Tube amp smells like something's burning/melting?

__HM__ · #1 03-19-2011, 12:13 PM

I just got a Marshall JCM2000 guitar amp, it sounds great (Although it desperately needs a new set of preamp tubes). It was switched to run at 8 ohms, instead of the recommended 16 ohms, when I got it. (Its a 2x12 combo amp, the 16 ohms is recommended for the internal speakers. I dont have a cab hooked to it.)

So an hour or so ago, I cranked it. Everything maxed. After a minute or 2, I start to smell burning, so I turn it down, put it on standby, check the back, nothings in flames, theres no visible smoke. (Its switched to the not-recommended 8 ohms at this point, btw)

So I figured "***?!"

I took the back off to investigate, when something caught my eye. I switched it from 8 ohms to the recommended 16. I cranked it again, and the smell is gone.. What the heck? Im kind of scratching my head here. What do ohms have to do with the amp burning? I assumed I had a bum tube or something was wired poorly.

Any input? Comments? Thanks!

JimmyM · #2 03-19-2011, 01:08 PM

keep it at 16 ohms?

T-Bird · #3 03-19-2011, 01:10 PM

Hi.

This is a joke? Right?

Regards
Sam

GrowlerBox · #4 03-19-2011, 01:10 PM

Quote:

Originally Posted by JimmyM

keep it at 16 ohms?

Yeah -- the impedance is too low, there's too much current passing through the power section, and it's overheating. It's only rocket science, dude!

__HM__ · #5 03-19-2011, 01:16 PM

Its not a joke, and that kind of hurt.. haha. I don't know anything about tube amps, other than how to change the tubes, bias 'em, and how to operate em. I dont really know anything about ohms, or impedance, or any of that mumbo jumbo.

I spose Ill keep it at 16 then!

__HM__ · #6 03-19-2011, 01:18 PM

Quote:

Originally Posted by GrowlerBox

Yeah -- the impedance is too low, there's too much current passing through the power section, and it's overheating. It's only rocket science, dude!

Thank you very kindly! haha. Go easy on my guys, I'm a guitard.

GrowlerBox · #7 03-19-2011, 01:21 PM

Quote:

Originally Posted by __HM__

Thank you very kindly! haha. Go easy on my guys, I'm a guitard.

That's cool, man. Interesting that a guitarist (I don't like guitard -- some of my best friends are guitarists!) should come to TalkBass for "technical" advice. Word!

BurningSkies · #8 03-19-2011, 01:28 PM

In reply, "ohms" (actually known as impedance) has everything to do with your amp burning up.

Please match the speaker load to what your amp is set to see, otherwise you could do serious damage to your amp or kill it outright by trashing your output transformer. Impedance is the 'load' your amp sees, and depending upon the amp, they're pretty finicky about seeing too much or two little.

In layman's terms, think of the impedance as how hard your amp has to work to 'push' power through the speakers. Too much or too little can be a problem.

__HM__ · #9 03-19-2011, 01:30 PM

Quote:

Originally Posted by GrowlerBox

That's cool, man. Interesting that a guitarist (I don't like guitard -- some of my best friends are guitarists!) should come to TalkBass for "technical" advice. Word!

Ha, I just like the community here better, as opposed to TDPRI or some of the other guitar forums. Besides, I rock a pretty mean 4 string, too

BurningSkies · #10 03-19-2011, 01:30 PM

So, really what we should suggest is that you take some minutes and review some of the info found here:

Ohms FAQ

You'll find much that will make things clear, help you in the future, get to know your gear way better and operate it correctly and more efficiently AND you be able to amaze and impress your bandmates with your technical prowess.

BurningSkies · #11 03-19-2011, 01:33 PM

Quote:

Originally Posted by __HM__

Ha, I just like the community here better, as opposed to TDPRI or some of the other guitar forums. Besides, I rock a pretty mean 4 string, too

The TDPRI are cool cats too. Some of us also are members there.

Bass players just tend to be a bit more tech minded than a lot of guitar players... so you've come to a good place for answers and many answers apply to both guitar and bass.

__HM__ · #12 03-19-2011, 01:34 PM

Quote:

Originally Posted by BurningSkies

In reply, "ohms" (actually known as impedance) has everything to do with your amp burning up.

Please match the speaker load to what your amp is set to see, otherwise you could do serious damage to your amp or kill it outright by trashing your output transformer. Impedance is the 'load' your amp sees, and depending upon the amp, they're pretty finicky about seeing too much or two little.

In layman's terms, think of the impedance as how hard your amp has to work to 'push' power through the speakers. Too much or too little can be a problem.

That was ridiculously helpful. Its just 4th grade math.. lol. Two 8 ohm speakers add up to 16, run the amp at 16 ohms!

(I know, I know! I'm brilliant.)

Thanks man

__HM__ · #13 03-19-2011, 01:38 PM

Gah! I cant keep up with all these posts. lol.

Im a member of TDPRI also, I just prefer TB for just that very reason you mentioned. Bassists know gear. I've never met a serious bass player who doesn't know his stuff.

I know guitars inside and out, but amps are totally new territory for me. So Im open to all the help I can get. Thanks for the link to that FAQ!

GrowlerBox · #14 03-19-2011, 01:39 PM

Quote:

Originally Posted by __HM__

That was ridiculously helpful. Its just 4th grade math.. lol. Two 8 ohm speakers add up to 16, run the amp at 16 ohms!

If they're wired in series, yes. Or it's 2 x 32 ohm speakers in parallel. Inside that cab, you're probably correct, but it's important to note that the jacks to external cabinets are virtually always wired in parallel, so that adding speakers lowers impedance, potentially causing you wind up with similar problems.

s_mcsleazy · #15 03-19-2011, 01:42 PM

my bassman smells like a stoners flat. good reason for that though. the previous owner told me he once left a bag of green in the amp while it was running

__HM__ · #16 03-19-2011, 01:43 PM

It is 2 8ohm speakers wired in parallel in this case, I know, I swapped out the speakers last night. lol. As for adding a cab and lowering the impedance, would you care to elaborate on that? How does wiring another cab in parallel lower impedance?

KramerBassFan · #17 03-19-2011, 01:48 PM

Both of my old SUNN amps have very strange smells to them.

GrowlerBox · #18 03-19-2011, 01:50 PM

Quote:

Originally Posted by __HM__

It is 2 8ohm speakers wired in parallel in this case, I know, I swapped out the speakers last night. lol. As for adding a cab and lowering the impedance, would you care to elaborate on that? How does wiring another cab in parallel lower impedance?

For parallel connections, total impedance (R) = 1/[(1/R1) + (1/R2) + ...]

If you have 2 x 8 ohm speakers wired in parallel, the impedance is 1/[(1/8) + (1/8)] = 4 ohms, not 16. The impedances are added when wired in series, which this would be if it is indeed 16 ohms total.

The same principle applies when you're adding cabs, just substitute the total cabinet impedances for R1, R2, etc.

__HM__ · #19 03-19-2011, 01:54 PM

Ah, we're up to some 7th grade math, I see.

Thanks a lot man. Im gonna go ahead and bookmark this thread.

BurningSkies · #20 03-19-2011, 01:55 PM

Quote:

Originally Posted by __HM__

It is 2 8ohm speakers wired in parallel in this case, I know, I swapped out the speakers last night. lol. As for adding a cab and lowering the impedance, would you care to elaborate on that? How does wiring another cab in parallel lower impedance?

Two 8 ohm speakers in parallel is actually a 4ohm load. Remember, resistance in parallel will be LESS and resistance in series will be MORE. So if you wire in series your 2 8 ohm speakers present your amp a 16 ohm load.

There are some nifty formulas out there to show how this all works, but there are also some quick and dirty calculators online that will do the math if you'd rather.

Tube amps are usually OK with a bit of a mismatch...within reason. In this case, your amp found that seeing a 4ohm load instead of a 16 ohm load is more than 'within reason'. You sort of told your amp it would have to push using a certain amount of effort, but then gave it waaaay less to push against and your amp didn't like it.