I know that technically there is little difference between AC or DC heating of the tube - if it gives proper voltage everything is fine... Except the hum: I have read in various sources that some preamp tubes are very resistant to AC hum and can be properly heated with it and some are somewhat terrible with the hum and are needed to be heated with filtered DC if you want to achieve noiseless performance.
The question is.. How do we know which types of tubes are affected by the hum and which are not? Can you find out without testing? I have researched various datasheets but found little help.
The problem it appears is that in most datasheets it is not stated if the heater is isolated or not. (Obviously an isolated heater will produce way less hum)
I plan on working with ECC85 tube (similar but not the same as 12AT7/ECC81). (Maybe some of you had experience with this specific tube?) In this tubes datasheet it is stated that it can be heated with both AC and DC but then again... if we are working on somewhat high-end headphones amplifier is the noise going to be an issue with AC heating? And how bad is it going to be? Here is the datasheet: https://frank.pocnet.net/sheets/010/e/ECC85.pdf
There is a wonderful source of information in this website (tubes enthusiasts - you must read this! It's a gold mine) and it basically answers my question (use DC and you can not go wrong). http://www.valvewizard.co.uk/heater.html
Of course you can always go the guaranteed road and filter AC to DC (cmon, it's easy isn't it?) but I am really curious whether you always need DC heating for high end performance... And how do you find it? - the answer is still somewhat a mystery to me.