Switched DC heater vs AC

[Bigun]

there are also switched electronic transformers which can be adapted for use, something I am considering to heat an 845 tube

 

[audiowize]

The gain is -20dB from 0V to 200mVrms, and 0dB from 201mVrms to 2Vrms, with an output impedance of about 3-4 ohms.

There isn't going to be any noise.

 

[astouffer]

there are also switched electronic transformers which can be adapted for use, something I am considering to heat an 845 tube

I would question if the output is totally isolated. If you need DC just run two 5v switchers in series.

 

[Bigun]

The output is usually a small toroid, good isolation. Plenty of folk have used these to light up their bottles, a few years back, possibly mostly over on audioasylum

 

[6A3sUMMER]

At the higher switcher frequencies that are used today,
be careful.
A few pF of stray capacitance makes for a lot of noise injected into your amp.

 

[TMM]

Also with those types of switching lighting transformers having active power factor correction and being dimmable these days, there is no guarantee that the output won't have 50/60/100/120Hz ripple on it, nor that it won't produce audio frequency signals or RF.