I take it you are asking about tube amps, probably the buzz you are hearing is hum from the AC cycle, this normally comes from one or any combination of the following, Filter Capacitors, they degrade with age and get noisy, Pre-amp Tubes, cheap or failing tubes can be noisy even though pre-amp tubes are humbucking by design, Power Tubes, which are not humbucking by design, can be noisy for a variety of reasons, the DC heating element circuit in most amps will cause this, for one reason being that 50% of the time they are wired wrong from the factory, some amps ( Like Fender Silverfaces ) have a bias-balance circuit that bring alot of noise into the tubes, and cheap power tubes can be noisy too. The good news is that all of the above and more can be fixed by the normal maintenance that a tube amp should given to keep it healthy...................