IMPORTANT! 2x8 ohms in parallel = 4 ohms

NickS · Post by **NickS** » Mon Feb 04, 2008 9:14 am

One of the Sunday night jam crowd turned up last week with a vintage Marshall 100 top that he hadn't used in 2 years instead of his usual Mesa Boogie Dual Rectifier. Last night the Mesa was back from the workshop and the repair bill was in.

He'd been using it for 2 years with 2x8 ohm cabs, on 8 ohms output. WRONG!! Difficult to say what went first, but all four output valves had gone, the output transformer was shorted and the overheated valves had baked and warped the PCB that sits underneath them - a couple of diodes had also gone. Replacing that lot = Â£422. Ow.

Sloan · Post by **Sloan** » Mon Feb 04, 2008 9:21 am

what an asshole

Post by **Mike** » Mon Feb 04, 2008 9:22 am

Oh dear. That's some schoolboy shit.

The worst part is that the Mesa manuals are really good and make that very clear.

Gavin · Post by **Gavin** » Mon Feb 04, 2008 9:25 am

NickS · Post by **NickS** » Mon Feb 04, 2008 9:35 am

Sloan wrote:What an asshole

Expensive lesson, but I think he can afford it. He came back from New York at New Year with a Zak Wylde Les Paul and told me his kids "bought about Â£1000 worth of clothes" while they were over there. Unfortunately, although Zak Wylde had signed the control access plate, he's played the guitar so much since that the signature wore off before he could get a replacement access plate.

Sloan · Post by **Sloan** » Mon Feb 04, 2008 9:41 am

what a cocking asshole

he could have sold the WYLDE STAYLKION GUITAR for milionz.

Post by **Mike** » Mon Feb 04, 2008 9:42 am

Zakk Wylde sucks so bad.

kim · Post by **kim** » Mon Feb 04, 2008 12:57 pm

pffhaaahaa.. he almost had it right

.. i checked the manual online, it's so very clear about this

most guitar amps don't take 4ohms but i think it's cool mesa's do most of the time.. i think the dual rectifier does..more guitar amps should do that

it's cool if you're using bass cabs too since those are max 8ohm most of the time so you can use two if you want to....what a schmuck though

euan · Post by **euan** » Mon Feb 04, 2008 12:59 pm

To 8ohm cabs so he should have used the 4ohm outputs?

kim · Post by **kim** » Mon Feb 04, 2008 1:00 pm

euan · Post by **euan** » Mon Feb 04, 2008 1:00 pm

Stop posting so fast that by the time the pages refreshed you've replied. Its creeps me out.

euan · Post by **euan** » Mon Feb 04, 2008 1:02 pm

Mike should delete his comment to be courteous to Kim.

Be a gentleman for once god dammit.

kim · Post by **kim** » Mon Feb 04, 2008 1:56 pm

wtf no way, that'd be shit

anyway.. hopefully i'll be a little bit less stupid than this dude and connect the cab the the right speaker out later on so i don't blow stuff up, fuck taxipost being a bitch again, i'm gonna go pick something up ...

Post by **Mike** » Mon Feb 04, 2008 1:56 pm

wtf are going on about, euan. You're crazy.

euan · Post by **euan** » Mon Feb 04, 2008 2:09 pm

NickS · Post by **NickS** » Mon Feb 04, 2008 3:41 pm

euan wrote:To 8ohm cabs so he should have used the 4ohm outputs?

From Kim's pic I can't say I'd have understood that without RTFM.

Post by **Mike** » Mon Feb 04, 2008 3:48 pm

Most guitarists don't know anything about impedance, let alone impedances in parallel.

analogsystem · Post by **analogsystem** » Mon Feb 04, 2008 5:47 pm

You'd think people could at least read....but no.

I told the guitarist from the other band we share a practice space with that he could use my JCM800 head for a couple rehearsals because the shitty solid state combo he usually uses was broken. I told him to make sure to always set the head to 16 ohm and then use the 16ohm input on my marshall 4x12.

Well I didn't realize he was still using it a few weeks later and I was rockin' my 800 on my other little 2x12 cab so I had set the Marshall to 8ohm. He took the head and put it back on my marhsall cab and plugged into the 16ohm input!

I, of course wasn't there but I figured it out later when I saw that the head had been moved and was on the 4x12 instead of where I'd left it on my 2x12!!! He told me they had like a 4 hour practice too!

There doesn't seem to be any damage to my head, but I couldn't believe how dense this dude could be. There is a selector on the head....make it match the input! It's not like he was using 2 cabs or anything.

NickS · Post by **NickS** » Mon Feb 04, 2008 9:12 pm

analogsystem wrote: I had set the Marshall to 8ohm. He took the head and put it back on my marhsall cab and plugged into the 16ohm input!

I, of course wasn't there but I figured it out later when I saw that the head had been moved and was on the 4x12 instead of where I'd left it on my 2x12!!! He told me they had like a 4 hour practice too!

There doesn't seem to be any damage to my head, but I couldn't believe how dense this dude could be. There is a selector on the head....make it match the input! It's not like he was using 2 cabs or anything.

That's not so bad, you won't get damage from plugging a 16 ohm cab into an 8 ohm output on a valve amp, you'll just get less power out. Plugging 4 ohms into an 8 or 16 ohm output, on the other hand, draws too much current and, in the long term, willl lead to roasting. If you can't make it match an output (like you have a 6 ohm cabinet), go for the next lower output (like 4 ohms). It doesn't hurt to give him a verbal beat-up on the basis that he could do it the other way next time.

Solid state amps are basically voltage amplifiers and with no matching transformer on the output they'll drive anything you plug in down to a minimum impedance (usually 4 ohms, though weirdly some WEM amps used to have "6 ohm" outputs). Max. power is usually into the lowest impedance.

Post by **Mike** » Tue Feb 05, 2008 9:21 am

Marshall transformers can handle a 16 -> 8 mismatch.