#1
okay gang, my 50 watter jcm 900 keeps blowin fuses and i dont know why. i took it to a tech who said he ran it on an oscillator (i think thats what it was) on full power for 4 hours and couldnt get the fuse to blow. he didnt find any problems with it whatsoever, but low and behold, whenever i play it the damn fuse doesnt last more than half an hour. ive already blown the output transformer and had to replace it.. at least fuses are cheap. by the way its the "output valve fuse" i keep blowing. so i just assume i have it set up wrong.. help me out? heres how i have it running

the back of the head: IMPEDANCE SELECT i have the choice between "(4)16" or "8". im not an ohmage expert, but im only running one speaker cable, so i have it set on 8.

i also have a choice of MODE: high or low. its on HIGH. maybe thats the problem?

so ill have one speaker cable running just out of one of the 50w RMS output jacks (they dont say anything to distinguish a difference so im assuming theyre both the same) into my jcm 900 lead 1960 cab: so the choice between STEREO or MONO is pretty obvious, i have it on MONO cuz im only using one cable. correct? but then heres the confusing part:

the input jack on the left says "4 ohms" above it and "8 ohms right" below it. the jack on the right says "16 ohms" above it and "8 ohms left" below it. so why the hell does the left one say "right" and the right one say "left"? thats just confusing. and i dont know which one to plug into. help me out? the amp tech said the head is just fine and he couldnt get a fuse to blow, so its obviously the way i have it set up. thanks in advance to any person kind enough to walk me through it

edit: by the way, before the question comes up, yes its a speaker cable. im sure. it says speaker cable right on it. thanks!
Last edited by jhole at Feb 27, 2009,
#3
Umm, it could be the power in your house. If that amp is getting more than (insert country regulated voltage here) then you could be lucky its just blowing fuses

I wouldnt run an amp without a power conditioner....
I've bought, sold, and traded more gear than I care to admit.
#4
power conditioner meaning like a surge protector? i dont think its that because i first played the amp at my parents house and blew a fuse.. then in our practice space at my buddies house and blew another.. and now in my new apartment it still blows. so i dont think its the power coming out of the wall.. plus i use surge protectors. dont know if that helps at all.
#5
Surge protectors are MUCH different, however if it's happening at several different places, its possibly the way you are setting the amp up....

*reads above post*

I would think you should run this 8 ohms from your head into the 8 ohms right on your cab. Usually the right side is intended for mono...however...if that cab is only wired for stereo...you might have an issue.....
I've bought, sold, and traded more gear than I care to admit.
#6
I had that problem with my 5150 II

It turned out to be one of the power valves was faulty, so replaced them all.

It could be something else for you though, what usually triggers it i found was turning the volume up enough that you got power amp dirt.
#7
Good call Sora

I was going to mention power tubes, but I figured since the tech didnt have an issue...but to run into the oscillator he didnt need to turn it up....

How loud is it when the fuse is blowing? Does it blow when you just turn it on and leave it sit?
I've bought, sold, and traded more gear than I care to admit.
#8
Quote by Brendan.Clace


I would think you should run this 8 ohms from your head into the 8 ohms right on your cab. Usually the right side is intended for mono...however...if that cab is only wired for stereo...you might have an issue.....



that could be, but dont you think if the cab has a stereo/mono switch that switching it to mono is okay? lol. and by 8 ohms right, do you mean the actual jack on the right side (which says 8 ohms left) or the jack on the left that says 8 ohms right?

thats the problem with playing music these days is they expect you to have a damn major in electronics to get your amp working properly.


edit: and i dont know if it would blow if i just let it sit there, ive never tried that. i usually only have it up around 4 or 5.. maybe ill set it up and let it chill for a few hours and see what happens.
Last edited by jhole at Feb 27, 2009,
#9
LOL it's usually just tube amps that have these kind of issues

I would think that the input that says 8 ohms right (thats ****ed that its on the left), but yeah, as long as the cab and head are set for 8 ohms mono, all you have to do is match.....
I've bought, sold, and traded more gear than I care to admit.
#10
Sounds like a power tube issue, don't keep replacing the fuses, next time you play watch the power tubes, see if theres any arcing around the tube or the socket or if there is any redplating on the tubes.
#11
the funny thing is when i bought my first setup.. spider II and behringer cab .. i noticed the spider output jacks said the same thing. and i was like "hahaha, what a stupid amp". but now that my new cab says that too.. i wonder if theres something more to it?

yea, the switch from ss to tube is totally worth it for half an hour at a time. lol but all kidding aside i love it.
#12
Quote by al112987
Sounds like a power tube issue, don't keep replacing the fuses, next time you play watch the power tubes, see if theres any arcing around the tube or the socket or if there is any redplating on the tubes.


yea that makes sense, but dont you think the tech wouldve caught that? i mean i understand there are some idiot techs out there and he very well may be one of them, but he ran it for hours and didnt have any problems.. i would think if it was the tubes he wouldve seen some kinda problem?


edit: and any theories on changing the mode from high to low?
#13
Quote by al112987
Sounds like a power tube issue, don't keep replacing the fuses, next time you play watch the power tubes, see if theres any arcing around the tube or the socket or if there is any redplating on the tubes.


I agree...though it could also be an impedence issue....

I say check both!
I've bought, sold, and traded more gear than I care to admit.
#14
the reason i think its the impedance and the way its set up is cuz i blew the O/T a while back. i dont really understand ohms but the way i think of it is, electricity through the cables is like water through a hose, and the size hose corresponds to the ohms. so if you have this electricity going through and the ohms arent right, itll become backed up like a kinked hose and destroy the first thing going backwards through the system. and isnt the O/T the last thing that the signal goes through before leaving the amp and entering the cab? so although i got it replaced doesnt mean its safe from harm. i think i just dont set it up right.. i mean ill buy some new tubes, was lookin at the el-34s anyways, but im not made of money to keep throwin into this thing if its just the way i have it set up.
#15
This is how your impedance needs to be set.

Put the selector on your amp to 16 and plug your amp with a speaker cable into the jack on your cab that is labeled 16 ohms and put the switch on mono. The way the jacks are labeled is that each pair of speakers is wired for 8 ohms, so if you were to use the cab in stereo, you'd have an 8 ohm load per side. The switch puts the two 8 ohm loads in either series or parallel to get either 4 ohms or 16 ohms if you're running the full 4x12 in mono.

Match the impedance between head and cab. If you're load impedance is too high you will get reduced output and bad tone. If you're load impedance is too low, you will 1) burn out tubes very fast 2) put a large amount of strain on your OT.

Don't try to make sense of it with your current understanding of what impedance means. Trust me, it's not intuitive unless you understand how the OT works. Unless you do, just make sure you're matching your impedance. OTs are destroyed when people don't match their impedances. If you had to have yours replaced, I imagine something like that is what happened.
Last edited by al112987 at Feb 27, 2009,
#16
thanks man, thats probably the most helpful thing ive heard all day. i really figured high impedance meant burning out tubes and **** like that, which is why ive always been afraid of the 16 ohms. lol but thanks a lot, ill switch it up and see what happens. i was playing it a few minutes just now and now im gonna leave it on but in standby for a few hours and see if the fuse still blows. ill switch it over to 16 first though, thanks man
#17
what is important is making sure the head and cab are set to the same. Setting your head to 8 ohms and your cab to 16 ohms will not do anything bad usually, it just results in some very bad tone. But the other way around, bad things can and generally do happen.
#18
Try putting a spike buster before it, or plug it into some other plug point in the house and see.
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#19
well thats very possible to whats been happening. i knew i had to match it up, but never understood the difference. so this is how i have it now: (4)16 on the head, one cable to the 16 on the cab, set to mono.

and still in high mode instead of low. but now i guess thats a good thing.
Last edited by jhole at Feb 27, 2009,
#20
Hey I kinda stopped reading after the 5th post just to answere one question youve kept asking.

The High/Low switch switches off some of your tubes, which makes it quieter. I have the 100 Watt JCM 900. and yea my HIGH/LOW switch changes the amp to 50 watts by turning off valves.

The only downside is you give up a bit of tone, so I don't use it. Its just set to High
"if i were you I wouldn't fiddle with the feet etc. as that nullifies warranty but yeh, just get up in digitech's grill and demand they make you a custom whammy that is purple and gold and fires rockets when you bend +2 octaves"
#21
thanks man, yea i was wondering what that did. and ive left the amp in standby for the last few hours and every time i play it it seems fine, so the fuse was only dying while i was playing it. we'll see how long it lasts running 16 ohms, hopefully that solved the problem
#23
i had it at 8 ohms and mono, didnt realize 8 was for stereo and 16 was mono. the mode was always on high though, i didnt have to change that
#24
Ok yeah, if you're running the head at 8 ohms into a 4 ohm cab, you're really putting a lot of stress on your OT and is really working your power tubes as well. You could be doing some real damage to your OT.
#25
haha well i already paid to replace it once so its good that i got that problem resolved lol
#26
Quote by goonerbynature
Try putting a spike buster before it, or plug it into some other plug point in the house and see.


obviously did not read the whole thread

I've bought, sold, and traded more gear than I care to admit.