Marshall JCM 900 SL-X Problems

[Aso]

A buddy and I were jamming today and my JCM 900 SL-X decided to blow all three fuses. Well I after trying to find new fuses with no luck because the local music store doesn't carry fuses anymore, I decided to open up the back panel check the tubes. Attached is a picture of what I found.

Looks like it's going to need to be retubed even though the tubes didn't have more then 10 hours on them but they were 10 years old . Now I am wondering if the new Soldano cab I got isn't 16 ohms like it is supposed to be. I got the cab used from a Soldano dealer but he wasn't sure what speakers were in it and when he pulled one out he said they weren't labeled. Anyway I can check the impedance of the cab? Anything else I should look at? Anyone know of a good shop in the Southern Minnesota or maybe Minneapolis area to work on it?

Here is a photo of the carnage.

[jgcable]

Check the impedance of your cab and make sure you don't have a severe mismatch.
Also you really need to have the plate voltage checked. Your bias could be set very high (hot). Get that amp checked out before you throw new tubes in it. Thats a catastrophic problem when a power tube explodes.

[Cleveland Metal]

I'll gamble it took some supporting resistors out as well...

Never seen an impedance mismatch do that but never know...

I've seen new tubes just crap out on their own (MESA sourced 6L6's) and thro sparks all around inside like a a water filled snowing christmas decoration. And, taking out some resistors in the process. After actually watching that happen, I could imagine they could explode as well like yours did.

If you want to check the cab's impedance, get a Digital Voltmeter set to impedance and a plug a speaker cable into the cab and connect to the bare ends of it. It won't be exact like say a 16 ohm cab may read 14.7 ohms or whatever but at least you'll be in the area. I've noticed they usually read a lil under what it is. 4ohm=3.somethinohm, 8=6.somethin ohms.

Don't think mismatched impedance would do that really. Mostly it kills transformers in my experience. But, seriously mismatched tube amps are a bad thing and failures can really be a result.

[Spivonious]

Holy Crap!

It's a good thing those fuses did their job or I imagine your amp would be toast.

[Aso]

Holy Crap!

It's a good thing those fuses did their job or I imagine your amp would be toast.

It did sorta smell like toast. I think the magic smoke that makes electronics work got loose.

[Aso]

Check the impedance of your cab and make sure you don't have a severe mismatch.
Also you really need to have the plate voltage checked. Your bias could be set very high (hot). Get that amp checked out before you throw new tubes in it. Thats a catastrophic problem when a power tube explodes.

Is there an easy way to check the impedance of the cab without pulling all the speakers and figuring out how they are wired together. Like I said the cab is supposed to be 16 ohm just like my other Soldano that I bought new 15 years ago but the dealer wasn't 100% sure what speakers are in the cab so there may be a difference in speakers and/or wiring.

The funny thing is the amp went poof while the volume was turned off on the guitar and I was looking something up on the net. So I wasn't even playing through it at the time it popped put the power was on.

[Chad]

You said "Attached is a picture of what I found." That tube is not only blown, but is OUT OF ITS SOCKET. I assume you meant to say it's a pic after you pulled the tube's base out of the socket? Or did it literally blow the tube out of the socket?

[Aso]

You said "Attached is a picture of what I found." That tube is not only blown, but is OUT OF ITS SOCKET. I assume you meant to say it's a pic after you pulled the tube's base out of the socket? Or did it literally blow the tube out of the socket?

I pulled it out of the socket so it would be easier to see. Not sure if any other tubes were damaged or not.

[DonP]

You can check the impedance of the cab with a DMM. It might read an ohm lower, like 15 instead of 16.

I noticed you are in MN, the cold capital of the US. Was the amp real cold when you turned it on? Tube are made of glass, and when glass goes from real cold to real hot, it can crack and break.

[Aso]

You can check the impedance of the cab with a DMM. It might read an ohm lower, like 15 instead of 16.

I noticed you are in MN, the cold capital of the US. Was the amp real cold when you turned it on? Tube are made of glass, and when glass goes from real cold to real hot, it can crack and break.

I will grab a Fluke and check the impedance. Just go from the center conductor to the outside of it right?

The head was room temp. It hasn't been transported or outside for three or four months.

[Cygnus X1]

Most important, what jgcable mentioned second.
Get it biased.

[chrisolson]

Savage Amps is one of the best in MN.

[Aso]

Most important, what jgcable mentioned second.
Get it biased.

Ya I am going to get it looked over but trying to decide if I should just retube and rebias it or send it off to Jerry @ FJA Mods and have work his magic with it.

[DonP]

I will grab a Fluke and check the impedance. Just go from the center conductor to the outside of it right?

The head was room temp. It hasn't been transported or outside for three or four months.

Yes, measure from tip to shield.

After having a Cato like that, I'd take it in to make sure some insides didn't get fried. resistors could blow, etc.

[Cygnus X1]

And, sounds stupid, but I have made this mistake before: Unplug the cab from the amp when checking impedance.

Another thing...somtimes I'll get a wild reading from a cab or speaker.
I'll "bump it" with a 9 volt battery, and it will fall back in place for the reading. It has happened too many times on too many different speakers for it to be a dropped voice coil. Just one of those things, I guess.