V1, V3, V5 not lighting up

Turned my MKV on this week and about 15 seconds later the power light and footswitch lights went out. Checked the rear of the head and the power/rectifier tubes were lit & not redplating. Smelled a burning electrical smell & noticed a few wisps of smoke coming out from behind the OT so shut the amp off (was in standby the whole time). Fuse didn't blow.

I pulled the amp from the chassis & put it on the bench. I immediately saw that V1, V3, & V5 didn't light up. Switched a few tubes around & those individual tubes would light in other positions.

The amp is under warranty so I'll call Mesa on Sunday to start that process, but just curious if anyone had any ideas? Thanks.

Best to have it fixed by Mesa certified tech.

If you lost footswitch and three tubes, it is related to the 12VDC regulator. V1, V2 and V5 use 12VDC heaters, same 12V circuit also feeds the 5V regulator for the switch logic controller for the footswitch and channel selection. It could be the 12V regulator or the tube that has caused the short resulting in smoke (overloaded the 12 regulator JM2940), The 12v DC cooling fan also runs on this circuit. Best to have the amp serviced as there may be a cause for the fault that occurred. This will not trip the power fuse as it is generally low current and not enough to burn the slow blow fuse.

If you lost V3, V4, V6, V7 and all 4 6L6 heaters would be off as they share a common 6.3V ac circuit. Rectifier heater is part of the high voltage power input section and is part of the tube diode (creates the cathode circuit).

The standby only applies to the high voltage circuit (used for plate voltages for the power tubes and preamp tubes ). The low voltage circuits are active at all times when the amp is turned on (standby can be on or off).

FWIW it was the low voltage rectifier diodes.

conrad79 said:

FWIW it was the low voltage rectifier diodes.

Click to expand...

I have a feeling I've read about this type of problem before. I think it has to do with a particular batch of diodes that where bad.

Hmmm....interesting, thanks for sharing. My amp is dated 10/31/13