24.18.227.0
In Reply to: RE: Ping Paul Joppa on BR for SE math posted by uW on May 04, 2009 at 13:48:48
It's simple enough. The electrical Q (usually called QE) is increased by the ratio of resistances:
new QE = old QE * (RE + Rout)/RE
where RE is the speaker's DC voice coil resistance and Rout is the amplifier's output resistance.
Rout can be closely estimated as the sum of the tube's plate resistance plus the output transformer's primary DC resistance (both scaled by the impedance ratio) plus the transformer's secondary DC resistance. It is usually between 1/4 and 1/2 of the nominal load impedance; I usually assume 3 ohms on an 8 ohm tap for a typical SET.
Once you know the new QE you can use any box design software.
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