Superfuzz, diode voltage drop

[yeeshkul]

Guys did you measure what is the voltage drop across the germanium diodes in an original unit? I have
a bag of Ge diodes and wanna pick the best pair.

[cortezthekiller]

The diodes in my superfuzz measure .36 volts, in circuit, on the diode checking function of my multimeter.
This one is the big red box with the blue foot pad, containing c536 transistors (for reference).

[R.G.]

Guys did you measure what is the voltage drop across the germanium diodes in an original unit? I have
a bag of Ge diodes and wanna pick the best pair.

Best matched for voltage? Best *mis*-matched for voltage? Best "sounding"? 


... "sounding" ? What's that mean?

[Thecomedian]

Voltages and currents may all be "in the pipe" and working great, yet it might sound bad because it's not properly biased. Kind of like how many of the original fuzz faces sounded bad, and some sounded good, due to transistor and resistor tolerances.

[yeeshkul]

Cortezthekiller, thank you mate, thank you a million!!
R.G. i was clearly asking for voltages. You are pushing me somewhere where i don't wanna be.

[R.G.]

R.G. i was clearly asking for voltages. You are pushing me somewhere where i don't wanna be.

Not really, although it may have seemed that way. The point was to wonder how voltages related to best sound.

Germanium diodes will measure widely different voltages at different test currents, because their conduction voltage starts so low and their conduction knee is relatively big. With a simple voltage test, the unspecified current used in the test will make differences. With only a meter testing at an unspecified single current, about all you can really get it whether they match or not, and whether and how much they mismatch.

[yeeshkul]

I can't define the best sound. I just wanted to know what diodes to pick to get the same sound - thus the voltage. You are right about the currents. I was
just expecting my DMM using similar current to the other DMMs.

[R.G.]

That makes sense. In general, germanium diodes will show 0.2 to 0.4V depending on the forward current and sometimes the packaging. The packaging sometimes has more resistance than with modern silicon diodes.

Unspecified multimeters will tend to have similar test currents, and 1ma is common, but there's no way to know other than testing the meter. So the tests are consistent within measurements in the same meter, but not necessarily across meters.

It's a worthwhile thing to do to test your own meter for what current it uses for testing in the resistance tests, at least in the low ranges.

If this seems circular, it is. Knowing what your meter does is a big deal at unexpected times.

The sound will vary more on how well matched the two diodes are than on the voltage level. It would be good to pick out several with low, mid, and high voltages on the meter, then experiment with matched low, matched mid, matched high (which ought to differ primarily in loudness) and various mismatch combinations, which will toss some even harmonics into the mix noticeably.

[yeeshkul]

Hey Cortezthekiller, can you hear it? Mate can you use another DMM to measure the forward current that goes through the diode while you measuring it by the first DMM?
That would give us some bulletproof results 

[Mark Hammer]

Not to be a naysayer, but given where they are in the circuit, I don't think it really matters.  If you have a pair with a 380mv forward voltage, and you want to know what a 330mv FV would sound like, just turn up the drive knob a little.  They're just clipping whatever comes out of the doubling portion; if it's hotter, they clip more, and if it's cooler they clip less.

On the other hand, you'd be pleasantly surprised with how much it clips even without ANY diodes in there.  Louder, too.

[cortezthekiller]

Hey,

I haven't got another meter to test simultaneously, though I do know that the meter I tested them with uses 1mA current as R.G. mentioned.

The test was done with the pedal off / nothing plugged in or signal present.

[yeeshkul]

Cortezthekiller: Brilliant, thank you.
Thecomedian: sure, that is quite fundamental . Also the octave doubler should be well balanced for maximum octave.

By the way here is my little article on tuning up the octave with pictures and sound samples.
http://www.diystompboxes.com/smfforum/index.php?topic=94633.msg817034#msg817034