Mark III oscillation

[Nigel Tufnel]

Hey guys I've been a lurker here for awhile but this is my first post. Anyway I bought a Mark III bluestripe rackmount awhile back and it sounds great but it squeals like crazy when I use it for high gain. I don't use super amounts of gain either, right when I get to that sweet spot on the lead channel where I get that great mark series sustain it wants to go nuts. My main gripe is that it won't stop the feedback when I mute the strings, and when I'm playing longer sustained notes they seem to bloom into feedback before they shoud. These problems only occur once I open the master volume up. I've talked to some other mark III owners on other forums that don't have the same problem at all and they run more gain than I do. I've swapped tubes like crazy, I've tried many different guitars with different pickups and it's just too unstable. The last gig I used it at the master was just over 2 on the dial and it was feeding back everytime I muted the strings and I was a good 15 feet from the amp. If anybody has any insight I would surely appreciate it, I want to love this amp but it's driving me nuts, I've looked inside and everything looks just as it should. I'm fairly experienced with modding and tweaking amps but this one is far more complex than what I'm used to dealing with. Anybody had this kinda problem before? Any usual suspects in the circuit of these that can make them unstable? Thanks in advance.

 

[murray]

hello there - have you tried a SPAX7 in the V 1 spot - these are special low noise 12AX7 with heat shink wrapped around the glass - even seen some amps with these valves in every pre amp socket , hope this helps mk111s are great amps regards Murray.

 

[Nigel Tufnel]

Actually I have tried one of those, the guy that I bought it from shipped it with a bunch of tubes, there was one of those spax7's in there. I've tried every tube under the sun in this thing nos and new production and I can say for sure that it's not a tube issue. It really seems like instability in the circuit, I've goofed with enough amps hot rodding them beyond their limits to know what it sounds like :wink: Only I'm pretty sure it's not a design flaw causing the instability as I have done learning as I go in the past, I think this is related to something in the circuit that was right as mesa intended but has gone south at some point in this amp's long life. Thanks for your thoughts though, I do appreciate it.

 

[Buddy]

If you nix all effects from the signal chain , does the feedback remain?

 

[Boogiebabies]

The pull bright on the lead channel will get you every time with high master settings.

 

[Nigel Tufnel]

I don't use the pull bright either, I've tried every solution possible, different cables, different guitars, different tubes(a crapload of em), I swear something's just not right with the amp. My settings go something like this, volume 7, treble 7-pulled, mids 6, bass 2-3, master 2ish-pulled, lead drive 6-7, lead master 5-7 wherever I need it to get balance between clean and dirty, and I usually keep the presence off. I use the graphic eq as well. Those settings don't sound too crazy do they? Any other ideas fellas? Thanks for the help so far.

 

[Boogiebabies]

Don't take this wrong, but are you using a speaker cable as a guitar cable?

 

[Nigel Tufnel]

No offense taken man, I know it sounds unusual but this thing is driving me nuts. I've tried several of my cables from cheapo $5 jobs to my higher end lava cables, no speaker cable. I was wondering about shielding too, I was thinking that maybe the input grid wire's shield had come loose but it's all there, all factory lead dress and zip ties are in place too, it looks untouched inside, no burnt resistors, no bulging or visably leaky caps, everything looks original and good. I guess it's time to take the multi-meter to it and see if I can find anything way out of spec. Thanks again.