Green Russian Big Muff Pi Transistors Issue/Question

[canman]

Hey everyone,

I built this Green Russian:



And I'm having an interesting issue.  When the transistors are oriented (2N5089) correctly as per the layout and pinout from Tayda (yes, they are from Tayda) I get a bizarre, almost octave-ish effect and when you pick lightly it's really farty and glitchy.  The volume also reaches unity very quickly, and it has more gain than most BMP I've played/built.  Odd, because the Green Russian is supposedly has a bit lower gain.  So, when I flip all the transistors around, everything works as it should!  Unity volume is around noon on the volume knob, no glitchiness...very interesting.

Could it be the transistors from Tayda just have a backwards pinout than what the datasheet lists?  Or should I be looking for something on the board that is in the wrong spot?

[duck_arse]

stick the transistors into yr multimeters hFE tester, or knock-up RG's tester, and see what they read one way, then reverse them and compare.

[canman]

Did a little more digging, my transistors are in fact normal, but I still can't kick the oscillation.  I checked my voltages and found that compared to the voltages listed at GGG, I'm getting almost exactly the same voltages:

Q1-3
C: 3.8v
B: .6v
E: .03v

Q4
C: 4v
B: 2.5v
E: 1.8v

(The above voltages are from GGG)

My Q4 is different...I'm getting about 3.8v, 1.8v, and 1.2v (CBE, respectively).  I suspect maybe the base is where my problem is coming from?  I'm really bad at interpreting voltages and where the problem may be, so any help here would be appreciated.

EDIT:  I've been digging through different Green Russian schematics online...I don't see any 390k values on any of the schematics (in reference to Q4, and the 100k resistor connecting to the base)

EDIT 2:  I did some audio probing and the oscillation begins in the feedback loop of Q3:



But I can't figure out which component it is.  I'll try jumpering each component in the feedback loop and see if I can narrow it down further.

[duck_arse]

Mr. Ohm has a law that covers Q4 very nicely (give or take). work out the expected voltage at the base. how close is your measured? subtract the Vbe drop 0V7, do Ohm's law to find the current in the E resistor, use that current to work out the drop across the collector resistor. how far out are they?

Q2 and Q3 have the same component values, so the voltages should be very very similar. jumpering in the f/b parts will result in badness. check yr tonestack values/connections.

photos of your build?

[canman]

OK so I did all the math (at least, I think I did) and I'm not really sure what I'm supposed to be looking at, hehe.  I believe the voltage drop across the collector resistor came out to 8.1v, assuming my math was right. 

I ended up building a second one and it fired up right away, so there's no *need* to debug, but I'm still interested in the math and thinking process you are walking me through!

[Tony Forestiere]

I ended up building a second one and it fired up right away, so there's no *need* to debug, but I'm still interested in the math and thinking process you are walking me through!

Great you got one working. If you still have the clunker, make your measurements of the differences to expound upon your troubleshooting skills and maths learning.

[canman]

So, the "differences" is where I'm getting caught up.  What exactly am I measuring and comparing the differences of?  And what are these differences telling me?

Unfortunately, I used the caps and transistors from the clunker so I can't really make any more measurements...but the theory is still interesting. 

[duck_arse]

show us your calculations, and maybe PRR will appear and provide some whys and wheres.

[canman]

OK, so looking at Q4, I subtracted the Vbe drop of .7v as you said earlier, from the 2.5v that is supposed to be correct at the base.  So, then I plugged in that 1.8v into Ohm's law to find the current in the E resistor and got about .00082 amps.  Assuming I used the correct resistor value (the 2.2k resistor), of course.  Then I plugged in that current into Ohm's law to work out the voltage drop across the collector resistor and got 8.1v...and whether or not that's correct, I really don't even know what that calculation is telling me!

[MaxPower]

Those voltages for Q4 you posted from GGG look wrong to me. Must be a couple of typos. Based on the schematic the voltage at the base should be around 1.5 V , not 2.5 V. That means the voltage on the emitter should be around 0.9 V.

I'm getting about 3.8v, 1.8v, and 1.2v (CBE, respectively)
That actually looks okay. That part of the circuit should be working fine.


And your math was fine, giving you a voltage drop of 8.1 v across the collector resistor using the GGG values. Anyway, you subtract the voltage drop across the collector resistor from the power supply voltage giving you the voltage at the collector: (Vc = Vcc - Ic x Rc).

[duck_arse]

bearing in mind some wobble on the numbers (and I always manage to exclude the base current in figuring), here is how I do it:

with 470k//100k and 10k + 2k2 and 9V supply:

1/(470k + 100k) * 9V * 100k = 1V58 [the base voltage should be around this value]
1V58 - 0V7 = 0V88 [the emitter should be about this]
0V88/2k2 = 400uA [emitter current], 10k * 400uA = 4V [across the collector resistor]
9V - 4V gives Vc of 5V.

so, when you see this type circuit (Q4), you can work the around-about voltages yourself, and see if your readings are near-enough, or volts out. none of these numbers will come out exact, one build/resistor/transistor to the next.

[PRR]

Family matters.

> I always manage to exclude the base current in figuring

Base current is only 1% of emitter or collector current. Yes, you ignore it most times.

Emitter and Collector currents are =equal= (but opposite) for practical purpose (1% different).

You can sometimes skip computing current.

If the Emitter is showing 1V across 1K, and collector resistor is 3K, then the *drop* across the collector must be 3V. (Really 2.988V, so what?) If the battery is 9V, we expect 9V-3V= 6V.

If instead we find the collector at 8.7V, which is 0.3V down from 9V, then we wonder if the "3K" resistor might be really 300 Ohms (0.3K), or the "1K" resistor really 10K. In some color-codes (and some light!) this is too easy to do.

I picked round numbers for easy concept. In real circuits they may be 1K and 4.7K. Well, 4.7/1 is between 4:1 and 5:1. You can multiply by 4 and then again by 5, assume the answer is somewhere in the middle. If it seems close, this is probably not where your problem is; often if it is wrong it is very wrong and no fussy arithmetic is needed to see that.

[canman]

I love reading this stuff.  It all makes sense and confuses me at the same time!  I'm always humbled every time I come here for help...I know next to nothing about this stuff, so much to learn!  Thanks for the detailed posts and theory, very cool!