Mesa Boogie MKIV bias issue

[Gregg]

Concerning the bias it's easy to make it adjustable so google "mesa mark IV adjustable bias mod" and you'll find plenty of info and pics on that.

[Ampzone]

Concerning the bias it's easy to make it adjustable so google "mesa mark IV adjustable bias mod" and you'll find plenty of info and pics on that.

Right, this is not my amp, just wanna get it safe as is and out the door. So where is the best location for the 6l6gc?

Also can bias probe socket or 1ohm resistors (tied to pins 1& 8)be added to this circuit to get some readings?

[bob p]

6L6 always in the middle, 6L6 or EL34 on the outside.

if you're not going to be fussy like me, and you just want to get the amp up and running safely and you don't want to order mesa tubes, then try this: find one pair of 6L6 tubes that are reasonably well matched, and another pair of 6L6 that are reasonably well matched. put one pair on the inside and the other on the outside and take bias voltage measurements. then switch the pairs and take measurements again. you want to populate the ones in the center positions with the best match possible for the amp's fixed bias voltage. the outer pair will be biased colder, so they're not going to be as critical. it's sort of a balancing act where it's impossible to get a perfect result.

if that doesn't work then your best options are to find a different set of tubes to go into the middle position. if you can't do that then you can install a variable resistor as sort of a master bias mod, which controls the middle pair and lets the outside pair run cold without directly adjusting them. optimally, two separate bias pots would be best, but as you know it's damned tight in that chassis.

i'd resist the temptation to bias hot. the amp sounds better biased cold. the lead channel can get particularly ugly sounding if you use tubes in the middle that bias too hot. that's why i recommend keeping the amp cold biased. we've discussed this before, so there are threads about this here if you want to dig for them.

[Ampzone]

6L6 always in the middle, 6L6 or EL34 on the outside.

if you're not going to be fussy like me, and you just want to get the amp up and running safely and you don't want to order mesa tubes, then try this: find one pair of 6L6 tubes that are reasonably well matched, and another pair of 6L6 that are reasonably well matched. put one pair on the inside and the other on the outside and take bias voltage measurements. then switch the pairs and take measurements again. you want to populate the ones in the center positions with the best match possible for the amp's fixed bias voltage. the outer pair will be biased colder, so they're not going to be as critical. it's sort of a balancing act where it's impossible to get a perfect result.

if that doesn't work then your best options are to find a different set of tubes to go into the middle position. if you can't do that then you can install a variable resistor as sort of a master bias mod, which controls the middle pair and lets the outside pair run cold without directly adjusting them. optimally, two separate bias pots would be best, but as you know it's damned tight in that chassis.

i'd resist the temptation to bias hot. the amp sounds better biased cold. the lead channel can get particularly ugly sounding if you use tubes in the middle that bias too hot. that's why i recommend keeping the amp cold biased. we've discussed this before, so there are threads about this here if you want to dig for them.

Thanks for helping, simple is all I want, the amp is PITA to work on way too much crap inside (too tight). so I have a new matched set of 6l6 GC JJ tubes. I will have to order the other tubes. Can you use a bias probe on this amp? Or can you add 1 0hm test resistors for MA check?

[nevetslab]

There's some interesting info in this post. I have a Mesa Stereo Simul-Class 2-Ninety power amp, which DOES have room inside, though no room on its' rear panel. I had picked this up at CenterStaging's garage sale back in 2010 for $75, and the first time I had powered it up, I recall it motorboating...watching the speaker cones slowly movoing like so many of the Dual and Tripple Rectifier models do. I had set it aside all these years, until last week, and put it back up on the bench to have another look at it. This time, both channels were stable, no motorboating and sounded quite respectable.

Basically two channels of what's in the Mark IV, but without the options of changing the operating class....this one's fixed in Simul-Class mode. I hadn't thought about tryng it with EL-034's on the outside pair.

Like you, I hate having to tear into Mark IV's, as everythigh is just so crammed in. Even trying to get the tubes out of the bear clamps hurts your fingers.

[Ampzone]

So I have a new matched set of 6l6 GC JJ tubes. I can order the other tubes. Can you use a bias probe on this amp? Or can you add 1 0hm test resistors for MA check?

[bob p]

either method will work.