Strange Fuzz Face problem

[Dooter]

I have built a fuzz face clone (mayer hendrix version) which has had problems. When first built it sounded as if only one of the AC128 Ge trannys was working as I only got nasty raspy distortion when strumming hard. Single strings just did not get though. Sounded like a bad noise gate. volume wos only close to bypass level when both fuzz and volume pots were on full. The bias trimpot made no difference to the sound and swapping the trannys led to a silent box.

However the following morning things were much better (suggesting a temperature effect) but still not quite right. output now much higher, (louder than bypass when volume pot is at 50%) but still not right. Sound is muddy and raspy when driven. the top E string appears not to have enough output to get through, only thicker strings generate a raspy tone.

However when putting an MXR noist gate before the fuzz It sounds fantastic! as good as any boutique fuzz on youtube. But only when the noise gate is engaged. When noise gate is set to true bypass then fuzz is again muddy and won't let the top E string through. It's as if the input is not quite high enough to turn the transistor 'ON'


Any Ideas?

[seedlings]

Picture of your circuit board or breadboard?  What transistors are you using?

See Solidhex's response here:
http://www.diystompboxes.com/smfforum/index.php?topic=95119.msg823084#msg823084

CHAD

[Dooter]

component side
https://picasaweb.google.com/lh/photo/xFKXf_n2U62g15K2odhf2fHuC7_QlIVvSFK9UTHlQjM?feat=directlink
flip side
https://picasaweb.google.com/lh/photo/xFKXf_n2U62g15K2odhf2fHuC7_QlIVvSFK9UTHlQjM?feat=directlink

Germanium AC128 trannys

[Dooter]

I've done some probing with the multimeter although I have ni Idead what any of the voltages ought to be apart from the 2k trimpot which is meant to supply bout 5v bias. It vaies from 8.8v - 8.4 volts on max / min settings, so all is not well there I suspect.

[Dooter]

https://picasaweb.google.com/lh/photo/UphgOOrx8MbAPXmygjHxLPHuC7_QlIVvSFK9UTHlQjM?feat=directlink
sorry, both links in earlier post were of the component side of the board, this should be right.

[LucifersTrip]

I've done some probing with the multimeter although I have ni Idead what any of the voltages ought to be apart from the 2k trimpot which is meant to supply bout 5v bias. It vaies from 8.8v - 8.4 volts on max / min settings, so all is not well there I suspect.

if you search the forum, you'll find a ton of troubleshooting threads with some proper FF voltages..

here's a recent one with close to the voltage readings you should get:
http://www.diystompboxes.com/smfforum/index.php?topic=95119.msg823399#msg823399

If you hooked it up properly and are getting 8.8v - 8.4v on Q2 collector, you need to choose different transistors.

Do you know the gains/leakages of those transistors? Did you breadboard this first to test & tweak it?
http://www.ebay.com/itm/1pcs-700-Point-Solderless-PCB-Breadboard-SYB-120-board-/220890973119?pt=LH_DefaultDomain_0&hash=item336e20c3bf


good luck

[pinkjimiphoton]

sounds like classic case of misbiased transistors. does the mxr have buffering in it? that may be why the fuzz sounds better.
fuzzfaces are strange animals...moody.

[petemoore]

Germanium 'AC128' trannys
  Mention of the moniker reminds me of threads and AC128 tranny's I bought once, which did basically nothing good except when using the internal diode as a passive component, I couldn't find an active circuit in which they would 'do' or do well, so I say this after personal experience and experimentation. The experiments were basically all flops, perhaps these are also 'new production' 128's like I made the mistake of buying a batch of, sometime ago.
  Replacing the 'part# moniker' with leakage and gain numbers as #1 and only real priority for Ge transistor selection has resulted workable if not wonderful results.

[Dooter]

I did not breadboard test first as this was a kit bought off the web which seemed idiot proof. The tranny's were tested by the seller to have Hfe's of 101 and 97. I did not get leakage data, but the seller says he screened out tranny's with excessive leakage before selecting them for sale.

[LucifersTrip]

I did not breadboard test first as this was a kit bought off the web which seemed idiot proof. The tranny's were tested by the seller to have Hfe's of 101 and 97. I did not get leakage data, but the seller says he screened out tranny's with excessive leakage before selecting them for sale.

just because it's a kit doesn't mean you shouldn't breadboard, but that's cool. If the transistors really do have gains of 97 and 101 and aren't very leaky, you shouldn't get 8v on Q2's collector...so there is another problem.

refer to the debug thread:
http://www.diystompboxes.com/smfforum/index.php?topic=29816.0

...and post the exact schematic you used with the six e,b,c voltage readings.   ask if you're not sure what to do.

good luck


edit:
check to see if the Q's are in the right way. I see the Q1 & Q2 tabs are facing different directions. I don't build on pcb's, but I would think the designer would have all the Q's oriented the same

[petemoore]

  Q1E goes to Gnd., easy to test for, the Emitter pin goes to ground.
  Under the transistor is an orientation mark, the flat part of the circle printed on the board is the 'front' of the transistor in the case of both Q1 and Q2, so both transistors tabs should orient the same way in regards to the flat-side of the circles.
   Get Q1 right and Q2 should follow suit, but testing for continuity 'to whatever' [is shown on the schematic] from a given circuit node, using the correct schematic as the guide is the best way, in FF Q2 emitters case, it goes to the gainpot and feedback resistor [100k].
   PNP FF Schematic:
   http://fuzzcentral.ssguitar.com/fuzzface.php
   Also the 2.2uf and 22uf capacitors may be damaged by exposure to reverse polarization.
   
   

[Dooter]

The trannys are meant ot be facing opposite on this board
Schematic:
https://picasaweb.google.com/lh/photo/dzkotYf1hg542dRQlP-Ye_HuC7_QlIVvSFK9UTHlQjM?feat=directlink

off board connections:
https://picasaweb.google.com/lh/photo/qkopxc_TnuQUE3Bo5tzxMPHuC7_QlIVvSFK9UTHlQjM?feat=directlink

I tried fitting the q1 high frequency filet cap ( someone on another forum said it cured his similar problem, probably coincidentally but I tried it anyway to no avail)
I reflowed the tranny connections and managed to kill all sound so not really worth posting ecb voltages just yet, although last night I got
Q1 E:0 B:63.1mV C:85.1 mV   Q2 E:77.5mV B:88mV C:8.4V
I pulled the trannys to see if I'd cooked them.
My multimeter does not have tranny tester, just a diode tester here are the values:

Meter open 1.064 
Q1:   .395 (+ve on E, -ve on B).  1.048 reversed  &   .392 (+ve on C, -ve on B).  1.045 reversed    .392 (+ve on E, -ve on C) .983 reversed
Q2:   .399                                 1.052               &   .397                                 1.049                  .385                              1.008
   

[Dooter]

So looks like both trannys are bad :-(

[LucifersTrip]

here's an easy transistor tester used my many, if you'd like to double check or check future ones...

http://www.geofex.com/Article_Folders/ffselect.htm

[Dooter]

Thanks for the tranistor tester schematic. I came across an old Bang and Olufsen TV remote Circuit board which proved just right to build the circuit on (and even supplied me with some really good sockets that I was able to put on the fuzz face board for when some new trannys arrive). I used combinations of resistors and a cermet trimpot to get to within 1% of the resistance tolerances.
One of the AC128s reads a gain of 96 and a leakage of 580uA, the other reads a gain of 98 and 748uA leakage! bretty bad.

https://picasaweb.google.com/lh/photo/u0EC64Q4jV-2LmM_PNXZ-3LPvCyhF1lUNexWYvHET_w?feat=directlink

Thanks for all who helped and a happy new year.

[Dooter]

Just one more thing - will the transistor tester give the gain of silicon transistors ?

[LucifersTrip]

Just one more thing - will the transistor tester give the gain of silicon transistors ?

yes, but it will be slightly (not enough to worry about) off....and there will be (almost) no leakage so you only care about the reading after the button is pushed

[Dooter]

OK thanks for that.
I have bough 10 AC128's and 10 AC128K's off ebay.
I tested them all and have got nice tones from a couple of combinations. I have a 22k variable resistor suppling vaiable bias to Q2's collector but can only get close to correct bias by using the lowest leakage tranny's I can select (<200uA leakage) and the trimmer on full tilt. This means I have had to use trannys with true gains of 70 for Q1 and 90 for Q2, slightly lower than the target gain suggestes by many websites. and bias is still 5.5V rather than the suggested 4.5V
would there be any benefit in using a higher biasing trimpot i.e. 100k? would this allow me to get the biasing voltage down? What would be the side effects? Output is nice and loud on 50% vol pot setting I could easily cope with a dropped output if that is what would happen. I have loads of trannys which have leakages in the 200-300uA range of gains around 100, but all sound nastily raspy in the current setup and get biasing voltages around 7.5-8V. I find that Q2 can be a bit leakier than Q1, but Q1 certainly needs to be as low leakage as possible.
sorry for the barrage of questions but my knowledge of electronics is very basic.

[LucifersTrip]

OK thanks for that.
I have bough 10 AC128's and 10 AC128K's off ebay.
I tested them all and have got nice tones from a couple of combinations. I have a 22k variable resistor suppling vaiable bias to Q2's collector but can only get close to correct bias by using the lowest leakage tranny's I can select (<200uA leakage) and the trimmer on full tilt.

honestly, 22K is a bit high for the "best" sound. I believe most shoot for the 4k-15k range, therefor a common setup is a 4-5K cutoff with a 10k pot

This means I have had to use trannys with true gains of 70 for Q1 and 90 for Q2, slightly lower than the target gain suggestes by many websites. and bias is still 5.5V rather than the suggested 4.5V

4.5 to 5.5 is a big difference in sound. it'll be less soft & mushy...and darker

would there be any benefit in using a higher biasing trimpot i.e. 100k? would this allow me to get the biasing voltage down?

yes (voltage down), but most likely you won't get a great sound...try it...only your ears count!

[Dooter]

OK thanks.
I raided the stores at work today and found we had six vintage AC128's they seemed better quality than the tungsram ones i got off ebay. Two were dead on testing but the other 4 had leakages of 200uA or less. The gains were 99,100,140 and 200; so I should have a decent pair now.

I also found about 30 bc108's some with gold pins some with silver. I tested 3 and got gains of 200,400 and 1000!? what's the ideal gains for a pair in a silicon fuzz face npn setup?