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I'd like to build a pair of BR cabinets for FE207E to use exclusively with my parafeed 2A3 amp. In this post you talk about accounting for the higher Q that comes from using an SET amp. Can you talk me through the math? (You've probably explained this eleventy times elsewhere, so I apologize for overlooking those posts-- please throw me a link or a search string.) Thanks!
I've been following this thread and went back to look at the original posts regarding the speakers using the Fostex FE207E drivers and Parts Express enclosures/ports. I have absolutely no experience with speaker construction but this looked extremely doable for me. I want a pair of small speakers for my work area where I plan to put another set of 300B Paramounts. Are there any obvious pitfalls to avoid here? It seems that all I would need to do is make the cutouts for the drivers and ports.
Conrad
Paul,
Sorry, I only asked because I'm blind and can't work with the tube manuals. Should have made that clear.
-- Jim
Oh - I apologize. Nobody can tell on the internet!
Plate resistance from the data sheet at the SEX operating point is about 2600 ohms. I measured 258 ohms for the transformer primary, for a total 2858 ohms. Scaled by the impedance ratio of 8/8000 that's 2.858 ohms. Add the estimated secondary resistance of 0.258 ohms to get 3.116 ohms, or very close to my usual assumption of 3 ohms. So the electrical Q goes up by 11/8 or a factor of 1.375.
Hope that's more helpful!
Paul,
Yes, in some ways it's the great equalizer and other times it means that I can't assume people know and should remember to point it out. Maybe I need some sort of obvious avatar or something -- maybe an eyeless Hagar the Horrible with a white cane instead of a spear, or something like that :-).
Thanks much for the information -- I'll save it in my project file.
-- Jim
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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