Can I run a 16 ohm and a 4 ohm cab together?

Kiwi

Silver Supporting Member
Messages
4,081
Can I safely play a 16 ohm and a 4 ohm cab at once?

My amp math says (16+4)/4 equals 5 ohms, so set the transformer switch at 4 ohms.

The amps involved have two speaker jacks, can I assume they split the load evenly?

Any reason to not do this?

Kiwi
 

Slick51

Colonel Curmudgeon
Messages
522
George

My math says 3.2 ohms...

for the special case of two speakers the math collapses to

R1 x R2
------------ = 64/20 = 3.2 ohms.
R1 + R2


Still the 4 ohm tap, but closer to No-Go-sville, IMO.

and I seem to remember that the 4 ohm speaker will draw the lion's share of the current. I don't remember that math, so I'll stop here.

Slick51
 

John Phillips

Member
Messages
13,038
No reason not to do it.

The total impedance is 3.2 ohms, yes. That's completely safe from a 4-ohm output on any tube amp - the safe range is about half to double, ie 2 ohms being the lower limit... 3.2 is well above that (it's the proportion that matters, not the absolute value).

The power is divided in reverse proportion to the impedances, if the cabs are in parallel (with almost all amps they are) - so in this case 4/5 of the power goes to the 4-ohm cab and 1/5 to the 16. This is quite a big difference, but it probably won't be as noticeable as you'd think... depending on what, and how many, speakers are in each cab.


(You can actually use this unequal power distribution for attenuation, if you have the right combination of loads - a 16-ohm cab and a 4-ohm dummy load in parallel works well, and reduces the volume quite a lot - about 7dB.)
 

Slick51

Colonel Curmudgeon
Messages
522
Thanks, John...I've alwas operated on the 'never under 4 ohms' wife's tale; it's nice to hear the truth. I knew the math, but not the reasoning.

Ditto on the current draw...it's a straight ratio as well.

That's why I hang around here...someone will always come by with the answer!

Slick51
 

Kiwi

Silver Supporting Member
Messages
4,081
Thanks, JP and YoSlick! For stuff like this, I always ask here first.

GeoKiwi
 




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