There is no specific time as such for changing tubes, i mean generally preamp tubes may last for years in a stretch with high or medium usage, Power tubes such as 6V6's also last plenty of years and may even outrun the person using it. When to change a tube depends on various factors.
Originally Posted by mymusicmyguitar
Some of the factors that i know of are below.
1) When your amp generates uncontrollable high pitch noise or say squealing.....(Also known as microphonics or microphonic tube), this can happen any time, even when a tube is right out of the box.
2) Even when you raise the volume to 10, you still feel that the overall output has gone down considerably, this is when a tube needs to be looked at and replaced.
3) Loss of gain, or drive also means a faulty or worn out tube and needs replacement.
4) The tube colour changes from orange, to blue or purple also is a sign to replace it, it may work fine but it at the end of its life. and replacement is recommended.
5) Known settings on the amp suddenly has a lot of hum or noise to it, Replace the tubes.
6) When you look at the tube physically, it does not appear transparent and has a milky or cloudy appearance to it, Time to change the tubes.
Hope this helps.....there are many more signs when a tube needs replacement.....
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