big muff voltage doesn't seem right...

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big muff voltage doesn't seem right...

Postby alange5 » 03 Jun 2014, 03:14

Hello, first post here. I just built a green muff and it sounds awesome, but it has a little less "oomf" than my original. I opened up my original, and voltages all fall within the documented specs. On my build, however, the collector voltages are all a bit high. Here are the voltages on my build:

Q1
C 6.75
B .63
E .08

Q2
C 6.33
B .64
E .1

Q3:
C 5.98
B .63
E .09

Q4
C 6.86
B 1.08
E .53

Power supply is about 9.48V where it meets the board. I've checked and re-checked the layout (http://diy-layout.com/9) but to no avail. I can't find any shorts or continuity problems. Resistors are within spec. Any ideas where to look for a problem?

Many thanks
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Re: big muff voltage doesn't seem right...

Postby galmar » 03 Jun 2014, 14:07

First of all, you should give us the theoretical voltages that you refer to, so that we can judge on our own. You should also show the exact schematic that you followed, and your parts list, to see whether something is going on. :)

Minor differences are never a problem. Consider the fact that the DC bias point is for example dependent on the absolute value of hfe of every transistor - this depends on temperature, type of transistor etc. Changing the hfe from 100 to 400 can sometimes shift your voltages more than 0.5V, for example.
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Re: big muff voltage doesn't seem right...

Postby alange5 » 03 Jun 2014, 14:25

I posted the link to the layout I used, which contains a full parts list. http://diy-layout.com/9

and the schematic: http://www.generalguitargadgets.com/pdf/ggg_bmp_grus_sc.pdf

Here's some info on Big Muff voltage. http://www.generalguitargadgets.com/pdf/ggg_bmp_instruct.pdf

My collector voltages are at least 2V higher on all 4 transistors
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Re: big muff voltage doesn't seem right...

Postby alange5 » 03 Jun 2014, 17:18

update:

I swapped in the transistors from my original big muff and the voltages are fine. But I'd rather keep the old transistors with the old muff.... what can I do to lower the collector voltages using my newly purchased 5088's? is it as simple as increasing the resistor values on the collectors?
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Re: big muff voltage doesn't seem right...

Postby galmar » 03 Jun 2014, 17:33

Sorry, I thought you had posted the link to the software you used!

As I said before, the most likely culprit are the transistors. Please measure the transistors you just bought, and see whether they have a high hfe as advertised! For example, I ran a DC simulation using a 2N5088 with an hfe=50 (very low), and the result was indeed a high collector voltage.

If they turn out to have such a low hfe, either they are faulty, or damaged or what. Measure the hfe of your transistor either directly with a voltmeter (if you got such an one), or directly on your build: calculate every current by measuring voltages on resistors etc and find Ic/Ib. I would try both to compare results. :)

Also, make sure these 12k resistors are indeed 12k. Check the color code, measure them etc. Do it for the emitter resistors, too.
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Re: big muff voltage doesn't seem right...

Postby galmar » 03 Jun 2014, 17:50

I did a quick check on the numbers that you posted, and they seem to justify my claim: that your transistors have very low hfe, and they shouldn't.

For Q1: Ic = (9.48-6.75)/(12k) = 227.5 uA
current through 470k = (6.75-0.63)/(470k) = 13 uA
Ib = current through 470k - current through 100k
current through 100k = (0.63)/(100k) = 6.3 uA
so: Ib = 13 - 6.3 = 6.7 uA

Thus: hfe = Ic/Ib = 227.5/6.7 = 34

Which is very low, and indicates that something is wrong with your transistors. Tell us where you bought them and look up for the specs of their manufacturer.

To confirm that the transistors are the issue, confirm that the resistors are indeed correct - color code + measuring. :)
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Re: big muff voltage doesn't seem right...

Postby galmar » 03 Jun 2014, 22:32

I did a small approximation above (that does not alter the conclusion), so I will post the detailed calculation for clarity:

For Q1: Ic = (current through 12k) - (current through 470k)
current through 12k = (9.48-6.75)/(12k) = 227.5 uA
current through 470k = (6.75-0.63)/(470k) = 13 uA
Ic = 227.5 - 13 = 214.5 uA
Ib = (current through 470k) - (current through 100k)
current through 100k = (0.63)/(100k) = 6.3 uA
so: Ib = 13 - 6.3 = 6.7 uA

Thus: hfe = Ic/Ib = 214.5/6.7 = 32

Same result, without the approximation I used in the first place: hfe seems very low.

Did you work it out? :)
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Re: big muff voltage doesn't seem right...

Postby alange5 » 03 Jun 2014, 22:41

wow, this has been incredibly helpful. I purchased the transistors from Mammoth Electronics. Here's the link: http://www.mammothelectronics.com/2N5088-FSC-NPN-General-Purpose-Amplifier-p/100-1000.htm

I've got a friend who is kindly offering me some 5088's. I'll be picking them up tonight. I'll swap them in and report back. Again, you've been very helpful and I appreciate it. And thank you for laying out the math. I'm still in the learning stages and every bit of info helps.
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