Ge fuzz auto bias??

[1wahfreak]

I have a question regarding biasing Ge fuzzes. If premium sound depends on the voltage at Q2 to be at about 4.5v, is there way to regulate that voltage without affecting the circuit? First I started to think about putting an LED in my enclosure which would tell me what the voltage is at Q2 at any temp. When the volatge would drift one way or the other, you adjust a trimpot and be done with it. But then asking our EE at work, he said it is possible to actually regulate the voltage using an op-amp with positive feedback and historesis??? Not sure if that's the right spelling. He was trying to explain everything to me and it was going way over my head so I really didn't understand all that much. I'm still pretty much "painting by numbers". Forgive me if I'm missing something, but I would assume that some of you guys would have thought of that if it were possible. Can it be done??

[brett]

I'm sure that something like that is possible, but it kind of negates the simplicity of the fuzzface.  

However, a simple 4.5V indicator could be made by taking the collector voltage and feeding it through a 1M and 91k resistor to 0V.  Feed the base of an NPN tranny from the junction of the 2 resistors, and connect the emitter to 0V and the collector to 9V via an led.  That led will light whenever the collector voltage is at or above 4.5V (approx), giving you an indicator that you can use to trim the bias.  (this works because the transistor switches on when the base voltage is 0.6V or above).  If the led turns on significantly lower or higher than 4.5V adjust the 91k resistor up or down a few k until it is turning on at exactly 4.5V.

Alternatively, you might try replacing your unstable Ge trannies with ultra-stable Si, using the "piggyback" technique, discussed in the forum a couple of weeks ago.

Have fun.

[The Tone God]

Do a search for "servo bias" or along those lines in the archives. There was a couple of threads on this awhile ago.

Andrew

[bwanasonic]

I would also advise against blind adherance to the 4.5v value. Depending on a number of factors, you may find the sound you like at a different bias voltage.  About as fancy as you need to get with a FF is a trimpot on the collector of Q2, although the various *boutique* placements of adjusment pots might suit your needs.

Kerry M

[petemoore]

I think somebody was doing this. Cant'  remember how it was wired but had to do with a GE tranny and an opamp.
  Lotta help I am...lol
 Oh yeahh that Pig face thing...
 I think you could work that piggyback tech. into any NPN or PNP FF device using SI, GE [anything that gets base current?] transistors.
 Any more FF's I build will have the PB mod at it's disposal.
 I use an 8 pin socket for Q1 of FF [holds up to 2 Q's and a resistor] and an 8 pin IC socket also for Q2 position...you could tweak till your mind melts...I decided on a Tone that HAD to be boxed after hours of tweaking...I'm sure there are alot of 'inbetweens' I went past again too.
 I had one boxxed after a while, with the board mounted its pretty easy to swap & tweak.

[Doug B.]

I would also advise against blind adherance to the 4.5v value. Depending on a number of factors, you may find the sound you like at a different bias voltage.

Agreed -- a bit of experimenting in light of individual taste (and individual components) is always in order!

I believe that Gary Mould's FF servo bias circuit could be easily adapted to set different voltage levels.  I don't have it in front of me right now, but IIRC, there was an equal-resistor voltage divider on one of the opamp inputs that set the voltage at half the supply (4.5v).  It's easy to alter the value of those resistors to set a different voltage... or make it adjustable by using a potentiometer.  

IIRC, this is how it works.  The opamp "compares" the voltage at the Q2 collector with the reference voltage set by the voltage divider, and outputs whatever voltage is necessary to the base of Q1 to make Q2's collector equal to the divider voltage.  So if you alter the divider voltage at the opamp, it alters the Q2 voltage... and tries to keep it there!  

- Doug B.

[1wahfreak]

Point taken about the 4.5v. I guess I was just trying to figure out why the circuit couldn't (or isn't being) made a little more stable. If the critical factor is maintaining the same voltage (whatever that is) at a given temp that might drive the rest of us to rig some way of maitaining  it. I wasn't sure if the project of maintaining the voltage would interfere with the circuit itself. Just thinking out loud.....

[RDV]

Speaking of FF (in)stability, when I got my first Ge FF(Analogman Sunface), we played outside as a cold front moved through. The temp dropped about 25 degrees in about an hour and my fingers started to go numb. Just as my own physical condition was deteriorating, so was my Sunface, just splattering and spitting, and grinding and wheezing till finally it slowly faded out to no sound at all. We all thought it was really neat to listen to it die as we were playing to nobody as they had all been driven inside by the cold. Gotta love that Ge.

Regards

RDV

[petemoore]

A Ge heater on a thermostat !!!
 AC though might take a little bigger box.

[bwanasonic]

I was thinking more along the lines of a FF built into a beer cooler or Thermos bottle to insulate it it.


Kerry M