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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.
So Paul,
What would be the numbers for the s.e.x. with the iron upgrade?
I've just been pondering designing a small set of desktop speakers to go with my amp.
-- Jim
The tricky part is usually the plate resistance. The GE data sheet has plots though (I think it's the last page). Figure 28mA and 350 volts - just use the 300v line, it's close enough.
It's easy enough to measure the transformer primary resistance. The secondary may be too low to measure accurately, but you can approximate it to be the same as the primary after correction (i.e. multiply the primary resistance by (8/8000).
(nt)
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