Oh No!

[michael_dba]

Played an old theater converted to a stage today and had two JMP amp issues. The '76 JMP 1959SL first blew a fuse after cranking it up about halfway, then just kept blowing fuses when powered up.

Subbed it with a JMP 50 1987 clone (at about 7-8 volume) which worked well until about 45 minutes in and it suddenly got louder. It kept going, but after the show I could smell a burnt odor from it. I haven't opened it up yet, but where would be a good place to start with either amp?

Oh yeah, the 1987 amp might have had the tubes biased a little hot, but they did not look red to me.

Both amps were used with the same 2 4x12 cabs.

Any suggestions on where to start inside the amps?

[surfnorthwest]

I first have to ask if those cabinets were yours or were you just using them for the gig?

[michael_dba]

Hey Surf, these were both JCM800 1960 cabs with G12T's in them, they have that stereo/mono switch. This is the lead guitarists rig...with the 1959SL. We *thought* we had them set up for running at 8 ohms each, and set the amp's impedance switch to 4 ohms. Same setup for the subbed amp.

[surfnorthwest]

A 1960 cabinet is 16 ohms and therefore your amp should have been set to 8ohms for the pair. That "could" be your problem right there. I hope you didn't fry a tranny.

The JMP 50 1987 clone is probably fine if it kept working but I am sure it got hot, the '76 JMP 1959SL will need to be serviced.

[michael_dba]

Man, I hope that trans didn't fry. I found a pic of the switchplate on the cab, since I don't have one on my 1960BX cab. Looks like we had it plugged into the 4 ohm (mono) side on each cab....so it must have been running at a 2 ohm load...and we had the amp set to 4 ohms.

Strange how the 50 watter chugged on while the 100 watter tripped fuses immediately.

(Guess my old Super Reverb with 2 ohm output would be happy at that load!)

[surfnorthwest]

Doesn't necessarily mean the tranny blew, but I would have it looked at. You can run ohms mismatches but for how long is anybody's guess. Always best to know for sure what the impedance of the cabinets are when you use one that doesn't belong to you. I suspect the other amp is fine, it just got really hot.