Big Muff issues only if playing the low E string

[Chris S]

Hey all, against my better judgement trawled the internet and found an unverified (as far as I knew) vero strip layout of a Big Muff based on the GGG schematic. (See below)

I changed the input and output caps to 0.1uF which tightened up the sound and it sounds great when I play the 5 highest strings but when I play the low E string there's a muffled sound (maybe like something is being overloaded) until the signal decays a little and then it fuzzes like all the other strings. I've had a look this guide http://www.dallashodgson.info/files/BigMuffCircuitGuide.pdf to see what I should be modifying and I'm not sure what to touch.

Thanks in advance.


https://docs.google.com/open?id=0B5sOv5hO4idbcjA1UDBNc3B1SjA

[Brainbender]

I had the same problem and as far as I remember, too high gain transistors and slight gain modifications were causing muffled sound.

[LucifersTrip]

1st step: what are the voltages on the transistors?

[Chris S]

Q2 (input stage usually Q1 on most schematics)
E: 0.02
B: 0.60
C: 6.82

Q1 (1st clipping stage usually Q2 on most schematics)
E: 0.07
B: 0.67
C: 4.30

Q3 (2nd clipping stage)
E: 0.07
B: 0.67
C: 4.29

Q4 (output stage)
E: 1.02
B: 0.06
C: 4.31

Ah so problem with the input stage? - there all 2N5089 transistors
double checked that the resistor connected to E is 10k as per layout

Cheers

[hannibal827]

Your voltages for the input stage [Q2] and the output stage look like they're off.

The collector on that input stage transistor should be at about 3.8 VDC.
The base on the output stage should be at least 0.5-0.6 volts higher than the emitter voltage, whatever the case.  The collector on Q4 should probably be closer to 4.0, but 4.3 volts is probably okay for that.

[Chris S]

Sorry just flipped input stage and first stage readings around. (my bad)
So it's the input stage that's off. So to reduce the voltage to the input stage I increase the value of the resistor going into the Emitter?

Thanks

[hannibal827]

I would try increasing the resistor between the collector and V+ to get the collector voltage down.  Hope this helps.

[Chris S]

Great. Thanks! Roughly what sort of value should I look at using (currently it's 120R and I don't have a low value pot lying around)

[hannibal827]

No, I'm talking about R5 in that layout.  It's listed as 10k; I don't know what value you used. 

I don't have a specific recommendation for the increased value; maybe someone more clever than me will.  I have always used 10k in that spot and the voltages have always been in the ballpark.  If it were on the breadboard, I would just keep trying higher values until I hit the target.  Sorry I can't be more precise.

[Chris S]

Whoops got my Emmitters and collectors switched. So yes it is currently 10k. Will play around with some other values tomorrow.

[pinkjimiphoton]

check your caps, what you describe could be leaking dc, too...that causes a peculiar "muffled" distortion, particularly if it's only really an issue with certain frequencies..

[Kesh]

Q4 looks fried from Vbe

[Chris S]

Thanks Kesh, I'll replace Q4.

pinkjimiphoton, Are the leaking caps likely to be around the faulty reading transistor or anywhere on the board?

Thanks again

[Chris S]

Thanks again Kesh. Q4 replaced Q1 readings back to normal