Re: OPT equalising circuit

"Alex"
>
> I noticed that in most cases a capacitor or a RC circuit is connected
> across the primary of a tube OPT. It is apparently to compensate for
> the leakage inductance of the transformer.


** Partly for that  PLUS the fact that most speakers have a sharply rising

impedance curve at high and supersonic frequencies.   A nominal 8 ohm 
speaker at 250 Hz can easily be 100 ohms at 50 kHz.


> However, I am not sure how to correctly calculate this RC circuit.


** You are not alone ....


> If I know my load impedance Rl,


** No you don't.

You just know the  " nominal " impedance =  useless.

What you need to make sure of is the amp does not break  into HF 
oscillations ( parasitic or continuous)  when there is no load  OR  when 
under test with simulated ( or real ) speaker loads.

The vast majority of tube amps have no need of such a compensation 
etwork  -  as the OT has little leakage and the NFB reduces to low value
at 
supersonic frequencies.




.......   Phil