#1
Hi!

UPDATE - Today i tried to switch the amp on again and the tube in question lit right up again and blew up! I really don't know why this happened as this is only the 3rd time i've had it running since new. I hope it's covered on the warranty as i don't fancy paying £50 for a new one!

I just got myself a brand new Jet City JCA 100 head and decided to give it a spin. I don't have a speaker cab but i have a 100 watt marshall MG combo which I plugged it into. The marshall said 4 ohms on the back, so this is what output i used on my JCA amp.

I used it earlier like this and it was fine, played it for about an hour on about 1 volume (I've never owned a tube amp and didn't want to break anything!)

I just fired her up again, letting her warm up on standby mode for a about 5 minutes. I turned standby off and began playing, again on a level 1 volume but i noticed a not so nice burning smell, along with a weird crackling noise coming from the back. I looked at the tubes and one of them had lit up absolutely white hot so i turned my guitar off, hit standby to let them cool down and then turned it off.

I have no idea what happened or what this means and i'm playing a gig Tuesday, will it be ok to use then? As i said previously i have no real knowledge of tube amps as i have never owned one, but this surely doesn't look like a good sign! Could one of the tubes be dodgy?

Thanks in advance for any help!

James
Last edited by Trevascus at Dec 4, 2011,
#2
dude on the mg combo i'm SURE is an output not input...

but hey, i might well be wrong...

if you're that concerned, take it to a guitar tech or shop and ask them to check it out.
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#4
Please tell me that you didn't plug the head into the input of the mg? Unless your mg has a input jack for your speakers which I doubt, then you probably did. You may have even damaged your output transformer.
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#5
Nooo hahaha! My MG has an input jack on the back to allow the speaker to plug into a separate amplifier! Otherwise how would i have got sound without a speaker!? Yeah it was working fine at first and was sounding good and i still get sound from it now, it just lit up really bright and was really hot, smelled pretty tasty too!
#7
No i used the cable, which is a cable running from the speaker, and plugged that into the back of the JCA head. I didn't plug anything into that red hole!
#8
Ok. Now, where you read 4 ohms, was it on one speaker or on the amp somewhere?
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#9
what's the model of your marshall?
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#10
It says on the back of the amp, '100 watts output into 4 ohms'. The JCA has a 4, 8 and 16 ohm output on the back, so i plugged it into the 4 ohms to match it up. And Dempsey, the model is a Marshall MG100 DFX.
#11
Well, I'm stumped to be honest. As long as the head output ohms matches the speaker ohms, then it should run fine. I'd say wait until someone with more knowledge comes along, hopefully its nothing too serious.
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#12


so you plugged the cable coming out of the speaker into your JCM head... i get it now.

should be fine if you plugged it into the 4ohm output of your JCM

sorry, can't help you from here, all i can reccomend if you check the t00bz are in properly and if so, take it back the shop and get someone to check over it.
Belief is a beautiful armour but makes for the heaviest sword.
#13
If you did everything correct hookup wise, then i'd suspect a faulty stock tube. I bought a Bugera once...brought it home, jammed for a bit, decided to turn it up some(not cranked) and instantly the Power Tubes lit up bright, then brighter, and then they lit up like a frakken Light Saber and popped!

Fuse Blew, and new tubes were needed...new fuse too of course...maybe you got away with just tubes going bad and not Blown...?
#14
Thanks for the help guys! It's strange cause i also used it to play a show a couple weeks ago, cranked about half way through a 4x12 and it was absolutely fine! I'll see how it goes on Tuesday and go from there, i'll keep an eye on it while i'm playing. If not i'll have to take it to my local guitar shop, or maybe get a new amp on the warranty! I'll keep you guys posted!
#15
From what I read, TS plugged the output of his JCA into the output of his Marshall.........what are you guys reading??????
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#16
yeah if you took the lead/cable coming off the speaker and plugged it into the 4 ohm extension speaker socket on the jet city head (assuming the mg's speaker is 4 ohms) that should be ok. mightn't sound amazing, but shouldn't do any harm either.

maybe you just got unlucky with a dodgy valve or something.
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#17
Quote by JKHC
From what I read, TS plugged the output of his JCA into the output of his Marshall.........what are you guys reading??????


He explained in a few replies i think, that he basically took the speaker lead cable from the MG and connected it to the head...which should work aslong as the 4 ohms is matched with 4 ohms on the head.
#18
Yep walt is correct. I don't want to try it on a louder volume on a bigger cabinet and it go boom! This tube business is a bit scary for noobs! haha
#19
did you buy it in a store or online?

i'd say take it to the store and show them what's happening with it plugged into a cab.
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#20
I bought it from eBay but it was brand new, from a company selling them half price! It came with a year's warranty and they said they were a main importer of Jet City amps for the UK. Thinking about it, i hope i haven't been done over with a dodgy amp!
#21
Quote by Trevascus
I bought it from eBay but it was brand new, from a company selling them half price! It came with a year's warranty and they said they were a main importer of Jet City amps for the UK. Thinking about it, i hope i haven't been done over with a dodgy amp!


if you return it or have it serviced, even though it shouldnt have hurt it, i would keep the Jet City + MG ordeal out of any conversations with the company etc...dont even mention it on paper or email. They might accuse you of damaging it even though it probably wasnt your fault.
#22
half price


sounds like you had a power tube red-plate and die. The smell is probably the smell of burnt electronics like screen grid resistors, wire insulation and PCB.

return it under your warranty and make sure all the tubes are pushed in real good on the next one. good luck.
#23
You should always have spare tubes. If you had some you could have tested the one you suspect is faulty by now and you wouldn't be here. With new amps you should always try pulling out the tubes and plugging them back in, they can rattle loose in transit.
My guess is that it came with a dodgey tube. Contact the supplier, that's what warranties are for. If a simple tube swap doesn't fix the problem then it's time to take advantage of the warranty.
Which tube is suspect btw?
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#24
Check the bad tube for cracks. Tubes often burn white when they lose their vacuum.
#25
oh if you got it new, yeah, time to make use of teh warranty, i think. no point in trying anything else in case that voids the warranty.
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Quote by K33nbl4d3
I'll have to put the Classic T models on my to-try list. Shame the finish options there are Anachronism Gold, Nuclear Waste and Aged Clown, because in principle the plaintop is right up my alley.

Quote by K33nbl4d3
Presumably because the CCF (Combined Corksniffing Forces) of MLP and Gibson forums would rise up against them, plunging the land into war.

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Et tu, br00tz?
#28
My bad, my computer went weird and didn't show the second page, sorry!
Last edited by Trevascus at Dec 5, 2011,
#29
well after reading your update, it sounds like a tube was going bad. Now it appears to have gone completely. The good news is that you can probably replace the tube that blew up, and be on your merry way. If you replace it, and still have problems, then you'll most likely have to either use the money you saved off the price and take it to a tech, or try to get a refund.

MOST of the time when a tube amp has issues, it's just a bad tube. Sometimes people buy a set of brand new tubes, and have one bad right from the factory. Don't assume that a bad tube in a new amp means you got a lemon. I had a similar issue right after I bought my mark IV, and I just about had a heart attack when the tube blew. Popped in a new set of glass, new fuse, and she's been good as new ever since.
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#30
Quote by Trevascus
Bump!

You bought it off source from ebay correct?
Just hit them a message and use your warranty
they are good peoplez
#31
Quote by JoePerry4life
Was it this?

http://www.elderly.com/new_instruments/items/images/130N/MG100DFX_back-detail-2.jpg

If so, that is not an input to the speakers, its an output from the power section of the amp.

Errr......what?


Anyway, it sounds like you did nothing wrong, just send it back under warranty. Don't mention anything about ohms etc - in the incredibly unlikely event they ask, just say it was plugged into a cab of the correct impedance.