How can you tell a tube or tubes are going bad?
I think my amp is creating more hiss/fuzz noise than it used too.
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My tech did a surprsing thing to test my preamp valves the other
My tech did a surprsing thing to test my preamp valves the other day when I thought I had a dud one. He knocked them with a screwdriver (while plugged in and on - be careful if you try this there are some very high voltages inside - if in doubt, take it to a tech). The good one made no noise through the speaker, the bad one made a ringing sound - real noticable.
I swapped it out and sure enough - she's working fine now. Not sure if this is true of power tubes (EL34 and the like)...
G
Very simple test..tapping the tubes. I use a pencil, a screwdriv
Very simple test..tapping the tubes. I use a pencil, a screwdriver takes a bit more finesse. Chances are the bad one is a 12ax7-type of preamp tube. Power tubes can go microphonic, too, but they are the ones more likely to drop power and/or tone. Tube testers out there these days run the gammet from poor to fair....
That's usually a pretty good sign. Others include volume swells/
That's usually a pretty good sign. Others include volume swells/fades at random times, dull/lifeless tones - regardless of how much you EQ your amp, pops/crackles/etc...
Devices exist to test tubes - i'm not sure how much they cost - but as a general rule, if it's been a while since the last swap (1 year + of moderate-to-heavy use) and you're experienceing one or more of the above symptoms, it's time for a change.