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Okay so I'll be going through some of the methods of supplying the tube filaments of vacuum tubes and the advantages/disadvantages of the different methods.
The first one (the one at the left (orange) ) is usually the best method when it comes down to hum, but the diodes waste power and bring down the power factor as a whole. It should be used only in very sensitive devices like preamps and such. The power tubes usually need a lot of output signal to drive relatively low power output loads, so even up to a few volts of hum won't be noticeable at all, after the output transformer steps it down, but at the preamp stage it's very important to keep noise away from everywhere you can think off (supply rail noise filtering , heater noise suppression, good quality components and such). I recommend to use very high value filtering capacitors for this method as shown (with supercapacitors for instance), cuz dirty filtering can sometimes cause even more problems than with no filtering at all.
The second method at the top right (red) is somewhat adequate when it comes down to noise suppression, but I personally don't prefer it, because one end is grounded, so the two current carrying wires are not symmetrical, so no matter how much you twist 🔀 the wires, you won't get any electromagnetic emission suppression and other parts of the circuit may pick up the noise and amplify it. It's a cheap and dirty solution but it works sometimes.
The third method is the one I'll be concentrating most, as it provides very good noise suppression and can be used to elevate the heater supply voltage which is sometimes needed to not exceed the Vkf rating of the tube (maximum cathode to heater voltage). This is especially important in parts of the circuit where the cathode voltage is expected to be high (cathode followers and some types of phase splitters). Just flip the switch and you'll see how the voltage floats above some preset voltage level. When the wiper is grounded it creates a virtual ground and you can find the best symmetry for the transformer output where the two voltages are equal in amplitude and opposite in phase (which cancels out any EM emission). This gives best noise suppression with minimal components and is the method I usually use when working with noise sensitive tubes. I'm currently using this method and managed to bring down noise to a completely inaudible level.
Some other tips:
- Always usr tightly twisted cables when powering tube heaters, as this helps cancel out any noise along the wires.
- Stick to using balances feedlines and keep out of using the second method I showed. It does the job, but is dirty and I don't really recommend it.
- A little bonus tip. If you're using 12AX7/ECC83 use the series heater configuration and ground the mid point or use it to elevate the supply voltage. This creates a virtual ground and basically replicates the third scenario I described here.
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