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OHMS

Longwheelbass

Longwheelbass

Sep 27, 2008
709
2,340
Devon UK
#1
I am ohmically challenged so I was wondering if any of you amp and speaker guys and girls out there could answer this question: If I have a tube head with a single 4 ohm output going to two 12" 8 ohm speakers wired in parallel to match the head's aforementioned impedance, and I wanted to add a 15" speaker to this set-up for more bassiness
A) is this possible?
B) what resistance rating (ohms) do I need for the 15?
and C) can it be hooked up with a series/parallel arrangement or if I got an 8 ohm 15" speaker can I just wire that in parallel with the two 12's? Would this still give me the required 4 ohm load?
Any assistance to solve this resistance will be greatly appreciated.
 
#4
mmbongo said:
Not possible. You're already at 4 ohms with 2 speakers, you can't add any more.
Click to expand...
Well, he could add a 15" 4 in series and get to 8 total, but then how to get back to 4? Add an 8 ohm 15" in parallel? Feasible, but easier just to buy a new cab, I would think.
 
#5
bholder said:
Well, he could add a 15" 4 in series and get to 8 total, but then how to get back to 4? Add an 8 ohm 15" in parallel? Feasible, but easier just to buy a new cab, I would think.
Click to expand...

Problem with that is that two of the speakers would get twice the power of the other two. It will cause more problems than it will fix.
 
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#7
The first problem I see is the premise that a 15 will add more ‘bassiness.’ It might but then again it might not. Speaker size has no connection to how much lows the driver can put out.

Tube amps can be very forgiving with respect to the load. That said it shouldn’t be taken for granted that a 4Ω tap will be able to push a 2.67Ω load (3x8Ω). Like as not it won’t blow up but the output tubes will have a much shorter life span.
 
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#8
This sounds like one of those math questions that you get in school.

Any chance that the amp has another speaker tap besides the 4 ohm?

It could work well and have the impedance matched if you had an 8 ohm tap and the right speakers.
 
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#9
Personally I see no problem to give a try for 2.7 Ohm nominal impedance.
Tube amplifiers are tolerant to some impedance "mismatch" especially if the nominal impedance of the cabinets is south of the amplifier output impedance.

If you dig in some "bass" with the EQ your cabinet impedance may rise beyond 2.7 Ohm "nominal" anyway. No surprise if (in total) you get 4 Ohm average in "practice" rather than 2.7 Ohm nominal impedance.
 
#13
So by reading this thread, am I correct in deducing that a head rated at 4 ohms 500 watts or 8 ohms 300 watts would be a perfect match to drive one cabinet rated at 8 ohms and 300 watts ?
 

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