Sunn 300T Impedance Issue

[Sloom]

Hey folks,

I'm doing some reggae gigs this summer, and am using a (Fender) Sunn 300T bass amp.  There's an Impedance-Matching Switch on the back panel, and I have a question concerning the use of speaker cabs with un-matched ohm-loads.  I currently have a Schroeder 1210-R at 4 ohms, and an 8 ohm 215 cab (a modified Peavey 215 loaded with 2 BW 1505-4 ohm speakers wired in-series, so it's now an 8 ohm cab.  I got some help from a local on TalkBass with WinISD to re-vamp the Peavey- it used to be an RBS-1 215 sub- and I'm still in the tweak-stages with it, but it seems to work acceptably).

I'm considering pulling together some scrap resources here and building a 4 ohm 1x15 box, to go with the Schroeder instead of the Peavey 215  8 ohm cab.  That would be most ideal, as this here question could then go away!  But as long as this is where I'm at right now, I pose the question:  Can I run the two mis-matched cabs together safely?  I understand that a tube amp is 1) not particularly affected by the ohms load as concerns volume (like in a solid-state head) and 2) a tube amp tends to be more forgiving of things like impedance at the output.  Right?...

So, summarized:  If I were to use the 8 ohm cab from the Main Speaker-Out jack, and the 4 ohm cab from the Ext. Speaker-Out jack, with the Impedance Selector Switch set at say 2 ohms, would I be perfectly fine, and not needing to be concerned about losing my head (so to speak!)?  *Will there be a slight loss in output if I'm running a load of 2.67 (whatever!) and the switch is set to 2 ohms?  Can the switch be set to 4 ohms?  Bad idea?*

Thanks for your time! 

Quick note:  This amp has "Fender" all over it.  Is it really the same as a Sunn 300T?  What is the actual relationship with Fender and this head?

Thanks again!

[Isaac]

Some say that the actual load impedance should be lower than what the selector is set for, if you can't match it. Others say higher. Most agree that a mismatch of less than 100% is not likely to be an issue. I know that I've failed to check mine and run two 8 ohm cabinets (total 4 ohm load) on my Model T with the impedance switch set at 8 ohms and had no issues.

In your case, I'd probably go with 2 ohms. Others might prefer 4 ohms. "Will there be a slight loss in output if I'm running a load of 2.67 (whatever!) and the switch is set to 2 ohms?" Maybe. I doubt you'll notice it.

Impedance isn't resistance, and speakers aren't resistors. Impedance varies quite a bit with frequency. At the lowest, your two cabinets in parallel will probably get down to about 2 ohms. At higher frequencies, quite a bit higher.