Name:    Steve Kennedy
Email:   steve@pacair.com  
Subject: Hi/Lo Power settings
Thread:  113
Time:    Tue, 17 Jul 2001 00:39:03 UTC

Depending on the age of your amp you might notice several different things.


On the later amplifiers (with the solid-state phase splitter) the Hi-Lo switch simply drops the B+ voltage to the plate of the output tubes by 1/2. This is done by selecting a different secondary winding on the power transformer.


High power is about 700Vdc plate voltage, low power around 350-400Vdc.


At low levels, you won't notice any difference because you are not driving the amp hard enough. If you play it REAL loud, you will notice that the output tubes will distort earlier when in the low power mode.


Unless I need high power, I always run my MM amps in the Low Power position. This puts less strain on the output tubes, components connected to the plate circuit and the output transformer primary windings.


The older MM amps with the 12AX7 phase splitter circuit uses a different circuit. The Hi-Lo switch actually changes the PRIMARY AC winding tap of the power transformer. This has the effect of not only reducing the B+ voltage to the output tubes, but can also reduce the low voltage DC supply to the solid state preamp (under low line conditions). Because this is undesirable, they changed it to the switched secondary tap in later models.


Steve