Monday, April 27, 2009

Are these two resistors different?

One of the first steps you'll read in any set of tube amp troubleshooting instructions is "visual inspection". It just means looking over the components for any obvious signs of stress. Here I've stuck two 100K resistors in a piece of foam for the sake of comparison.

Fender '65 Twin Reissue failed plate resitor

Looking at the two 100K resistors pictured an astute observer might notice the slightly darker midsection of the upper resistor. It's fairly subtle here and even more so when it's still in the circuit, not positioned right next to a normal looking one.

These resistors are plate resistors taken from a Fender '65 Twin Reissue that came in with a broken Normal channel. The resistor that shows slight signs of overheating was completely open. It was passing no current at all, effectively shutting off the tube. It might as well have not have even been in the circuit. I replaced it to get the channel working again.

So why did I take out both resistors? Well the second one is actually completely open as well, it just doesn't happen to show any visible signs of fatigue.

That's the trick with visual inspection. It's a very good first step. But troubleshooting effectively means knowing how to use your meter to see for you. Parts can fail in many ways that you never have any hope of catching with your eyes.

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