For those that don’t know much about valves, the heaters are a fundamental part of what makes them work as amplifiers. Valves control electron flow a bit like a tap controls water flow. However electrons only flow in conductors like metal wires and, as the bright sparks like Edison, De Forest etc realised in around 1907, in a vacuum heated to just the right temperature e.g. 250 degress C for a KT88 (glass bulb). So when you turn your amplifer to ON to what you are actually doing is warming up the valves ready to control electron flow – into your loudspeaker.
To raise that kind of temperature valves contain a heater filament. It is rather like a mini electric bar fire and it is the central tube of the vacuum tube; it is the bit that glows orange when the amp is turned on. Most amplifiers use an AC power supply to feed the heaters as this is the simplest design – just take the mains supply, transform down to what the valves like, typically 6.3 volts and you are done. However, the down side to this simplicity is that it is possible for the the 50 or 60 Hz hum to end up coming out of your loudspeaker due to pickup i.e. when the heaters or their interconnecting wires come near to the wires carrying your precious guitar signal. Just think for a minute about how sensitive guitars are to picking up interference from transformers, fridges, dimmer switches, mobile phones etc.
Well the same goes for the hum from the AC heater supply. The only way to minimise heater interference is to carefully dress the leads away from the sensitive parts of the circuit and you can usually see this in your amp as a twisted pair of wires in a chain going from valve to valve. However instead of taking the most direct route often you will see dressed carefully away from the valves and tucked into the amp’s chassis. Twisting the heater wires together carrying AC reduces hum transmission as a little cancellation occurs on along the same principal as humbucking pickups. However inside the valve itself the signal and the heater are just a couple of millimetres apart and there is nothing you can do about that except consider redesigning the amp to use a DC heater supply.
Many higher quality amplifiers are now using DC heater supplies to produce much lower hum at the speaker the reason that this works is that a high quality DC power supply has no hum. So why do we not see this more often, well the reason manufacturers tend to avoid DC is presumably to do with cost. DC power supplies are a little more complex requiring extra components i.e. rectifier and smoothing circuit i.e. some diodes and smoothing capacitors and that may effect the price point of the amplifier. Also when an amplifier is being played at volume hum becomes insignificant. Boutique amplifiers such as Carr use DC supplies presumably as they are not trying to compete on price just on quality.
So what is the downside to a DC heater supply well the only one I can think of is that you can’t always hear when the amp is on!