped in the ge's (I don't have enough additional components at the moment to make breadboarding easy).

Voltages now:

Q1 (71hfe)
C 0.819v
B 0.038
E 0.0001

Q2 (117 hfe)
C 3.75
B 2.995
E 2.990

Battery voltage = 8.35. Looks to me like the base and emitter of Q2 is far too high compared to the recommended values above. Actual measured resister values:

463
32k8  when unsoldered, but when in the circuit: 27k50?
7k57

The 100k resistor also measures closer to 56k6 when in the circuit; is this because it is measuring resistance through the transistors or does it point to a potential source of error?

[LucifersTrip]

q1c and q2b must be =

[lmorse]

Hmm, yes. I should have spotted that. I suspect some sloppy measuring I did in a hurry. My wife was pushing me to go out

[lmorse]

Hmm, yes. I should have spotted that. I suspect some sloppy measuring I did in a hurry. My wife was pushing me to go out. I'll re-measure when I get home. I guess that they are the same, and I accidentally touched Q2e.

[lmorse]

OK, without being sloppy and in a rush 

Q1 (71hfe)
B 0.036
C 0.832
E 0.0001

Q2 (117 hfe)
B 0.832
C 3.747
E 0.717

[LucifersTrip]

well, you should have sound, just thinner and lower volume.  either you need to choose a couple different ge's or tweak the resistors.

start with a trim/pot instead of the 33K and slowly increase. you may also want to slightly decrease the 8.2K, but not too much. I wouldn't go much lower than 5 or 6K, unless you don't mind an overall tonal change.

[lmorse]

Yes, I do get sound, very quiet, fuzz is barely discernible.

So I need to look at boosting the gain? You don't think there is a problem elsewhere? I have been tracing the layout for hours and cannot find any issues.

[LucifersTrip]

Yes, I do get sound, very quiet, fuzz is barely discernible.

I think you should have more than that with the voltages you have, so most likely another problem...you can easily get an OK fuzz (albeit less than average and weaker) with Q2C voltage even lower than what you have.  

So I need to look at boosting the gain?

you shouldn't have to with a ff

You don't think there is a problem elsewhere?

good chance...especially if you've tweaked the resistor(s) I mentioned above and are closer to good working voltages.
check offboard wiring and/or post a clear closeup of your board and someone may spot something.

for instance, are you sure the jacks are wired properly?

edit:
I also suspect 242uA leak for Q2 is a bit too high for an optimum ff. (the higher leak, the lower Q2C voltage)

[pinkjimiphoton]

you said you added a diode for polarity protection... did you reverse it when ya went from npn to pnp?

also... GE transistors sometimes are better BACKWARDS...  reverse beta'd. if you find you're not getting the sound you expect, try flipping them 180 degrees.

i often find ge's that show a gain of 5 or 7 or so "right" that are closer to 100hfe backwards.

fets can work the same way. 

the voltages look way too low to me for a decent FF. i'm betting that diode you added is backwards, and starving the circuit.

[lmorse]

My latest instalment....

I replaced the 33k and 8k2 resistors with 50k and 10k pots. I can get Q2C voltage to around 4.5v, but then I can't get the Q2B below 1.3v. These measurements are taken without connecting the input\output jacks to amp and guitar.

The PCB is getting a bit past best now (I'll have to etch another). So I have gone back to a simplified breadboard version, removed the polarity protection diode, the 1M resistor and the 100uf cap, and have gone back to using the silicone transistors. Again, if I adjust to get Q2C at 4.5v Q2B won't go below 1.3v. If I adjust to get Q2B to around .580v, then Q2C won't go below 7.4v?? Verified on both the PCB and the simplified breadboard version.

My understanding is that the 33k and 8.2k resistors are working as voltage dividers. The capacitor with the output volume pot would have some high-pass filter effect? What I cannot see is where the rest of the 9v is going if not through the voltage divider (4.5v + 1.3V = 5.8v??). Would some of it be going to ground through the volume pot? I do understand that this is voltage potential and not current.

you said you added a diode for polarity protection... did you reverse it when ya went from npn to pnp?

I am happy that the protection diode is the correct way round, I have used pnp transistors throughout, so no need to reverse. I also get the same readings on the breadboard with the diode removed.

also... GE transistors sometimes are better BACKWARDS...  reverse beta'd. if you find you're not getting the sound you expect, try flipping them 180 degrees.

Very Interesting, not read this anywhere - I will experiment! So e becomes c, and vice versa?