DC/DC converter for 200VDC tube pre. Avoiding converter noise (digital switching noises and hiss)... Possible?

[777funk]

Insulation can fail. A battery powered device is no in any way connected to the mains, unless you plug it to a charger.
Let's not forget this is DIY, not design houses operating with processes and procedures, certifications etc.
I do not say everybody has to follow the advice, I say that is what I recommend doing.

Definitely a valid concern. Of course any power supply could have insulation fail and somehow get line voltage into what's supposed to be the "low voltage" side of things. An earth ground connection certainly minimizes that risk.

 

[Globulator]

a SMPS for boosting from 24V to 300V

For low power applications it might an simpler to use a mains transformer (at 50-60Hz) 'the wrong way around'
For battery applications, a class D amplifier running from a 50/60Hz oscillator might provide a good source of AC..