Burning Smell From Cab With 400+

[grue]

So, I picked up a used 8 Ohm Mesa PH212 a few months ago. Right away, I noticed a burning plastic smell when I was running fuzz. I assumed this was a crossover issue, so I took out the crossover (not a big tweeter guy anyway).

Problem is I still get the smell when using heavy fuzz or playing aggressively. My volume is loud enough for rock, but not extreme IMO. My guitarist and drummer agree that this is true.

The cab still sounds fine, but I'm sure it's a matter of time before it stops working or I poison myself and the audience with burning plastic fumes.

My guess is that either
A: the speakers were damaged when I got them and just need to be replaced.
B: there is a problem with my head or pedals.

I'd rather not drop a bunch of cash on new speakers if they just get fried again because there is an amp problem or whatever.

Anybody got any advice?

[BassmanPaul]

A tube amp cannot output DC like a SS amp can. My gut feeling is too few speakers for the volume you need. Perhaps a second cabinet will solve your problem.

[Downunderwonder]

Is the amp fan working?

It might just need a clean out.

Not running off a 4 ohm output? That'll make your tranny hot, not in a good way.

[Downunderwonder]

Bad connection at the cab?

[JimWasHere]

The cab is new, and you've had the head awhile? When you got the cab, did you find you had to turn the head up to get the same volume?

[grue]

OK, I'll try to answer everybody in one post.

Yeah, it's an 8 ohm cab plugged into the 8 ohm output.

I've had the head a while and used it with a bunch of different big and small cabs at both 4 and 8 ohms without experiencing this problem.

Fan works.

Haven't noticed any connection problems at the cab.

The cab was new to me, but not new.

It sounds like people think this might be the tranny. I hadn't thought about that. How do I rule that out?

[Mr. Foxen]

Seeing if the tranny is the source of the smell would be a good start.

[s_mcsleazy]

Originally Posted by Downunderwonder

Is the amp fan working?

It might just need a clean out.

Not running off a 4 ohm output? That'll make your tranny hot, not in a good way.

we all love a hot tranny

[T-Bird]

Originally Posted by grue

I'd rather not drop a bunch of cash on new speakers if they just get fried again because there is an amp problem or whatever.

Originally Posted by BassmanPaul

A tube amp cannot output DC like a SS amp can.

^This.

When people finally understand that the most common cause for mystery failure of speakers is DC component in the output, they automatically assume that's always the case. With a transformer coupled output section, a subtle amount of DC is virtually impossible. B+ in the speaker connection is obviously possible, but there's nothing subtle about that scenario.

If the amp would oscillate in sub-bass frequencies, You should see the cones move without any signal. If the oscillation occurs in the audible freuency range, You'd hear it.

If that's the only cab You have, and you want to be on the safe side, you can get the amp tested.

Originally Posted by Downunderwonder

Bad connection at the cab?

^This would be my first guess if the speakers really still sound good. Cooked voice coil doesn't produce too pleasurable sound.

I Don't know what jacks Mesa uses, but I've replaced many vintage tube amp output jacks as well as Marshall cab jacks that have just melted as a result of increased resistance caused by oxidation.

While You're at it, it wouldn't be such a bad idea to upgrade to Speakons if possible.

Regards
Sam

[anderbass]

Are you sure your speaker cable isnt actually a guitar cord that's getting hot and melting?

Originally Posted by grue

I assumed this was a crossover issue, so I took out the crossover

Did your crossover smell burnt? Did you re-wire around the crossover or actually remove it. If you re-wired maybe the signal is still getting into the defective crossover.

Whatever's causing this should still retain a burnt smell even after you've stopped playing. You should be able to take the cab apart and sniff out the problem. Check all the cabs wiring and jacks for signs of heat too.

[grue]

Definitely using a real speaker cable, and I physically removed the crossover from the box.

I also replaced the jack in the cab when I took out the crossover. The smell was happening both before and after, so probably not a bad jack...

I guess I will have to do some sniffing. It will be a couple days, though. I just got back from tour and our gear is still stacked in a giant heap in the middle of our space.

[wcriley]

This probably doesn't apply in your case, but:

The first time I ran my Acme B-2 at decent volume, it gave off a burnt plastic/chemical odor through the ports. Exchanged emails with Andy Lewis and he couldn't explain it, other than the possibility that maybe some cab adhesive had dripped onto the crossover during assembly and was getting hot when I pushed the cab hard. Never happened again after the second time and I never figured out what had caused it.

[BassmanPaul]

Originally Posted by wcriley

This probably doesn't apply in your case, but:

The first time I ran my Acme B-2 at decent volume, it gave off a burnt plastic/chemical odor through the ports. Exchanged emails with Andy Lewis and he couldn't explain it, other than the possibility that maybe some cab adhesive had dripped onto the crossover during assembly and was getting hot when I pushed the cab hard. Never happened again after the second time and I never figured out what had caused it.

Never had this problem with any of my four. Mind you the lamp lighting scared the crap out of me when it first happened!

[BassmanPaul]

OP have you tried the battery test to make sure the drivers are correctly wired?

[BassmanPaul]

There seem to be a question whether the amp or the cab is producing the smell. Put the cabinet as far away from your amp and try to replicate the smell. That should tell you which is the smelly one.

Have you racked the amp? A dozen 6L6GCs produce a lot of heat and even more when you push it. If it's in a plastic rack case that could be a source of the smell.

[BassmanPaul]

I STILL feel that your speakers are not enough to meet your needs. Two twelves are not that much.

[wcriley]

Originally Posted by BassmanPaul

Never had this problem with any of my four. Mind you the lamp lighting scared the crap out of me when it first happened!

I'd have REALLY freaked out if I'd seen the bulb light up at the time I noticed the burnt/chemical odor!

Sorry for the OT.

[Bassmec]

Originally Posted by BassmanPaul

I STILL feel that your speakers are not enough to meet your needs. Two twelves are not that much.

These big tube amps driven into healthy sounding distortion, produce far more real voice coil smoking power than the 256 watts RMS clean rating of the 400+ might suggest.
In my experience increasing the number of loudspeakers
an amplifier drives is the only way of improving tone, power and reliability. Call me old fashioned!.
The 600 watts voice coil meltdown rating this 2 X 12 carries sounds about right, sadly mesa boogie have failed to mention a horrendous stench threshold at half this power.
Neither have mesa provided any displacement limited power figures (usually about half the stench of death rating), thats the ones that tell you how much power it will handle before it sounds like total arse.

[grue]

OK, here's where things stand in the tale of the stench:

I was starting to get a little paranoid and my head was starting to sound a little rough, so I took it to the local tube amp guru and had it checked out. He cleaned it, replaced a couple preamp tubes, and tested the hell out of it. It passed with flying colors.

While the head was in the shop, I used an Ampeg portaflex (300 watts @ 8 ohms, pure SS, class D, no punch) as a backup with the same cab. Still got some smell. When I got the Mesa back, the smell was very intense.

I figured the speakers were just damaged beyond use (even though they still sound ok). So tonight I decided to try a different cab while I figure out what to do with the 212. In this case, it was a GK Neo 410, rated to 800 watts RMS, 1600 peak. And you guessed it, it started making a nice chemical smell about halfway through practice. I backed off immediately, but now I'm really at the end of my rope. How much speaker could I possibly need for an overdriven 260 watt tube amp?

Some details about my setup in case they are relevant: I'm using an old Gibson Ripper with standard tuning. The bass has extremely low output, so I use a (fairly cranked) Markbass booster pedal in front of everything else to get it up to a usable level. Also, one of the pickups is a little funky, which gives the bass a trebly, slightly phased-out sound. I do compensate a bit with the tone knobs on the amp, but I still feel like my sound has more mids and highs, and fewer lows than a typical bass. I love the broken-pickup sound, so I would prefer not to fix it.

What do I do?

[Gazman]

Have you tried a different speaker cable? Is it possible your cable from the amp to the cabs is an instrument cable?

If you rule out the head and the cab, it's got to be the cable right? Or ask another bass player to see if it your playing that stinks!