Guitar amps that use tubes are still great, and even after all these years, digital modeling amps still emulate the character that these amps possess. After a while these tubes start to “lose” their sound and then it is time to replace them. There are many opinions on which tubes are best and have the greatest tone, but it is not really that complicated. There are three basic tube types (by purpose) that an amp has and I will go over each one, its alternatives, and its tonal benefits. This will go much faster than you would expect.
The preamp tubes are the first tubes in the electrical signal path. These are the smallest tubes in the amp and are technically two conjoined tubes, a dual triode. Most common is the 12AX7, alternatives are the 12AU7, the 12AY7, and some other 12**7 tubes. They last longer than the power tubes so you won’t need replace these as often. Their differences can be explained best here.
For guitar and bass, I would stick with the 12AX7, it is easier to find since it comes standard on everything! You might think the alternatives would be a great candidate for a reverb channel or an effects loop, but no one would really notice a difference. If anything, you might have to convince yourself that it was worth the experiment.
This is what most people look for. These are usually the first to fail and the subject of much argument. Common types are 6L6, EL34, 6V6, KT88, EL84, and KT66. The first two in the list are in 90% of all amps, and bass amps usually go for the KT types. All have pentode sockets, with the exception of the EL84. You should only get the type your amp needs, or you can damage your amp. For example, if you are replacing KT88 tubes with 6V6 tubes and don’t have the ability to adjust the voltage, you may melt something.
These tubes are very voltage dependent and their output differences are mistaken for tonal variation. Buying matched sets is highly recommended if your amp has multiple sockets for power tubes. These tubes must be biased to work properly, that means they have to have their voltage “regulated” by a little knob so thy can be used to their full or desired potential. When the voltage is low, they are said to be “running cold”, and when the voltage is high, they are said to be “running hot.”
These tubes are the not something you want to do wrong. Replace these with the same type, or if you want to really experiment, check this site here for some basic info, and here for the hard data. If you are changing to some Webber Copper Caps, like I did, then use something comparable.
The rectifier tube can change the feel of your amp, but as I was told by an amp builder– once you crank the amp to make it loud, the whole “sag” thing kind of disappears because the voltage hitting the tube is so constant.
A Note on Brands and Different Manufacturers
You are going to see people say that JJ tubes are the best, and that Ruby Tubes have the best tone, some say that the NOS (New Old Stock) is the way to go. Before you dredge the forums and tear your hair out at Ampage looking for a solid answer to which tube brand sounds the best, ask yourself this question: which sounds better? A Sylvania light bulb or a Phillips light bulb?
Tubes are basically designed to do the same thing, so don’t expect one brand to eclipse the other in terms of tone and sound quality. They all are designing their tubes for the same voltage requirements, not tone. “But cathode bias is the best on a XYZ tube!” No. Unless you have a magic ear, you wont hear it. My friend swapped out all of his Sovtek tubes on his Laney VH100R with JJ tubes because he heard they had great tone. When he got them all in, it sounded terrible because it needed to be rebiased and he had a bad 12AX7. Once he had it all up an running, it sounded exactly the same! Let me end with this, tubes are designed for voltage, not tone.