Resistor drift under load

[toner]

i was wondering about resistor drift under load, and maybe partially from heat issues. i know i can test resisters with a mulimeter, but is that really a definitive test that they will work properly? i know CC's drift with heat, but im not talking about those, im talking about CF's, MF's, and MO type modern resistors. If im trouble shooting an amp, how can i know that for sure they will work in the circuit assuming that you aren't going over voltage or current spec's. Do these types of parts fail under load, or are these something that if they are the right value, and test correct with a MM out of circuit, can be guaranteed to work right? Ive had lots of bad caps..anyone had defective resistors that test ok out of circuit?
thanks

[Structo]

Sure anything is possible.
If you use 5% tolerance parts or lower, you stand a better chance of having stability in the amp.
Also choosing the proper voltage, current and temperature ratings is a must in tube amps where we see higher voltages than most consumer goods these days.

Think about those old Fender amps that people covet so much.
Their resistor values are all over the place.

I find the CC resistors will drift higher in value over time.

Personally, when I build a new amp or do any repairs, I always measure the component with a meter to verify it is what it says it is.
That way there are no surprises unless I make a mistake.

Caps can be a crap shoot because of their higher tolerance rating.
Thankfully most of the time they work good enough.

On tube amps I make sure that I use the proper voltage rating to ensure that the parts won't fail from excessive heat or other failure modes.

[Super_Reverb]

All resistors have voltage coefficients and temperature coefficients. That means resistance changes with operating temperature (ambient and power effects). Resistance also changes with voltage. Voltage coefficients are a source of nonlinearity and part of the "magic" that many perceive with carbon composite resistors. Tempco is primarily a material properly.

TempCo is measured in ppm or percent / degree

VoltCo is measured in ppm or percent / volt

This information should be available online for the resistor technology you are using.

cheers,

rob

[Zippy]

Ive had lots of bad caps..anyone had defective resistors that test ok out of circuit?

Yes. I've seen resistors that had bad lead connections or small cracks that, when heated due to high current (run close to maximum rated wattage) would swell and go non-linear. After I removed them from the circuit, they would often fall apart with little effort. I used to keep a can of freon on my bench to spray suspect components in situ.

If you suspect a bad resistor in a build, read the voltage across it and see if that changes as the amp heats up.

[Structo]

Yep, a can of cold air is a great troubleshooting tool, in tube amps as well as SS devices.