6SN7 load resistance

[boroman]

How that?

If I put 4ohm resistor with 4ohm speaker to get 8 ohm (and double primary impedance then), I will loose a lot ot of useful volume

[boroman]

OTs don't have an own fixed primary impedance.
Rather they have an impedance transfer ratio.

A 10k/8Ohm OT will provide a 20k primary impedance with a 16 Ohm load.

And yes, it is possible to combine 2 OTs, but there might be drawbacks.
The Marshall JTM45/!00 used 2 JTM45 OTs with paralled primaries.

Please specify desired primary impedance, load impedance and impedance ratios of your OTs.

Allright. I'll look what I have here tonight :-) Maybe I can do something useful with those with low ratio....

[Enzo]

The transformer does not have an impedance of its own, it reflects the speaker load back to the plates. SO if you change the load from 8 ohms to 4 or vice versa, that changes the impedance the plates see. Thus it is a way to avoid using two transformers or other similar complicated tricks.

[Helmholtz]

If I put 4ohm resistor with 4ohm speaker to get 8 ohm (and double primary impedance then), I will loose a lot ot of useful volume

I thought this was about a different speaker impedance.