Join Date: Dec 2000
Location: Longmont, Colorado
Thanks for the suggestions, and I know tube amps pretty well, I'm an EE and have built amps in the past but this one is driving me nuts.
A) I may pick up some tubes tomorrow but I don't think that's the problem. BTW, I prefer Svets, probably the best power tubes you can get today, followed by JJ, don't know where the Mesas come from. Tube matching is way overhyped, kinda like true bypass in pedals (when I get my pedal business going I'll put up a dissertation on that on my website, although I do use true bypass, there are times where it is good and times when it is bad for tone). The tubes in the amp are new, according to the shop, but they have no markings so I don't know where they came from (potential source of questions).
B) Mesa uses fixed resistors for the bias, no bias adjustment pot, I can make an arguement for either, the fixed bias makes tube changes easier but I am going to check the bias circuit, and yeah, they do run the bias a little on the cold side. Since it's fixed, I guess they want to be on the safe side.
C) Yes, it hums like a cheap hooker on Sunset even without a guitar cable plugged in (Mesa uses a shorting plug that shorts the input jack to ground when a cable isn't plugged in).
D) I have changed the output tranny, since the shop got it in with bad tubes I didn't want to take a chance that the previous owner had accidentally fried the output section by not having a speaker load hooked up, hence, why he got rid of it so cheap.
E) After some surfing around, this is apparently not that unusual an occurrence on these amps. Some of the construction techniques Mesa has used have me asking "What the h**l were they thinking?" The main board is a pain to get to, so servicability sux. I may layout a board for them to mount the controls to and send it to them. It's insane what you have to go through to get to the solder side of the board. This is also an older model so I don't know if they have rectified (no pun intended) some of these issues. I'm going to check for ground loops on it today, may put in a central ground and run lines to different ground points on the board. The next issue would be possibly the choke but I'm leaning more toward a layout/ground loop problem right now.
I know what causes Rectum Hum... Mexican food.
Thanks for the ideas, I'll let you know what I find out.