Things bread-boarding has taught me and a "Pot vs. Fixed resistor" issue.

[DIY Guy]

Hey everyone!

Beyond wiring my own guitars and changing out bad switches and jacks from circuit boards, I've not ventured into the how's and why's of circuitry.

Until I discovered Josh Scott's LPB-1 how-to video series last year. He made me WANT to try!

The Begining:
I built a "bread-board" out of stripped 14ga. solid copper house wire, some screw terminal blocks from a junked industrial furnace's control panel, a household light switch, my wah pedal's old 100K pot and a couple of old jacks.

Then I removed as many electronic parts as possible from old broken radios, DVD players, printers and such. Reading up on what all these parts were I cataloged them in a notebook and got started.

I eventually learned the transistors I have are either Japanese or Russian because the legs are in a different order.

THAT sure helped. lol

I learned that a wall wort power supply running through an ebay sourced $2
Boost/Buck circuit is not as noisy as expected, and has saved me a bunch of money on batteries.

Since my "breadboard" is all about soldering, I stole the four early 70's 500K pots out of my guitar project to more easily change transistor biasing values...which is where I started scratching my head.

When I found biasing values that sounded good via the pots, I'd measure their resistance and note the sound of those values in my book. Then I'd play around some more to see if I liked other combinations better etc.

Having settled on some values that matched resistors available in my stash, I removed the pots, soldered in the actual resistors and pthhhht. Vastly different sounds.

My meter has always given me values that very closely matched all the resistor's colour bands, AND, in testing I always set the resistance of those pots to match my stash's actual resistor values before committing to solder.

Oh, and I should add... I gave up soldering resistors in series to give me any desired values. Even when the measured result was as predicted, the sounds would be even more "off" than single fixed resistors...

What's happening here?

Cheers :-)

[Loudthud]

1) When measuring resistances with a DMM you must be careful not to touch the probes with your fingers. The higher the measured value, the more your fingers can affect the reading.

2) Resistors removed from old equipment can sometimes become damaged and change value when heated for soldering. When connected in series, you can still measure the resistance to see if it changed as long as both ends of the chain are not connected to a circuit.

3) Any diodes or transistors in a circuit can affect the readings of a DMM. Best to disconnect one end of a resistor to measure it's resistance.

[DIY Guy]

Thank you!

That's it right there. I never disconnected the pots so the tests were apples to oranges.

Cheers!

[g1]

Biasing by ear will not get you anywhere unless you can always duplicate the same signal level you biased with.
Transistors are usually biased to be in the most linear part of their operating range, to allow the widest variation of signal level.

[DIY Guy]

Ok! That makes sense, and I think I may have been choosing values that did accomodate the widest possible range of signal levels because I never wrote down any pot values that were only achievable in "one spot"... Only if all four bias resistor values were in the middle of a "wide zone" that sounded good from one extreem to the other would I write values down as "good combo's".

I will certainly give those thoughts more importance now.

Thank you!

[g1]

Also keep in mind that whether electronics, engines, etc., some times things sound the best right before they explode.   
It's best to know the theory first, safe operating parameters, datasheet recommendations (application notes), and then play around within those safe areas with listening tests.
Usually, compromises are made, in terms of tone vs reliability.