Mains transformer as a choke

[diydidi]

Hi Guys.
Is it possible to use one winding of a small mains transformer as a B+ choke on a valve amp?
I have read that chokes have an air gap in the core.
If not, why not?

[nickb]

You can use them but (1) only for low currents and (2) They might not the inductance you want (3) They tend to be heavier.

You can get a idea of the maximum current by measuring the peak primary current when hooked up to the normal supply voltage and with all secondaries unloaded. You can deduce the approx inductance as L = Vrms /Irms/2/3.14/f. f is 50 or 60Hz

[J M Fahey]

You NEED that gap, sorry.

IF you can pull EI laminations and reassemble, itīs easy, I cook them somewhat with a plumberīs torch to soften varnish and some sideways tapping with a small hammer loosens them; after you pulled the first couple, the rest is easy.
Then reassemble all E one way, close with a pack of I, insert a sheet of paper first between E and I mating surfaces.
Day and night.

The primary winding is then a good low current choke,think preamp or screens, not plate.