Need Help with Marshall 1960B Cab issue - blew Power tubes.

[sunjive]

I recently picked up a used JCM 900 1960B cab. Worked fine for a few weeks. I am running two 100W heads (in stereo) into the cab. All the ohms were set right (8ohms) and the switch was set to stereo. The other night i turned on the heads... hit the standby switch and heard a pop than a hum. Both heads blew a power tube simultaneously and one head blew a fuse.

I am trying to diagnose this issue. The fact that both heads went simultaneously, could it be the stereo/mono jack plate? Or could one of the speakers have blown or short circuited?

I'm afraid if i change the jack plate and the problem is not resolved, i will blow more tubes and possibly hurt the heads.

Any help would be greatly appreciated.

 

[joepete77]

I would check to make sure that the cab is wired correctly and also if speakers were swapped out for a different ohm possibly? I doubt its coincidence that both your heads blew a tube at the same time.

 

[charveldan]

Doesn't make sense to me to run two 100 watt heads in the same 4x12, why would you?

To much juice Holmes.

 

[glpg80]

did you or the person before you re-wire it to become a stereo cabinet by default? assuming it is loaded with 16 ohm speakers, each side would have to be paralleled down to 8 ohms independently with two seperate connections.

a whole different issue is the fact you are overpowering that cabinet in the first place. this way anyway. what you need is another cabinet.

your other option is to use a CAE amplifier switching unit and switch between them into a single cabinet.

if you're trying to mix sounds at the same time - even if you get two load boxes for each amplifier and run the line out signals into a third power amplifier you are still going to need two seperate channels to amplify both of them separately, however the difference here is that you will need to be using a solid state or 50 watt tubed amplifier per channel - 60 watts maximum per side to run to a stereo wired cabinet.

the final option is pull two tubes in each amplifier, half the ohm settings on each down to 4 ohms, and run that into each 8 ohm side of the cabinet.

you've probably experienced an odd form of voltage flyback when powering up the mains to the center tap of the OT.

 

[wIsEbLooD]

those switching jacks are garbage. i always hardwire new jacks in.

 

[Anonymous]

if there are t75's in that cabinet, then you aren't overpowering it.

as long as they're wired right, and not via that junky switch, your heads should work fine.

my guess'd be there's a malfunction somewhere inside the cab (wrong wiring, clip came off one or more speakers, switch is bad, etc).

i'd agree though, just get another cab. make sure the one you have is wired right, and get rid of that switch assembly..hard wire it.

 

[severinsteel]

I would check the jacks with a multimeter to make sure they are the correct resistance.

 

[LP Freak]

I would check the jacks with a multimeter to make sure they are the correct resistance.

+1. My 4ohm and 16ohm jacks were actually wired in reverse on mine. I was shocked when I put a meter on it.

 

[glpg80]

I would check the jacks with a multimeter to make sure they are the correct resistance.

+1. My 4ohm and 16ohm jacks were actually wired in reverse on mine. I was shocked when I put a meter on it.

wow, that is not good

 

[charveldan]

I would check the jacks with a multimeter to make sure they are the correct resistance.

+1. My 4ohm and 16ohm jacks were actually wired in reverse on mine. I was shocked when I put a meter on it.

When i bought this 1978' G12M Blackback cab the shop owner told me it was 16 ohms, got home to find out it was wired to 4 ohms parrallel, came this close to frying a JMP 2203.