Checking Resistor Values

[David Root]

To steal an old line, "Measure twice, solder once".

[Cliff Schecht]

It's not uncommon to have 10 or less exactly on spec out of 1000 units, but maybe as many as 100 within .05%.

Once soldered the values will be off by ±5% or more depending on the type and mounting method. Why bother?

Vintage amps used ±20% parts most of the time, ±10% for quality stuff and recent models. I use ±5% parts, test all of them and keep those in spec - something like 99.9%.

The obsession with measuring every resistor before putting it in an amp is an exercise in futility IMO. I just do not feel the need to spend the time measuring every resistor that I shove in an amp, especially if it's from a known good stock and I've used said resistor type from my stock/pile before. It's the manufacturers job to sort through the crap resistors, not mine. I can't think of a single time that I used a carbon film resistor from a known good stock and had it be so off value that I can't use it. Many times even the 5% tolerance turns into fractions of a percent of a difference in a circuit and is not audible. It's just not worth my time to endlessly tweak something that is going to drift over time anyways.

I have a lot of carbon comps that I don't even bother to measure anymore because I'm yet to have ONE that's bad. And I've used thousands of them by now. The parts I really focus on are the HV electrolytics, coupling caps, pots (especially if recycled) and then resistors if I suspect any of them. I probably spend more time verifying that the power stage is wired up correctly and safely over anything else.

[alanp]

Given that a certain amount of colouration is desirable in guitar amps (as opposed to supposedly neutral hifi amps) I would have thought that a certain ballpark tolerance would have been good enough for rock n roll?

[Structo]

Yeah it borders on obsessive but certainly doesn't hurt anything.

I find it kind of enjoyable when starting a new project.

[gearhead]

Mostly check the >1watt resistors to make sure I got the right value. Some color codes have down cold (the 1's) but others either don't remember off the top of my head or can't quite read the color (esp the blue-green-violet). Maybe I am color-deficient?

For the caps, jeez, they're all over the map. I got a batch of of power cap E's that are barely inside the 20% range. Ever last 12 dollar one of them.

[Cliff Schecht]

Something I learned to do from building stompboxes and other projects with lots of discrete resistors was to read resistor color codes quickly. Perhaps shopping at surplus electronics stores has also sort of forced me to learn. It's more associative than cognitive for me now, I rarely stumble across a resistor that I have to think more than a second or two about.

Remembering the common values in one range (e.g. 1k, 2.2k, 3.9k, 4.7k, 5.6k, 6.8k and 9.1k) will make life easy because you can reference the set of resistors that you know like the back of your hand to figure out the goofy ones. The numbers 1-9 correspond to at least one band on one of the resistor values above, and something like this is about the smallest range I can think of to remember such things. Multipliers are easy because they really only go up through blue, so that's all of seven more things to remember.

So yeah, it all becomes automatic after a while. It goes a lot quicker when you actually make a conscious effort to memorize everything (and of course checking your work ).

[Structo]

Yeah, when I was younger I had the color code down pretty good.

Then I spent a period of years where I did no electronics tinkering.

I've slowly gained back that knowledge but you are right, it takes practice to where you don't have to think about it much, if at all.

The real common mistakes are using a 470R instead of a 470K or other similar looking values.

Especially on 5 band resistors, which I'm not even sure were around when I first started in electronics.
Those sometimes still give me trouble if the tolerance band isn't noticeably wider than the first band.

[gary sanders]

I check them all too.Recently got a "2.2k" 2 watt that was actually 17k!!!!

[Structo]

I check them all too.Recently got a "2.2k" 2 watt that was actually 17k!!!!

You see, that right there is a good example why you should meter them.

You would install that resistor possibly on power supply string and you would be scratching your head wondering why your phase inverter is not working right.

Sure as soon as you measured your nodes you should figure it out but simply measuring them would have culled that resistor from the bunch.

[dynaman]

I occasionally have to meter some when switching from 4 to 5 band color codes.

OMG, y'all really are obsessive about tolerances. I'm good with +/-20% because I like surprises and sometimes I just use a particular value cuz I like the way certain band colors go together.

[selloutrr]

I was one of those head scratchers back when I started tinkering with amps in high school I was so excited and proud of this amp i fired it up for the first time and it was horrible... I had to take it to a tech. It looked right and yet didn't sound right. It turned out to be a bad resistor. Since then I check them, the piece of mind knowing what I use is correct is worth any time it might take.

It's not wrong to just insert parts and build, this is purely subject and personal preference.

It is however necassary to Meter parts when you are making a pair of identical circuits, Hifi or studio, stereo amps.. etc. or direct cloning. Yes it is possibly to buy matched iron and using the Amplitrex AT1000 I can test and tube and then search my data and match based on the test results, or print out the results and order a tube based on those specs. As well as adjust my test preferences to mimic the tester they are using.

[FYL]

sometimes I just use a particular value cuz I like the way certain band colors go together.

Zen and the art of amp building. That's why masters of the craft don't like the new Mallory 150s in their white guise. No soul, man, no tone.

[Structo]

Yeah, what's up with those white wrappers?

[ChrisM]

How is it I have built 60+ pedals and 25+ amps and never had a resistor was as out of spec as some of you claim? That's crazy, maybe my luck is just very very good?

[dynaman]

sometimes I just use a particular value cuz I like the way certain band colors go together.

Zen and the art of amp building. That's why masters of the craft don't like the new Mallory 150s in their white guise. No soul, man, no tone.

Uh oh, I've just enough wine in me to fess up to being one of those guys that has used/mixed certain brands of caps just to keep the caps' colors consistent. "When the amp looks good, you feel good. When you feel good, you play good....and when you play good, you might even think you sound good." New company motto....