First of all, it’s important to note that most tube amps are completely analog. Any analog circuit is going to carry noise. There are a few things that can interfere with the circuit and can cause noise such as florescent lights or dirty power. You need light to see, so this may be somewhat unavoidable. However, you can buy things like power conditioners to filter your power which will greatly reduce the noise carried into the amp via the A/C.
Another thing to think about is the interaction between your guitar and your amplifier. As stated before, analog circuits make noise. They have large transformers and other components inside. Now consider the pickups in your guitar. They’re magnets. When you walk up to your amplifier to change settings, you’re bringing the magnetic pickups closer to all those humming parts within your amp. By doing this, the pickups are going to “pick up” any noise those components are making. If the volume of your guitar is turned up, the noise is going to be amplified by the amplifier you’re plugged into. The type of pickup makes a difference as well. Humbuckers are designed to reduce the amount of noise that gets transferred to the amplifier by using two coils with opposite polarity. They aren’t perfect, but this essentially cancels out any noise that may come through. Single coil pickups, on the other hand, only have one coil, hence the name. These type of pickups are going to be much more sensitive to noise.
One last thing to consider is power and gain. If you have a 100 watt tube amp, it’s probably going to make more noise than a 15 watt tube amp. Also, if your amp is designed for high gain settings, it will most definitely make more noise than an amp designed to run cleaner. The higher you crank up the gain on your amp, the more noise it’s going to make. If you’re trying to turn up the volume, try turning down the gain and turning up the master volume. If you want more distortion and overdrive, then noise is an inherent part of obtaining that sound.”