Quote Originally Posted by Gtrjunior View Post
Tubes are easy to change but you absolutely need to bias them once installed (power tubes).
That means you’ll need to take measurements of your plate voltage with either a multi-meter or buy a bias probe.
Then there is an easy math formula to compute which will tell you what your bias setting should be.
I’d you have never done this before I highly recommend that you take it to a qualified tech.... there are very dangerous voltages present in an amplifier even if it is not turned on....you absolutely can get hurt or killed by these voltages. A Marshall will carry about 500V coming off the plates....get bit by that and you’re gonna have a bad day.

Preamp tubes can simply be plugged in and that’s all you need to do.

Preamp tubes usually last a very long time....decades sometimes.
Power tubes last much shorter. With heavy use maybe as little as a year or 2....or they may last longer. There is no set rule.

The amplifier uses some tubes for the clean channel stages and some for gain stages.

If you were happy with the JJ tubes, no reason to switch. They are well made, sound good and are reasonably priced.
The Marshall 900 has a SS rectifier if I am not mistaken