Valve amp output transformer Question.

[RODNEY]

Does connecting a 4ohm speaker to an 8ohm speaker socket have any effect on the output transforms' primary impedance so as to change current going through the output tubes? A video on youtube implies that the primary impedance does change because of the "turns ration" of primary and secondary but there is no physical (electrical) connection between the two.

[Mick Bailey]

There is a connection between the primary and secondary through electromagnetic coupling. A transformer's primary impedance is determined by the square of the turns ratio and the load on the secondary. When you change the load impedance on the secondary this is reflected by change in the primary impedance.

Because a valve amp power section needs to 'see' a primary impedance that's optimized for the specific valve type and configuration, any change to that impedance can reduce the efficiency of the power amp and therefore ultimately reduce the output power.

[eschertron]

And - not in all cases, but where the load lines are already marginal - changing the reflected impedance on the tubes to a lower value can push the operating curve of the tubes into melt-down territory. So if you choose to put a 4R speaker on a 8R connection, you may want to visually monitor the condition of the tubes as you "put the spurs" to them.