wiring toroidal transformer primary in parallel

[bikerhifinut]

If I had a Toroidal transformer with 2 x 115V primaries, i.e. suitable for 115V Mains and 230V mains as most commercially available ones are, then is there any reason why I can't wire the 115V primaries in parallel on UK mains in order to double the output Voltage?
I was thinking about the commonly and cheaply available 2 x 55V secondaries that wired in series will give 110V and if the primaries were paralleled, logically you'd get 220V which becomes a useful Voltage for Valve Circuits.
If it's ok to do this, would it be advisable to de rate the VA rating?
I figured that the insulation must be capable of withstanding UK mains voltage when wired in series, so there shouldn't be an issue of insulation breakdown, and I am not advocating this practice to get a higher voltage from a high voltage secondary.
It's just that 2 x 55V toroids are freely and cheaply available, whereas high voltage ones tend to be "specialist" windings and as one offs expensive.
I know some here can wind their own but apart from one instance of modifying a low voltage secondary under advice of the Gent who made it (Thanks Joe Bog) and it was a right old faff on if very effective, I have no real ambition to get involved in transformer manufacturing.

Regards and Thanks in advance.

Andy.

[bikerhifinut]

I should add, I know I could use a voltage doubler, but that means extra stresses on transformer and capacitors that I would rather avoid.
A.

[M0FYA Andy]

You'll certainly be overstressing the transformer applying 230v to a 115v primary. Having two windings in parallel won't alleviate this.

Andy

[Ed_Dinning]

Hi Andy, if it hase 2, 115v primaries they are meant to be wired in parallel FOR 115V ONLY, as otherwise the primary will be overloaded if only 1 is used.
Applying 240v to this 115 winding pair will push the primary well beyond saturation and cause mains circuit fuse blowing.

Ed

[Maarten]

You might be able to get away with it on a higher frequency, maybe 100Hz or 400Hz but that's very impractical.

[Glowing Bits!]

Me thinks this might call for a canister of CO2, it's good for putting out fires!
The primary should have two wires put together for 240v, not in parallel!
There is a way to up the output voltage using diodes but I can't remember how it's done, Japanese amps do things like this, it's 120v out to most valves except the output stage, diodes step it up to 350v.

[Ed_Dinning]

Yes, the voltage doubler circuit, but it does not have good regulation and at 50Hz and needing an amp or so of current will need pretty large caps.

Ed

[Glowing Bits!]

I can't remember what the caps are but 40uf springs to mind.
Me thinks it's time to dig out the circuit, the handbook has a nice A3 spread.

[bikerhifinut]

Thanks.
I knew there had to be a good reason why I have never seen it done!
OK thanks guys, we can put this one to bed. Be assured I WILL NOT try this at home!
Looks like the old voltage doubler from 110V if I go down this route.

Andy.

[kalee20]

If you use 230V on 115V windings, the transformer core will saturate and the no-load current will, instead of being negligible, be sky-high.

The only way it can be done, is if the frequency is also doubled. But getting 230V 100Hz is going to be more expensive than buying the proper transformer!

[Herald1360]

If you take care about VA loadings, you can use a couple of 230V:6V transformers back to back to give isolated 6.3V for heaters and (double) isolated 220V ish for HT.

A 25VA TX feeding a 12VA one would give you a couple of amps LT and maybe 40mA HT