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When to replace preamp tubes?

When do you replace preamp tubes like 12AU7sand 12AX7? Do you wait till they blow or start making noise? It seems I’ve had them in for a long time and are in use everyday. I have replaced them in the past when they become microphonic but should I wait that long, should I just replace them after a certain amount of hours?
djf1
inna
5,245 posts
 
Better NOS tubes last longer or it depends ?
bpoletti
1,004 posts
 
My opinions, partially based on experience from 55+ years of using tubes, building tube electronics (mostly from kits) and tweaking tube electronics.  I'm no expert, but I saw one on TV about 20 ago.

How to tell if a tube needs replacement?

Increased noise in one or both channels
Loss of sharp imaging  / soundstage
Tube fails to glow (burned out heater element - "hard down")
Persistent humming in one channel even when a non-connected source is selected
Tube goes microphonic (tapping on chassis can be heard through speakers)
Increasing distortion or output level in a single channel  
"Motorboating"  
Bright glowing of tubes, particularly accompanied by humming, sometimes LOUD humming (brighter in one tube than others)

Errant tubes can sometimes be identified by switching noisy or microphonic tubes to different channels to see whether the noise / microphonic follows the tube.



jab's avatar
jab
450 posts
 

This is why people fear tube equipment, little boys with little toys. My mother used tube amps and preamps, my grandmother used tube equipment and all was good. In my little picture next to my tag is the high end turntable from my grandmother, and it still works.

If you are willing to spend 10 minutes changing a set of tubes every 5-10 years or so, you will be rewarded with better sound, in my opinion.

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