Mark III problem

smk0000 · Jul 4, 2006

I currently have a Mark III and love it around Vol. 2... however, when I turned it past 2 on the Master, whenever I would play a muted note on overdrive, I would hear crackling and have, in the past, heard squeeling. It sounds great until I go loud with it. Is this a problem with the power tube, preamp tube or the speaker? I have always taken a working amp for granted and haven't had to deal with changing out tubes yet. If this is a problem that could be rectified by changing out preamp tubes, then it should be an easy fix. Could it be something more internal? How do I know when tubes aren't BAD and not working, but just need changing or are worn?

I appreciate any input.

Stephen

Boogiebabies · Jul 4, 2006

It could be a microphonic preamp tube. You can test this by swapping them out one by one with a new 12AX7. Mesa's with their high gain capabilities can make your pickups feedback as well. They are really touchy with the pull bright engaged. I usually run my lead master no higher than 3 or 4.

smk0000 · Jul 7, 2006

Though I will try replacing the preamp tubes one by one, I have noticed other problems. In the back of the amp, there is an effects loop in/out and a presence knob. There are no effects running into the unit, yet the presence knob effects the sound anyway. When the presence knob is turned to 0 it sounds bland, when the presence knob is turned to 10 it sounds sharp--again, no effects are being run into the unit. Coincidentally, directly beneath the presence knob is a 6L6 GC power tube. Now, I don't have much experience with tubes; however, while I would strike a note, while on overdrive, at a volume at which would come the buzzing (which sounds as though something is rattling, but more so internally and electronically than like a loose bolt rattling) I noticed in this particular tube, and not the others, a blue spot in the tube that, when this buzzing would occur, I would notice that this blue spot would vibrate, as well.

Now, as I said before, I have little experience with tubes and electronics. This is merely what I notice while I am having these problems. I will try to switch out the preamp tubes, and may just replace all of them... Should I be concerned with the 6L6 GC power tube and is it a coincidence with the blue in the tube, the buzzing sound at volume and it being directly under the presence knob which changes tone even without effects loop running?

Thanks for your help.

Stephen

Paul Secondino · Jul 7, 2006

The presence control isn't for the effects loop. It's a presence control for the actual amp just like any other head except that it happens to be located on the rear instead of the front.

As far as the blue light , sometimes that indicates a bad tube but blue glow from a vacuum tube usually isn't abnormal. A bad tube usually wont appear blue in my experience either. It's a wierd thing

smk0000 · Jul 7, 2006

A new, but possibly important discovery which will expose my ignorance but may also help in diagnosis. The amp only makes this buzzing sound when in Class A mode. I switched to Simul Class mode and I thought I heard a similar sound, switched it back to Class A and it was REALLY there. So, Class A is good for low volume playing (makes sense) and Simul Class for louder volume (again, makes sense). Another discovery in the manual is that EL34s have fewer incidents or malfunctions in the outer sockets (Class A mode). I currently have thin 6L6s in the outside and fat 6L6s on the inside (have ignorance in the difference, if any).

So, with the discovery that EL34s may be better for Class A (outer sockets) and that the problem pretty much only occurs in Class A mode and that the problem 95% goes away while in Simul Class mode (and any buzzing isn't too consistent) I am thinking that the problem may be rememdied by replacing the outer 6L6s with EL34s -- if it were a preamp tube problem, it should (correct me if I'm wrong) present itself at volume while in Simul Class mode, as well--which it did not.

Let me know what you think and thank you for any input.

Stephen