Fuzz Face Help!

[2yj1tj]

I have an early '90 Dunlop reissue, with germanium transistors.  Last weekend I did the Roger Mayer mods (100K volume Pot / 2K Fuzz pot / 470 ohm resistor to 1K ohm resistor / 8.2 K resistor to 18K resistor).  Everything worked wonderfully.  I bought a set of "matched" NKT275 transistors off eBay.  I put them in tonight.  Now for the problem,  if I turn the fuzz pot all the way up it feeds back really bad.  if I back it off a little its ok.  Same for the Volume Pot.  Any Ideas?  I'm NOT an electrician, so simple answers are GREATLY appreciated.  Thanks!

[LucifersTrip]

between the new transistors and the different value resistors, it seems you have biased it a lot hotter than before.

in order to troubleshoot, you're going to need to be able to take voltages. in this case, it would've been cool to know the voltages before you made the changes just for reference...but what is Q2 collector voltage now?

did you try lowering the 1K a bit?

[2yj1tj]

I have a good mutimeter, how would I check the Q2 collector voltage ( multimeter black to _____, multimeter red to _____)?  I don' t have any more resistors to lower the 1K.  I could buy one at radio shack tomarrow.  How far down should I go?  Is there a chance I have the new transistors installed wrong?  Thanks again!

[LucifersTrip]

I have a good mutimeter, how would I check the Q2 collector voltage ( multimeter black to _____, multimeter red to _____)? 

multimeter black to ground, multimeter red to all legs of each transistor, especially Q2C

I don' t have any more resistors to lower the 1K. 

http://diyaudioprojects.com/Technical/Electronics/parallel-resistor-calculator.htm

I could buy one at radio shack tomarrow.  How far down should I go? 

shouldn't have to go below the one you removed, but most likely somewhere in between

Is there a chance I have the new transistors installed wrong?  Thanks again!

doubt it since you have a hot sound

[2yj1tj]

I dont quite understand the calculator.  Do I input the voltages? Would there then be 6 significant figures.  After calculate, is that the resistor I need?  I'm trying to understand, electronics is really not my srong suit.  I am a Machinst.  I can make you anything out of metal.  Thanks for the Help!

[petemoore]

  RG Posted a detailed explanation' of FF activities at GEO, it is the most concise and complete one I've seen, as concise as it is, it is long and critical parts of understanding electronics then FF are dense also. I found it useful to gather plenty of reading material to help assist in trying to comprehend it...there's more at GEO, etc.
  You will probably have wished to have a working FF [if you're like me] before you gain mastery over electronics or even the two transistor feedback loop amplifier [FF].
  So...in the meantime, FF debugging tends to help develop 'chops'.
  The voltage measurements are critical to the math of finding the numbers that will tell you 'bigger or smaller' for Q2 collector resistor.
  IF the rest of the circuit is 'good', a variable resistance for Q2 collector resistor can bias all usable transistors:
  Starting with the 'base' resistance [stop resistor] of say 6k8 and adding 0.0k - 10k [a 10k pot does vary R from 0.0k - 10k].
  This gives 6k8 resistance with the pot turned to 0.0k, and 16k8 resistance with the pot all the way 'up' [6k8 + 0.0k = 6k8 // 6k8 + 10k = 16k8]
  With this collector resistor set up, dialing in anything between 6k8 and 16k8 is quick and easy, and allows biasing of most usable Q's for FF's.
  Q2 collector should sit at or around 1/2v of the power supply [that'd be 4.5v if using a 9v battery] allowing room for it to swing + and - from the bias point without 'hitting' the extremes of the PS: Ground and 9v...biased right in the middle works, adjust to taste from there.
  Because the diode in the transistors causes a diode drop and for other reasons, many like their FF's biased at 5v or 6v, also for assymetrical clipping [hits one of the power rails sooner/harder than the other]
  By posting the voltage or using the variable Q2cR depicted above, feedback about the transistor pin voltages will tell whether there's a problem, maybe even where there is a problem if there is one.
  Smaller resistances between Q2C and 9v 'pull the voltage up' toward 9v supply, larger resistances make Q2C voltage drop, adjusting the pot [trimpot or offboard] + 6k8 to some resistance which brings Q2C voltage near the middle of the supply voltage is what we're looking for. It'll do that if everything else is right.
 

[2yj1tj]

Thanks petemoore for the explanations.  I decided to come home and hook it up to my other amp (Fender Mustang I).  No feed back issues !?  OK . . .Well ?!  So then I hooked it back up to the amp I was using last night (9V Fender Mini Twin).  No issues! SWEET!  I remembered something I read while investigating mods, that Germanium transistors are temp sensitive.  Last night I had just soldered them in ( twice because i put them in backwards the first time ).  I would imagine that they still had some risidual heat built up inside them.  Mabie that was the problem.  What do you guys think?

[LucifersTrip]

I dont quite understand the calculator.  Do I input the voltages?

no, you put in two or more resistor values in and it tell you what the final value would be if the resistors were soldered in parallel.
you have a 470 and you have a 1K (1000)... put those in the calculator and you'll get 319.

Last night I had just soldered them in ( twice because i put them in backwards the first time ).  I would imagine that they still had some risidual heat built up inside them.  Mabie that was the problem.  What do you guys think?

very good chance if you plugged in immediately after soldering...the voltages will tell!

[petemoore]

 They cool off fast.
  Haven't heard about anyone burning them up soldering.. a warning about that..but doesn't seem to happen...
 Once the voltage measurements became known to debug the circuits most reliably, it got to be all about those V numbers on the Q pins, not the only thing...but is 'the' thing that tends to tell the story of troubles..the where and the why's, which leads to the problems [or goodness] which leads to the solution.