Mesa Boogie Mark IV tube problem?

[weldy]

My Mark IV played fine for a couple of hours last night. However, halfway into the gig I noticed (and the drummer also) that my clean tone had gotten pretty dirty on lower notes. I just replaced the power tubes a couple of weeks ago. Could this be caused by the preamp tubes? I believe that they are original to this early 90s amp. Everything I read talks about preamp tube lasting forever and making noise when going bad, not starting to distort or break up. Any ideas?

Thanks.

[Chuck H]

It may be your new tubes. They could be breaking in unevenly creating a mismatch or have started drawing more current than the last set. You may want to re check the bias (possibly on each tube) after the amp has been on awhile and played to simulate gig conditions. It may also be time for a new set of filter capacitors. These act as reservoirs for the power supply so it can smooth out ripple and supply extra current for dynamics. They last anywhere from ten to thirty years depending on many factors, but they do wear out and eventually need replacement.

Chuck

[Enzo]

Exactly. Just because the power tubes are new doesn't mean one of them didn;t fail. Look in the back. When this happens, do any of the power tubes look different? ANy parts of the metal structure getting red hot?

[weldy]

Thanks for responding. Sounds like you both don't think it could be the preamp tubes. I'll have to get everything set up again to give it a test. It took about 2 hours for it to start distorting - can I just leave it on that long, or will I need to be playing it to get it hot enough?

BTW, Mesa Boogie amps do not have a bias adjustment.

[Enzo]

It may not have an adjustment, but that doesn't prevent us from checking it to make sure it is working right.

I tend not to think it is a preamp tube, but one good spare preamp tube, substituted one by one down the row will tell you pretty soon if it makes a difference. It does sound to me more like a power stage issue, but anything is possible.

[weldy]

Thanks again for the response. Being just a "geetar" player....help me understand how to go about testing the preamp tubes. I understand that if it is a problem that occurs as soon as you turn it on - you just start changing 'em out. But it's gonna be reeeeaaallly hot after waiting an hour or so for it to mess up. Not to mention trying to maneuver around those power tubes. Any suggestions as to the process to check this out?

[Don Moose]

Use a clean sock or garden glove to handle a hot tube
- they do cool off pretty fast, but I think you want to keep the amp over all pretty warmed up for this.

The main thing is that you need a KNOWN-GOOD preamp tube - a new one isn't necessarily good.

Arrange things so that you can get to the preamp tubes easily - take off that piece of cabinet that runs across the upper back (if any) and get the amp up on a table with its back toward you.

With the amp off and cold, remove V1 to a safe, convenient location and replace with the K-G tube. Better? V1 was A (not necessarily THE) problem.

You now have the option of swapping V2 into the V1 slot (if V1 was bad, do this), or putting V1 back. In either case, the K-G tube moves to the V2 slot.

Keep going in this mode until you've tested ALL of the preamp tubes, noting which ones seem to be a problem. There is no reason that only one would go bad - they're all the same age if original, probably the same batch.

Hope this helps!

[weldy]

Thanks to everyone....gotta' get it figured out by Thursday.....or pick another amp.

[bob p]

I know this is an old thread, but here's a useful tidbit for all of you Mark IV owners:

You may not know it, but there are plastic clips to store 2 spare preamp tubes inside of the amp. You can't see them from the back. You have to pull off the front grille panel. Behind it you'll find a pair of plastic clips that are stapled to the amps wooden cabinet. Put a pair of spare 12AX7 in there so you'll always have spares handy.