# Help me with ohms!!!!



## JimiGuy7 (Jan 10, 2008)

Hi,

I have a problem, i have a early seventies Fender Bassman head. It runs at 8 ohms so when I plug a cabinet into it I have to make swure the speakers are 8 ohms. The problem I am having is I have 4 16 ohm speakers and I want to put them into my cabinet but I don't want to blow the head, which says use only 8 ohms speakers. What can I don to lower the ohms on the speakers to 8 ohms or lower so i don't hurt anything and it will all be full functional? Please help me, I have a gig coming up and the speakers I am using suck ass so I need to change them.

Thanks!


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## gtrguy (Jul 6, 2006)

This diagram will give you 4 Ohms- prevailing wisdom suggests this mismtch will be fine for the amp.


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## greco (Jul 15, 2007)

*Please wait for a tech/experienced person to confirm my thinking...*but I think that you would be OK if you just wired your 4 x 16 ohm speakers *in parallel and in pairs* and then the output of the pairs in parallel again.

It seems to look OK (to me) on a diagram....*but I am a beginner in electronics theory.*

gtrguy was posting as I was writing...essentially I was trying to explain (now that I read what I wrote...it isn't that easy to follow), what his diagram shows

Good luck with it

Dave


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## Wild Bill (May 3, 2006)

Do what gtrguy told you and you'll be fine!

With four 16 ohm speakers there is NO combination that will give 8 ohms total! You will end up with either 4 or 16 ohms for the combined load.

A tube amp like a Fender Bassman will work just fine on 4 ohms! The tone may slightly change, possibly for the better!

:food-smiley-004:


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## mhammer (Nov 30, 2007)

Note that the output will be divided amongst more speakers, such that each speaker will be effectively carrying/reproducing less of the signal.

Note as well that:
a) It takes roughly 10x the wattage, using the same speakers and input signal, to produce a doubling of audible level, so don't expect huge volume jumps when you drop from 8 to 4 ohms (although the cab required may well be more effecient and produce a volume bump of its own, but that's separate from wattage), and 
b) The usual rule of thumb that reducing the output load increases the available wattage is essentially a statement about how much current the circuit will draw. Of course, the amount it draws will only exceed the rated specs of the power and output transformer if you set things too high. While it would not sound all that great, you could run that head into a pair of 4ohm 5" speakers in parallel (each of them rated at 2W) with impunity, *IF* you kept the levels low enough that the output is not drawing too much current. As long as you resist the temptation to dime everythng, you should be fine.


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## nonreverb (Sep 19, 2006)

JimiGuy7 said:


> Hi,
> 
> I have a problem, i have a early seventies Fender Bassman head. It runs at 8 ohms so when I plug a cabinet into it I have to make swure the speakers are 8 ohms. The problem I am having is I have 4 16 ohm speakers and I want to put them into my cabinet but I don't want to blow the head, which says use only 8 ohms speakers. What can I don to lower the ohms on the speakers to 8 ohms or lower so i don't hurt anything and it will all be full functional? Please help me, I have a gig coming up and the speakers I am using suck ass so I need to change them.
> 
> Thanks!


Unless this amp has a replaced output transformer or is a modified Bassman 10 or 20, the output is 4 ohms not 8 ohms.


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## Ripper (Jul 1, 2006)

Are you sure your bassman is an 8 ohm out? Bassman amps traditionally are 4ohms for the main speaker out. You would use 8 ohm cabs if you were using two of them as the output jacks on those fenders are wired in parallel, giving you 4 ohms once again. If you find a schematic for a bassman you'll see that it is labelled for a 4 ohm total.


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## Wild Bill (May 3, 2006)

mhammer said:


> Note that the output will be divided amongst more speakers, such that each speaker will be effectively carrying/reproducing less of the signal.
> 
> Note as well that:
> a) It takes roughly 10x the wattage, using the same speakers and input signal, to produce a doubling of audible level, so don't expect huge volume jumps when you drop from 8 to 4 ohms (although the cab required may well be more effecient and produce a volume bump of its own, but that's separate from wattage), and
> b) The usual rule of thumb that reducing the output load increases the available wattage is essentially a statement about how much current the circuit will draw. Of course, the amount it draws will only exceed the rated specs of the power and output transformer if you set things too high. While it would not sound all that great, you could run that head into a pair of 4ohm 5" speakers in parallel (each of them rated at 2W) with impunity, *IF* you kept the levels low enough that the output is not drawing too much current. As long as you resist the temptation to dime everythng, you should be fine.


Mike, did you have your coffee yet?:smile:

Remember, this is a tube amp! Any tube data sheet will show that the plate load is far from linear with respect to power output. In fact, the power under such a mismatch might go up!

Now, if it were a solid state amp you'd be perfectly correct, as I'm sure you know. 

My brain used to work just fine, before the kids came!:smile:

:food-smiley-004:


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## mhammer (Nov 30, 2007)

I guess those damn cardinals singing in my window at 5AM today compromised my thinking. You're right, Bill. 

What IS true, though, and will remain so, is that it is always up to the amp-user to behave themselves when it comes to volume settings.


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## Wild Bill (May 3, 2006)

mhammer said:


> I guess those damn cardinals singing in my window at 5AM today compromised my thinking. You're right, Bill.
> 
> What IS true, though, and will remain so, is that it is always up to the amp-user to behave themselves when it comes to volume settings.


Well, again I have to disagree. I blame drummers!

In my whole life I can count on one hand the number of drummers that can play well soft as well as hard. Seems they all think they're John Bonham or something. 

:food-smiley-004:


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## Guest (May 26, 2008)

Also note that you could put a dead short on the output of your amplifier and your amp would survive, so going lower to 4 ohms isn't a problem since that amp could handle a short (not very musical, but it can) . With a tube amp what you must avoid at all cost is an open circuit when signal is driven through the amp. If a tube amp idles with an open circuit it'll be ok but not with a signal. This is the opposite of what would be expected from a solid state power amp.


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