[This answer is a little long – but I kept it as non-technical as I could!]
There are different types of amplifier, with different operating philosophy.
The main one you’ll find is “SE” (or “Single-Ended”) versus “Push-pull”
Single-Ended
A Single-Ended amplifier uses only one output valve for each channel, or sometimes they can be paired up but still used in single-ended mode.
The idea is that in a Single-Ended design, each output valve carries the whole signal. This makes the design of the amp much simpler, but SE amps have a significant penalty in terms of power and efficiency. Typically you need huge speakers to get the best from a SE amplifier because they’re usually very low-powered.
Some people feel quite passionately that the sound from amplifiers using the SE topology is much better. In fact it is not… SE amplifiers have their own quite pronounced distortion characteristics, but these are somehow pleasing to the ear.
Think of the SE sound as being like an Instagram filter for your sound. Some Instagram filters are quite pleasing, but each one is still a distortion in some way of the original picture.
The Audiophile community is very interested in Single-Ended amps, and as a strange irony, these are the ones that command the very highest prices, even though the SE design was invented as a cost-saving compromise back in the early days of valves.
Push-Pull
A Push-Pull amplifier takes the input signal, splits it in half, and amplifies each half separately, combining them again right at the end.
This way, each valve is able to be used much more efficiently, and the amplifier can be much more powerful as a result.
Power is usually a good thing, because these days we’re listening to music which has a lot more bass than in the old days, through speakers that are a lot smaller.
Push-pull amps do need to be set up a bit carefully though, because they can add a specific type of distortion to the sound if not adjusted properly which you’ll hear at high volumes.
All of the amplifiers we offer at ATRAD Audio are of the push-pull design, and we offer instructions on how to adjust these properly.
Hybrid vs. all-tube
Another design you may hear about is “hybrid”.
A Hybrid amp is one that gives the best of both worlds… part of it is valve-based, and other parts are using more modern electronics (called “Solid-state”, as opposed to vacuum-state).
There are many different possibilities in designing a hybrid amplifier.
Hybrid amps are usually smaller and cheaper than full valve amps. Sometimes, something might be advertised as a tube amp but in fact the tubes are only there for decoration. This is a fairly dishonest and unscrupulous practice, but at the cheaper end of the market it can happen.
ATRAD Audio amps are strictly all-tube, apart from some non-tube electronics in non-signal areas, such as the remote control or power supply circuits.