Holy shitee What Have I Done?

[indysmith]

okay - i got new valves for the jcm800 - whacked em in to give them a test - switched amp on on standby, left for 5 mins, turned off standby, played for about 10 seconds, sounded great, and then all sound just cut out completely.
I hadn't rebaised for these valves (i don't have a clue how, and i don't know anyone who does), but i thought it'd be the valves that would be damaged if i didn't rebias it properly and it was running hot. I've changed back to the old valves and still no sound at all. have i buggered the transformers or something? this sucks so hard. it's gonna cost me a bomb to fix isn't it?

[indysmith]

All the transformers and valves and everything look the same - there's nothing visibly blown up or anything. ARGH why do i break everything!?

[Dakine]

deep breath, calm down
have you checked fuses?

[indysmith]

deep breath, calm down
have you checked fuses?

Thanks man; so how do i check fuses - where are they and why would they break?

[Twinfan]

Should be two fuse holders on the back - one for the mains input and one for the power valve circuit.  They'll be in glass tubes so you can see if they've broken or not.

A faulty valve would blow the power valve circuit fuse, which I suspect is your problem.  A fault elsewhere is more likely to blow the mains fuse.

[indysmith]

okay the fuse that isn't the mains fuse (i think) has blown (i think) it's got some black burnt looking marks on the inside of the glass (which is intact) that the other fuse doesn't. I'll post some pics ina second - but they'll be shitee quality as i can't find the real camera

[indysmith]

I really apolagize for the rubbish quality of the pictures.
Does this mean that i just need to send the valves back and replace the fuse?

[Dakine]

well the fuse def. looks blown.
Maybe replace fuse and put orig. tubes back in and test.
Probably a tube blew it but why is another matter.
Bit hard to diagnose on pc :)
Are they diff. tubes that could have been needing a bias?
Try replacing fuse and putting orig. tubes back I would say.

[indysmith]

well the fuse def. looks blown.
Maybe replace fuse and put orig. tubes back in and test.
Probably a tube blew it but why is another matter.
Bit hard to diagnose on pc :)
Are they diff. tubes that could have been needing a bias?
Try replacing fuse and putting orig. tubes back I would say.

yeah they would probably be needing a bias - i'm off to town to get another 500mA fuse - thanks for your quick and useful help guys. Be prepared for a thread asking how to bias a jcm800 4104 :P

[TwilightOdyssey]

For ANYONE buying a valve amp, you can save yourself a lot of trouble by adding the following to your library:

http://www.amazon.com/Tube-Amp-Book-Aspen-Pittman/dp/0879307676/sr=1-1/qid=1162822065/ref=sr_1_1/102-0602884-5084945?ie=UTF8&s=books

http://www.amazon.com/How-Service-Your-Tube-Amp/dp/0962817007/sr=1-2/qid=1162822065/ref=sr_1_2/102-0602884-5084945?ie=UTF8&s=books

http://www.amazon.com/History-Marshall-Guitar-Amplifiers-1962-1992/dp/0793525098/sr=1-1/qid=1162822552/ref=pd_bbs_sr_1/102-0602884-5084945?ie=UTF8&s=books

This will solve a lot of panicked posting!!

[indysmith]

For ANYONE buying a valve amp, you can save yourself a lot of trouble by adding the following to your library:

http://www.amazon.com/Tube-Amp-Book-Aspen-Pittman/dp/0879307676/sr=1-1/qid=1162822065/ref=sr_1_1/102-0602884-5084945?ie=UTF8&s=books

http://www.amazon.com/How-Service-Your-Tube-Amp/dp/0962817007/sr=1-2/qid=1162822065/ref=sr_1_2/102-0602884-5084945?ie=UTF8&s=books

http://www.amazon.com/History-Marshall-Guitar-Amplifiers-1962-1992/dp/0793525098/sr=1-1/qid=1162822552/ref=pd_bbs_sr_1/102-0602884-5084945?ie=UTF8&s=books

This will solve a lot of panicked posting!!

haha thanks Ben - i might just have to look into those.

I went to the hardware store and apparently 500mA fuses are quite hard to come buy - i'm going to have to order one off the internet. Do you guys think the fuse blew because i didn't rebias?

EDIT: bugger, that wasn't as easy as i thought it'd be - i can't find one of the right values and the right size on the internet. I've emailed marshall - i guess i'll have to take it to MK to be sorted.

[Dakine]

Surely a fuse can be sourced here?

http://www.maplin.co.uk/searchtemplate.asp?criteria=500MA%20FUSE

Also, maybe best to let a shop bias your pride and joy. NO disrespect intended but some experience with electronics and some electronic test equiptment is a very good idea to enable bias'ing. This may turn out more costly initially for you, esp. if not used a whole lot thereafter.

Here is a article on bias'ing a JCM 800 (not only way to do it, but will give ya an idea).

http://tone-lizard.com/Biasing.htm

http://tone-lizard.com/Marshall_bias.htm

[Twinfan]

Have you checked the valve sockets (where the valves plug in)?  That's what caused a similar problem in my old Laney as they're a known weakspot.  However, mine smelled of burning really badly when it went and it went black.  I think you would have noticed that if it had gone on your Marshall...

If the power tube fuse has blown it usually means a problem with that part of the circuit - could be a bad valve or a blown component of some other description.

I'd replace the fuse (I get mine form Maplins via mail order, or call in if there's a shop near you) and replace the valves like Dakine says.  If it blows again, it's likely you've got a non-valve related problem...

[headtheball]

Re: The tone Lizard site.

I like that man's style. I've come up against a few Jedi amp techs in my day, and it's nice to hear someone at least trying to debunk all the surrounding cr@p.

[Johnny Mac]

P.M Bainzy, he is the amp dude in here!