Excessive hum

[ivox]

I have an (old?) Mesa Boogie, dual rectifier w/ 2-12" Green Back Celestions; 35 watts op and very little use. After using all the direct plugins, etc. on my mac, I decided I wanted to "push some air". So I pulled out the Maverick and much to my dismay, I get this loud hum. I've tried every setting; clean, lead, bright, rhythm-nothing but hum. Now when I plug in one of my guitars and crank the amp it sounds great, but in those bars I'm not playing, there's that hum. Any ideas, other than finding a guy that can work on this for hundreds of dollars?

[Twist]

With the volume on the guitar all the way can you control the volume of the hum with the controls on the amp? Has someone cut off the ground prong on the AC cord?

[ivox]

No. The problem seems to be coming from the amp as it has the hum even without a guitar or cord plugged in. The power cord is intact; all 3 prongs are still there.

[Twist]

When you turn down all the controls on the amp does the hum go away? I'm just trying to determine which section of the amp is causing the hum. Pay particular attention to the reverb level control. Also you may have some weak filter caps.

[ivox]

Even with everything turned down to zero, there is hum, altho much less, especially when turning down the output knob. Volume and Master do not add to the hum when the Output is down.

[ivox]

After further testing, Output @ 1/4, minimal hum, but as I turn up the volume, the amp begins to pop and crackle. Turning down the vol. to 1/4 and the Output past 1/4 results in the same thing. The Master knob doesn't seem to be involved, so mabe just new tubes?

[Mars Amp Repair]

hello,
for starters, I'd suspect a shorted K-G preamp tube. These Mesa's are so high gain that you often have to try a number of different preamp tubes & even swap them around to get them all balanced as far as hum is concerned.
You can try pulling the tubes one at a time from the 1st to the last (of course once you pull the driver tube you'll likely have no sound), to determine what stage may be generating the hum, however if it is a matter of filament balancing, that may not be conclusive.
I find having some Mesa or Ruby (Chinese) & J-J 12AX7's helps as you mix them up to what combination finally works.

In some amps you can build a little circuit to add a DC offset to the filament line. It works well on some of the old Ampeg stuff, but I've not had much luck on the Mesa's. It seems to be more a matter of 12AX7 issues. glen

[ivox]

glen, thanks for taking the time to provide such useful information. I'll give it a try with new tubes, in the order you suggest. Great info.
Again, thanks, Jim