Problem with a Mesa Boogie Dual Recifier

[carldacorte]

Hey everyone! Brand new to the forum, but it looks like I'll be spending a lot of time here!

I've got a Mesa 3 channel Dual Rectifier on the bench now. I'm getting tube arch on the further most power tube from the fuse. The amp is blowing fuses every time I take it off of standby (only in silicon diode rectifier mode) and in both tube rectifier and diode mode I get nothing but buzz.

All of this started after the customer flipped the switch from diode mode to tube rectifier mode while the amp was powered on.

I think the tubes are fine since the fuse only blows in diode mode... but I still only get buzz through the speaker cab whether its in tube or diode mode.

I'm not as familiar with PCB's as I am with vintage amps... which is why I'm having trouble with this one. I've opened the amp once to see if I could see any visible problems... everything looked fine. So I guess I'm just looking for a general place to start when I open the amp for a second time.

Given what I've described, what should I be looking for?

Thanks a ton!

[Enzo]

If a socket arcs, replace it. Ther is now a carbonised trace on it that will want to restrike the arc pretty much no matter what you do. Sockets are cheap.

All you get is buzz, yet the tubes are fine? I'd be trying a different set of tubes in that amp - power tubes anyway. You think the tubes are OK because they don;t blow fuses in some settings? Well they can still cause mighty hum without blowing fuses.

Isolate the problem.

Pull the PI tube, does that kill the hum? If so, then the power tubes are at least not causing it. And either the PI itself or something earlier is causing hum.

And do ANY of the controls have ANY affect on the noise? Any control that affects the level or tone or whatever of your noise is after its source. Any control that have no effect are either before the noise source or not in the signal path. (Trem oscillator tube for example is not in the signal path.)

[carldacorte]

I opened the amp up again this afternoon. Took the pcb's off to get a closer look at them. Again, didn't see anything out of the ordinary. I do however smell burnt plastic which seems to localize at the output speaker jacks and I do see discoloration on one of the 8ohm output speaker jacks like they had been burnt or something... Didn't see any melted plastic though...

BTW, all the taps in the OT measure the same resistance no matter where I measure from.

The diodes in the rectifier circuit all tested good.

Upon powering up the amp this afternoon I did notice that the buzz had been replaced by nothing at all. No sound at all... until I power up in diode rectifier mode and then that's when i blow a fuse.

And the tube rectifiers aren't glowing at all.

I cleaned the arc'd socket, and it doesn't seem to be arcing anymore, regardless that'll still get replaced if I turns out that the amp is worth saving. I would try a different set of tubes, but I don't have any handy.

Without having a tube tester, how can I test these tubes to see if they are ok?

[Jazz P Bass]

Mesa Dual Recto

I think you should stop plugging it in with tubes in it.
Start from scratch.
Remove the tubes.
Check the diodes in the power supply.
Check all your plate & cathode resistors.
Do you have a Variac. If not, make a lamp limiter.
Forget the 5U4 till you get it running.
Get the B+ up & running.
Check your -Vdc bias voltage.
Check your heater voltages.
Put in the preamp tubes one at a time while monitoring B+ & heater voltage.
When you get to the PI you should have a nice healthy Vac signal on the grids of the power tube sockets.
Put one output tube in and verify that you at least get a signal at your dummy load.
Check each output tube one at a time.