Fuzz Face and temperature

[momiel]

Hi everyone!
I built a Fuzz Face with a pair of selected ac128 I bought from banzaieffects.com, and i have a big problem.
I socketed the 8k2 resistor instead of using a trimpot, and changed the 470 to 1k to add a bit more volume. The pedal sounded very good at first try, even if I got about 7 volt at Q2 collector using a bias resistor of 10k.
The day after, I switched my pedal on and it sounded gated! I put it in the freezer for 50-60 seconds, and after this tratments it sounded very  good (and very cool...). I have to say I live in Rome, and in this period the day is about 30°C, and night about 20° (and the first time I played the fuzz it was in the night...)
so i tried to use a bigger resistor, and switched to 15k. i got a gig on the seaside last week; in the soundchek, about 6 pm, it sounded a bit gated. In the evening it sounded ok.
Yesterday I had a gig in another place. night temperature was about 15°C in the evening and the face sounded abit too muddy and compress.

And now, the questions:
-Do you think the transistors are good? Does high leakage maximize temperature sensibility?
-Can I solve the problem mounting a bias pot (10k with 2k or 3k in series)?

TIA

[GREEN FUZ]

Germanium transistors are very sensitive to changes in temperature so, yes, a bias pot is one way of compensating for this issue. It would be a good idea to mount it externally so you don`t have to keep opening the enclosure.

[mac]

Where I live temp varies a lot in a single day. A warm north wind during the day and suddenly a storm and south wind, 30C to 15C in minutes.
The only solution is an external pot to adjust the output voltage to 4.5v or so. Depending on your transistor's leakage, you'll need a 10k to 25k pot.

mac

[petemoore]

  Reverse diode seemed to fix it pretty good, same approximate value as, but opposite polarity to Q1's B/E...Ymmv.
  Hybrid Ge/Si is another option.
  Different transistors is an option.
  they all drift I think, some drift so little, if in the right other conditions you can't tell.
  Great sounding FF one day at an outdoor gig, got way too much sun and could only say "Gajkkkj j' j" through my amp, by the time I got to testing.
I forgot to bring ice cold drinks and an umbrella for it.
 

[David]

Seems to me that John Hollis created a FF clone he called the "Rock Face".  IIRC it's supposed to be a temperature-compensated FF.

[mac]

i breaboarded a rock face once. i finger heated the transistors and waited a lot to the ic to do its job.
i saw a reissue version with a thermistor also.

if you place a silicon at q1 you can forget about thermal runaway since it is q1 which makes most of the problem.

mac

[momiel]

10 k pot mounted, in series with 4k7...
Heated everything with a hair drier... it sound gated with the bias all up, it sound gated with the bias all down...

[momiel]

measured the first transistor: leakage 171 uA, gain 54,5. maybe the low gain for the first transistor is the cause of the strange bias of the circuit and the instability?

PS: R.G., you know you can save me...

[Serge]

check this article: http://www.geocities.com/sunsetstrip/studio/2987/britface.html

near the end of the article, you will see some notes on adding a reverse biased Ge diode at Q1 to increase the stability(as pete already suggested)

[petemoore]

  Yupp...the emitter base diode junction is marked on the schematic [pnp or npn], just grab a diode like what's in the transistor [actually you can measure some to get a close one GE threshold wise], then just install it in reverse to the transistors orientation.
  Find a miniature refridgeration unit w/thermal control...put that in there with the heat exchanger out the top.
  Warm it up to see if it works.

[mac]

measured the first transistor: leakage 171 uA, gain 54,5. maybe the low gain for the first transistor is the cause of the strange bias of the circuit and the instability?

Given your leakage, not hfe, you may need a higher pot 

Look for my calculator in the wiki http://www.diystompboxes.com/biascalc/

mac

[momiel]

Soldered the diode. Used a 1n34, higher leakage than 1n277. But the tone of the face has changed a lot...
I think I have to learn how to use mac's bias calculator...

[mac]

As I remarked on the page, calculations are very sensible to small errors in the readings, so use it to estimate.

Resistors are in K, so a 470ohm is 0.47k and a 1Mohm is 1000k. Also in the big muff and rangemaster section, you may find situations in which some resistors are not present, that is, an infinite resistor. In this case use a large number such as 9999999999999 or 11111111111111111.

Leakage is in mA, so 170microA is 0.170mA. For silicon use iL=0.

Vbe is the base - emiter voltage drop. For silicons this is between 0.55v to 7.0v. For most cases use 0.6v. Germaniums vary a lot. Most transistors I have, have a vbe about 0.07v to 0.15v. But maybe the DDM sucks current giving a false reading. Not sure.

Entering slightly different values than the actual ones, in particular Q1's hfe, leakage and vbe, may lead to deviations from the real voltages.

PM if you have some doubts.

BTW, the reverse diode is like a big resistor to gnd, it shunts some signal to gnd so the tonal change.

mac

[MartyMart]

I built a "RockFace" around a year or two ago and found it to be very stable.
Before being boxed up, I was playing through it whilst blasting it with a hairdryer just inches from the
circuit - it had to get VERY hot before it started to gate and fart, so it's a pretty good system IMO.

MM

[mac]

I built a "RockFace" around a year or two ago and found it to be very stable.
Before being boxed up, I was playing through it whilst blasting it with a hairdryer just inches from the
circuit - it had to get VERY hot before it started to gate and fart, so it's a pretty good system IMO.

How long did you have to wait to get 4.5v after heating? Do you have an idea of the dark-light resistance of the LDR you used? Many users say it works ok, I couldn't. 

mac