SOT: Cabk-toback transformers

[brett]

Hi.
I've used the technique of using step-down transformers to convert mains to a suitable voltage for a small valve amp (the Firefly).  

Two 240:12V transformers gave me 220V out from 240V in, and saved me $100 compared with a Hammond transformer ($10 each vs $120 for a Hammond 369AX).

Now I'm thinking of building a simplified 18W plexi-style amp.  The Hammond transformer (369EX) costs $147 here, so I was again thinking of using 2 cheap low-voltage transformers back-to-back.  In this case using a 240V:15V then a 9V:240V to get from 240V to around 380V.  

My only concerns are;
1. the lack of efficiency.  Can I assume 80% power efficiency, or thereabouts, in each transformer?  The voltage transfer through the 2 transformers in my firefly is about 80%.  So I'd need transformers rated at 25VA/(0.8x0. = 39VA ??
2.  any quality issues.  All the valve amp transformers seem to have black metal cans around them.  Is this useful or essential?

thanks for any help

[Peter Snowberg]

15 volts into a 9 volt winding = blue smoke  :shock:

[Regan]

Your overspecing the second transformer by injecting 15v into the 9v input. This is a no no.  You would be taking a huge chance of melting something. If the transformer is a 9v input, don't put more than 9v into it.
Regan

[Paul Perry (Frostwave)]

Yeah, the other problem is that the high voltage winding on the 2nd transformer is guaranteed for 230v , not for 380V.
Maybe you can find some old valve radios to hack??

[brett]

Your overspecing the second transformer by injecting 15v into the 9v input. This is a no no. You would be taking a huge chance of melting something.

Ok...Now I can see that if I put a *lot* more voltage into the secondary of the second transformer, I'm gunna over-rate the primary.  Oops!  But hey, I've just realised that what I'm shooting for is 360 to 380 V *DC* for the B+.  So I really only need 240V-260V out of the second transformer, so I can stay within or close to specs (Hey, my house gets up to 260V supply!!).  

I'm looking at both transformers being rated at 60VA, even though the maximum power draw is only about 25VA.  ie the secondaries of the transformers will be rated for 4 to 5 amps at the relevant voltage, but they will run at a maximum of 1.5 to 2A.

So will it work???  I'm still concerned that it will lack the shielding (??) that "proper" transformers have.

[Paul Perry (Frostwave)]

Sure, don't worry about that sheilding.