Mark V combo:1st power tube position blows fuse consistently

[DineroKartel]

Help! I just bought a lightly used Mark V combo and I recently started to notice a decreased performance in the gain saturation and the top end started to sound dry. I read that this was likely a power tube issue so I tried switching around the power tubes. The performance started to get better! I realized that one of the power tubes sounded like it had a filament loose so I put it in the first position (closest to fuse) by accident. This immediately blew the fuse when I turned it on. Since the amp was used I just decided to buy 5 new power tubes and replace all of them. When I started up the amp again with a new fuse, it wouldn't turn on! I did the whole power tube troubleshooting and verified that 4 of the 5 power tubes were working without a problem. When I tested the last one, it would immediately blow the fuse. I switched the last tube and fuse again, wouldn't turn on and blew the fuse. I then replaced the problem tube with one of the old tubes, and switched the fuse. Again the amp wouldn't turn on and blew the fuse. I checked for any burn marks but everything seems cleans.

Any of y'all amp gurus have some insight? Thanks!

 

[SamuelJ86]

There are only 4 power tubes. The tube that is furthest on the left as you look at the the amp from behind is a rectifier tube. It looks like the others but it should say 5U4GB where the others say 6l6-GC. If you're putting a power tube in the rectifier socket I imagine it's going to blow the fuse. Or a rectifier tube in a power tube socket. Hopefully it doesn't do any other damage. Am I understanding your post correctly? Did you know about the rectifier tube? You'll have to buy a new rectifier tube, 5U4GB. And any 6l6 you put in that rectifier socket might be cooked. Go check out the manual on mesa's website, it a good read.

 

[DineroKartel]

Jesus Christ, you're right! I feel like a totally idiot! When I got the amp it literally had 5 power tubes in it and it seemed to be working. So when I switched out the tubes I thought it also needed 5 power tubes. How would it be able to work at all without a rectifier tube?!

This is my first tube amp so I'm still learning the ropes. But needless to say this is the last time I'll buy anything this expensive off eBay.

 

[Nicklotsaguitars]

First amp :shock:
You've pretty much ended your amp journey before you've begun. No need to get another amp, ever

 

[domct203]

Jesus Christ, you're right! I feel like a totally idiot! When I got the amp it literally had 5 power tubes in it and it seemed to be working. So when I switched out the tubes I thought it also needed 5 power tubes. How would it be able to work at all without a rectifier tube?!

This is my first tube amp so I'm still learning the ropes. But needless to say this is the last time I'll buy anything this expensive off eBay.

Welcome to the forum.

As you are experiencing, a 6L6GC output tube in a 5U4GB rectifier socket should immediately blow the mains fuse, and most likely damage the tube.

I can't see how the amp could have ever worked that way.

If you don't have the manual for the amp, go download one from MesaBoogie.com. The manual explains all the functions of the amp, as well as the tube layout.

Dom

 

[DineroKartel]

Besides destroying two power tubes, is there any chance I damaged the amp?

 

[domct203]

Besides destroying two power tubes, is there any chance I damaged the amp?

Honestly, there is always a chance for damage in a situation like this, but Mesa uses good quality components & you may be ok.

Does the amp fire up OK with no power or rectifier tubes? If yes, chances are the amp is OK.

If this is your first tube amp, remember that you ALWAYS need a speaker load hooked up to the amp. Sending audio (AC voltage) through the amp without a speaker hooked up, or too much of an impedance miss-match can cause expensive damage.

Also, a tube amp operates on, and stores up to almost 500VDC, enough to stop your heart if you get jolted good. Don't go poking around in any tube sockets, or the open chassis, even with the amp unplugged as the filter caps are storing that voltage.

Let's hope the amp is OK. It's a great amplifier, one that will last a lifetime if used and maintained properly.

Dom

 

[DineroKartel]

Update: The rectifier tube came in last night and everything fired up fine and everything appears to be working correctly!

It looks like my ignorance didn't cause any permanent damage. Moral of the story: read the manual first if you don't know what you're doing.

This amp is a BEAST, what a difference upgrading your amp can make! I was up till 2 in the morning playing cuz this amp makes my guitars sound 1000x better!

 

[domct203]

Glad it all worked out.

Dom

 

[bandit2013]

First amp :shock:
You've pretty much ended your amp journey before you've begun. No need to get another amp, ever

He may have to get another one if the power supply is toast, may have killed a few other things if a rectifier was installed where a power tube belongs.

 

[bandit2013]

Update: The rectifier tube came in last night and everything fired up fine and everything appears to be working correctly!

It looks like my ignorance didn't cause any permanent damage. Moral of the story: read the manual first if you don't know what you're doing.

This amp is a BEAST, what a difference upgrading your amp can make! I was up till 2 in the morning playing cuz this amp makes my guitars sound 1000x better!

A thing about Mesa amps, there is always a metal plate on one side inside the amp shell that has a tube position and type diagram. If you cannot find the manual, (download a recent one from Mesa) but the plate will always be there....

Another trick, if you are replacing the power tubes, it is recommended to replace the rectifier too, remove only the 6L6GC tubes and replace those first, do the Rectifier last. If you pull them all out, the rectifier is the one closest to the power cord.