4X12 cab ohm experts HELP please....

[Ozz666]

I've got a 4X12 mono cab here with Celestion G12H-100's in it that are all 8ohms...it's been wired for mono, can someone tell me what ohmage I should set my power amp to? Would the cab be an 8ohm cab?

Thanks,
- Joel

[ April 24, 2003, 06:09 PM: Message edited by: Joel Drabicki ]

[rodrigo_echeverri]

Re: 4X12 cab ohm experts HELP please....

I don't know what you have... there is no way to tell. I do know what you need though... this is the only way to go for a mono 8-ohm cabinet with 4 8-ohm speakers:



Open your connection panel and follow the wires a bit. They don't bite... [img]images/icons/grin.gif[/img]

Another way to tell is to use an ohmeter... you might have one and if you don't, they are like 20 bucks at radio shack.

I NEVER CONNECT ANYTHING TO MY POWER AMP BEFORE CHECKING THE IMPEDANCE WITH AN OHMETER.

Good times...

[ April 24, 2003, 06:55 PM: Message edited by: rodrigo_echeverri ]

[Ozz666]

Re: 4X12 cab ohm experts HELP please....

Thanks Rodrigo,
It's a halfback cab so it's half open so I can trace the wires and take the bottom panel off as well and see how they're run. I do have an ohm meter...what's the best way to hook it up...plug my speaker cable into the cab out then touch the -ohm wire to the - of the cable and the + to the + on the cable?

Thanks,
- Joel

[parntz145]

Re: 4X12 cab ohm experts HELP please....

Dude,You dont have to go through all that.If you are running 1 8 ohm cab you set the head to 8 ohms.The rule of thumb is if you are running either 1 4ohm cab you set the head to 4 ohms 1 8 ohm cab set the head to 8 ohms.Hope this helps you paul.You said the speakers are 8 ohms right?

[Ozz666]

Re: 4X12 cab ohm experts HELP please....

Alright so I just hooked my meter to the speaker cable from the cab and I'm getting 7.1 ohms...is that a little low...should I expect a loss like that through the wiring? The speaker cable is 6ft. If I wired the speakers with let's say a 14g wiring would I probably see a better output?
The person who wired the speakers originally used just crappy generic (looks like 20-22g) speaker wire.
Thanks,
- Joel

[Ozz666]

Re: 4X12 cab ohm experts HELP please....

Hey Paul,
That's the problem...I don't know what the cab is...do I assume it's an 8 ohm cab since all the speakers are 8 ohm? I wanna confirm that before I set the ohmage on the power amp.
I'm finally going true stereo!

- Joel

[rodrigo_echeverri]

Re: 4X12 cab ohm experts HELP please....

Originally posted by parntz145:
Dude,You dont have to go through all that.If you are running 1 8 ohm cab you set the head to 8 ohms.The rule of thumb is if you are running either 1 4ohm cab you set the head to 4 ohms 1 8 ohm cab set the head to 8 ohms.Hope this helps you paul.You said the speakers are 8 ohms right?

<font size="2" face="Verdana, Helvetica, sans-serif">Paul: but he was saying that he does not know what he has and that is why I anwswered the way I did.

Joel:

Measure this way: Connect a 1/4 cable to the input of the cab jsut like you do when you connect to your power amp. Measure right on the jack that would go into the power amp.

When you measure with the ohmeter make sure your fingertips are not touching the tips of the probes... that would alter the values of impedance you read. Also hold the tips firm on the contacts...

The wire adds impedance on the system but not that much... the difference is probably just in the winding of the speakers... it doesn't seem to be a big deal in your casethough.

7.1 ohms sounds close enough to 8 ohms to me... how good is your ohmeter?...

you can try it either in the 8 ohm output of the power amp and the 4 ohm one and see what tone you like better....

If you connect it to the 4 ohm one your tubes will work less and be safer but your tone will probably suck.

[ April 24, 2003, 07:30 PM: Message edited by: rodrigo_echeverri ]

[rodrigo_echeverri]

Re: 4X12 cab ohm experts HELP please....

what power amp are you using?... not that it matters much... Mesas take small impedance mismatches like that one with no problems at all...

[ April 24, 2003, 07:35 PM: Message edited by: rodrigo_echeverri ]

[rodrigo_echeverri]

Re: 4X12 cab ohm experts HELP please....

do you see sparks yet?.... [img]images/icons/smile.gif[/img]

[Ozz666]

Re: 4X12 cab ohm experts HELP please....

Hey Rodrigo,
Thanks for the help. Depending on how well the leads on the meter were in contact with the 1/4 jack I was getting between 7.1 and 7.4 ohms with my Craftsman multimeter. I've got a VHT 2150 power amp and this was my spare 4x12 Mesa half back (straight) that I had sitting in my basement that I'm finally gonna hook up for true stereo with my other half back (slant). So far the 8 ohm setting seems dead on for this cab and I'm definitely digging the Celestions! I'm gonna hafta try the 4 ohm setting tomorrow to see what kinda difference there is.

Thanks Rodrigo,
-Joel

[lerxstcat]

Re: 4X12 cab ohm experts HELP please....

I am no expert, but reading similar threads here I've read that it's best to
match impedance of the head and cab if you can. The worst is to run a head into a cab with lower impedance than it can handle. A mismatch in either direction can cause damage though.

If your amp can be set for 8 ohms and the cab is basically 8 ohms, I wouldn't fool around with other settings, I'd just go 8 to 8. I'd also check the other cab's impedance to be sure, unless you bought them new and know for a fact that no one
has ever disturbed the wiring. 4 8-ohm
speakers could be wired to as little as 2 ohms, which would blow up most guitar heads. I don't know about your power amp.

[Chuckracer]

Re: 4X12 cab ohm experts HELP please....

That's both true and false. Depending on the quality of the amps components, mainly output transformer, you will either blow it up or not hurt a thing. Higher quality stuff, Mesa and up basically, will not be hurt in the least by an ohmage mismatch.
In the REALLY high quality land, one amp maker didn't even label the switch positions with ohms numbers, instead with looser/tighter markings and encouraged you to expirement with mismatch settings as they do respond differently.
Now, if you try this with your JCM900 or Blue Voodoo or whatever, don't come cryin' to me if it smokes the output tranny by playing with mismatches, but in general, as long as the load is higher than the amp thinks it is, you won't hurt a thing. Lower loads will cook your tubes faster.

[rodrigo_echeverri]

Re: 4X12 cab ohm experts HELP please....

if you connect a cabinet with higher impedance (lower load) into you amp then the current through the tubes goes down. That means they will not cook them by any means. However, the tubes react dynamically to these things and they seem to like a resonably high load applied to them... like I said in the other thread, I talked to the guy at Mesa and he told me the higher impedance in the cabinet will just color the sound differently... i.e. more bass and less high end... he encouraged me to rey it on me setup!...

As for the type of impedance mismatch that Joel has, since the impedance is on the low side, it is something to be a bit more careful with... but hey, he has an almighty VHT... those things must be able to take a small mismatch like that... they might even sound better!... (tube life might be reduced slightly)

Bottom line: He can do his little experiment connecting his cab to the 4 ohm and nothing bad will happen at all for sure...

If I was Joel I would call VHT to see what their specification in impedance mismatch on the low side is but I am sure he is within specification there.. doesn't seem like a big deal to me.

[ April 25, 2003, 05:35 AM: Message edited by: rodrigo_echeverri ]

[Denis]

Re: 4X12 cab ohm experts HELP please....

I just bought a randall head with an 8ohm cab, would I connect the cab to the 4ohm jack on the head?

[Ozz666]

Re: 4X12 cab ohm experts HELP please....

Denis,
If you only have 1 8ohm cab then you need to set the head to 8ohms...if you are using 2 8 ohm cabs then you would set the amp head to 4 ohms.

Lex, Chuck, Rodrigo...thank you for the help...I know I can always learn alot here from you guys! I've never been much of an electronics guy so impedence, voltage etc. aren't my cup o tea. I appreciate all the excellent information...I'm fairly positive the other Mesa cab I've been running hasn't been touched (that one is 4 ohm stereo or 8 ohm mono) but I think it may be a good idea just to check the impedence on it now.

Thanks guys!

- Joel