You can mount two small transformers back to back, but the smaller they 
are, the smaller is the ratio when used in reverse.

E.g. a 4VA print transformer with 220V/12V will not yield 220V when fed 
with 12V, it wil be more in the range of 150V. Keep this in mind. The 
4VA transformer will more likely give 220V on its output when supplied 
with 18-20V.
This ratio will increase with larger transformers.

Maybe this little transformer here will do the job? It's not 1:1 but 
close to it. In addition you'll also get a 6.3 winding for two ECC83 
tubes:

https://www.tube-town.net/ttstore/tt-powertrafo-ei-230-200-6-3-7-va.html

Thanks for the answers so far. I've had another idea this morning. Would 
it work when I use it as I've drawn it in the photo I attached? I am 
needing a symmetrical +-15V anyway for my project, so I was thinking 
about using two transformers each with two 115V primary windings. Are 
there any issues I'm facing when connecting it that way? I am only 
guessing I have to use transformers with double the rated power because 
I would be only driving one of the primary windings, the other one would 
be used to form a secondary for the tube plate voltage (HV)

-Valou

I doubt it is a good idea to abuse one half of the primary windings to 
get your HV supply. The transformer has a guaranteed insulation between 
primary and secondaries. This is not the case for the two primaries.

If you get a short between the primary windings you might end up having 
mains voltage without galvanic insulation on your HV supply.

In theory it would work though.

If you have different loads at the transformers the primary 
voltages/currents are not equal.

@Nils, I was suspecting the same thing already...
Plus the loads not being equal is indeed a real concern here.
I have to think this through again, thanks for the answers so far!

-Valentin

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