flipper wrote:

What are the disadvantages to using an output load different than
specified to get a different primary impedance?

For example. Say you want a 5k primary to 8 ohm but have a 2k5 to 4
ohm. Impedance wise it matches to just use an 8 ohm speaker on the 2k5
to get 5K, so what is(are) the reason(s) not to?

The transformer is specified for a specific RLa-a to secondary because the
information includes
that the primary inductance and winding losses will suit the RLa-a.
So a tranny with 2.5k primary will have 1/2 the inductance of the one with
5ka-a.
LF Stability with NFB could be an issue.

The 2.5k : 4 tranny may have 0.7 times the primary turns of the 5k tranny
because inductance
varies with the square of P turns. Therefore the 2.5k tranny may have
lower winding losses
when used for 5k : 8 .
The above is only valid for where the trannies are rated for the same
power.

In practice the 2k5 : 4 may be fine.

The ARC VT100 has load matches form 1.6k : 4 and 8 ohms.
When 8 ohms is used on the 4 ohm tap, the load the 4 x 6550 tubes sees is
3.2k, so the tubes are loaded
with 6.4k per pair which is the better load for more class A and less
distortion and lower Rout.
8 ohm speakers should only ever be used with the 4 ohm outlet in this amp.

I measured the power with 8 ohms on the 4 ohm tap to be 80 watts which is
still plenty.




And vices versa. Say you want 2k5 to 4 but have a 5k to 8. Why not
just use the 5k with a 4 ohm load, for 2.5k primary?

Winding losses will be twice as high when a 5k : 8 OPT is subsituted for
2.5k : 4.
Also leakage inductance can make HF losses worse.

Don't do this.

Use the right load for the tubes.

Patrick Turner



Or is it ok?