Biasing problems, FF circuit, Trannys: BC140

[ZtaRDuZT]

I built me some bias (trimpot resistors) circuits, both the classic FF NPN + PNP and a Rangemaster.
I'm now trying to bias my transistors in the NPN FF circuit, a couple of NPN BC140 I got from a friend.

Started out with resistor values: 330, 8.2k, 33k, 100k
+9V = 8.6v

hfe: Q1 92, Q2 110
Q1: E 0v, B 0.55v, C 1.8v
Q2: E 0.8v, B 1.8v, C 4.5v, after a slight tweaking of the 10k trimmer (8.2k resistor Q2,RC)

GGG Recommended should read:
Q1: E 0v, B 0.1v, C 0.5
Q2: E 0.4, B 0.5, C 4.5

Whatever I do I don't seem to be able to change the values on Q1
I'm trying to get it closer to the recommended Q1 values.

What am I missing here? I should be able to get Q1 down, right?
If I get Q1 down Q2 should look better and hopefully with a little tweak on the 10k trimmer get back in the 4.5v area.

What am I doing wrong? Please help!

[Steben]

Are you sure GGG wasn't talking about Germanium fuzz, BC140's are Silicon. With 0.1V offset at the base you won't hear a lot in those.
And after all: how does it sound my friend? 

[ZtaRDuZT]

It looks like the building instructions from GGG handle both NPN, PNP, silicon and germanium.

I'm trying these transistors out, I basically have no idea how they sound or if they'll work in a box.
Regardless of that, shouldn't I be able to bias the transistor anyway?
I'm trying to learn biasing here.

Don't know a whole lot about transistors, don't know what to look for in a spec, except maybe hfe.
Someone told me that any transistor should work if biased correctly.
It might not sound good but it'll work.

0.1V offset at the base? What does that mean?

[Steben]

I think it will rock n roll quite good.

With offset i mean the same as bias, sorry. I was referring to the 0.1V at the base, suggested by GGG. A Si transistor will be quite cut off at that voltage (as diodes do), a Ge transistor only a bit. Si transistors will be cut off a bit at around 0.55V, you see?!

[ZtaRDuZT]

If what youre saying is: Si trannys will read different on the pins than Ge, then I got it!

If that's so then the bias on my BC140 looks OK. Wasn't planning on going any further with
this setup at the moment, only wanted to see if I could bias them correctly.
This got me curious though, think I gotta go home and try them out.

Got a bunch of BC107,140,141,160,161,547, figured if I could get them to work OK
I'd have a whole bunch of projects work with, besides the ones I've bought from smallbear
wich I'm planning to start with.


Thx very much for the input!

[petemoore]

   FF's arent' 'wierd' to bias [well they can be] but they are: a two transistor feedback amp gain stage, Q2 sets bias for Q1...
  either I got...or never quite got...how Q1 can function with a such a small collector voltage, I think all it has to do there is have just a little 'twiddling' room to make Q2 swing around in voltage, relative to it but larger voltage swings.
  After all, Q1's input is tied to Q2's emitter, which is only '1k from ground' [through the gain pot].
  Read GEO Technology of the FF [or try] in it is an explanation of 'what swings where when' describing what happens in a feedback amp when it's amplifying an AC Source [like guitar].
  Looking at your pin voltages, I thought, where's the problem
  One FF WILL sound different than the next 'identical' one, not to mention the myriad modifications and transistors that can be used in it.
  By just trimpotting Q2 collector, and making sure all the other connections are made correctly, a FF should bias most transistors just with a trimpot adjust while looking for ~1/2v [a llittle more or less than 4.5v or 5v there, with a 9v batttery as PS] on Q2C.

[Steben]

Looking at your pin voltages, I thought, where's the problem

Exactly. As long as Q1 is a bit in the cut-off margin, there's the assymetric mushy magic.

PS.: Mr. Ztarman: you really should do the ear test along with the scope and multimetre.

[ZtaRDuZT]

I'll do the "ear test"!  Gotta send the kids to the playground first though. Otherwise I won't have a guitar to test with.
I've tried one out so far, a classic FF with BC547, too high gain for my taste, around 200-240 hfe. But I got sound out of it anyway.
That's gotta count for something concidering I'm a first time builder.


I'll give the Technology of the FF again since I've picked up a few things since last.

[bwanasonic]

Generally speaking, there is not much point in sweating bias readings unless the thing doesn't work or sound the way you want. I never bothered fiddling with the Q1 bias, and just twiddle the Q2's collector. BTW - I find I prefer a slightly higher than 4.5 reading on Q2 with the trannies I've used (5.5-6v usually). I use a trimmer to adjust Q2, and make sure I can adjust it above and below 4.5 a bit. Then I plug it in and see what it sounds like. The other voltages are only things I would tweak if I was breadboarding and *customizing*.

Kerry M