So I've got a tube head rated at 5 Ohms...

Running at 4 would be better than 8 or 16. On a tube amp you want as close as possible to the tap impedance. Best would be a 5.3ohm cab, i.e. 610 with 8ohm speakers in three paralleled series pairs, or a 310 with 16ohm speakers paralleled.

 

It'll be fine with a 4 ohm cab.
The impedence rating of a cab (or driver) is only an approximation. The actual impedence varies with frequency.
The same goes for amps.

 

Running it at 4 ohms probably wouldn't hurt it right away, long term potentially. I believe in order to change the output impedance, you'd need to change the output transformer. As said above, 5.3 would be your easiest route if you need to throw something together.

 

Pretty expensive. Take it to a tech.
You might be ok running 4ohms. Some amps can take it, some can't. I wouldn't risk it.

 

You'll be fine running the 5 ohm tap into a 4 ohm cab. Tube amps can handle that small a mismatch.

Chris

 

By the way, you can get your 5.3ohms running an 8ohm cab and a 16ohm cab in parallel.
If you don't have a 16ohm cab, take a 4ohm cab to a tech.
The tech can wire it (Or at least should be able to wire it) to switch between 4 ohms and 16 ohms. You can do this either with a jack for each impedance, or a DPDT switch.

 

Bummer!
Thing is you already have the hard-to-come-by bit. Grab an 8ohm Avatar for chump change and you're set, or just go on CL and grab the first cab you see for under $100.
It's the most common impedance by far, you should be able to get one pretty easily.

 

AMIS that I have seen use 6550's they are pretty stout tubes, they should not have any problem with the reduced plate load of 4 ohms. Going 8 ohms is a little chancey, 16 ohms could spell disaster. You may have other taps on the OT you don't know about?

 

You could always get a 4Ω cab and run it in series with 8 x 8Ω cabs wired in parallel.

 

Transformer amps are backwards, running at higher ohms than rated on the tap reflects back to the output devices and causes damage. Some amps amps actually have output jacks that short when no load is connected as this is much safer than no load. Check out the schematics
6 x 8 = 5.33ohms
or 3x16 = 5.33 ohms
You could wire 12 x 4 ohms to get 5.33 ohms
http://www.google.com/search?q=1/((1/(4+4+4+4))+(1/(4+4+4+4))+(1/(4+4+4+4)))&aq=f&oq=&aqi=

4 ohms would be OK but why mess with it

 

With tubes the rated impedance load is the recommended maximum, as oppposed to SS, where it's the recommended minimum. Anywhere between 2.7 and 6 ohms should be just fine.

 

I think with tube amp output impedance, it is better to think of it as a center average? Very little is absolute in audio amplification (related to impedance loading) and tube amp have the OT (Output Transformer) as a buffer to the output devices. SS requires a little more care in matching "rated" impedance as there is no buffer between the output devices and speakers.