Mark III Simul EL34 bias?

[VinylJunkie]

A few months ago I bought a Mark III which had an adjustable bias already installed. This bias mod has two trim pots, but it appears to be just a balance. Each pot controls one EL34 and one 6L6, not pairs, unfortunately. I'd assume that it would need a seperate bias circuit to control pairs. Anyway, I finally got around to messing with bias yesterday. After I biased the EL34’s to 35ma, the 6L6’s were only 17ma. If I run the 6L6’s up to 30ma, the EL 34’s are 55ma. I still had a long way to go before the 6L6’s were at 70% (42ma), but the EL34’s were already too high, so I went back to 35ma on the EL34’s and gave up for the time being. The high bias on the EL34’s would be normal if the EL34’s actually were running in class A, but from what I understand that’s not the case. Is that correct? What should the bias be for the EL34’s anyway? Unless I"m missing something, I don’t see any options other than trying to find a really hot pair of 6L6’s and a really cold pair of EL34’s. This is my first Simul Class amp, so it’s a bit of a learning curve.

 

[morgan138]

The high bias on the EL34’s would be normal if the EL34’s actually were running in class A, but from what I understand that’s not the case.

I think you might be wrong there - I believe that simulclass amps always run two tubes in class A (they're normally EL34s but I guess in some older Marks they might be 6L6s), and then adds in a pair of A/B 6L6s when you switch to normal mode. Because of this, their rated wattage is 75-80 watts with all tubes running, rather than the 100 watts you get from a 60/100 model.

 

[HelpingFriendly]

I also have been trying to balance good readings between the inner and outer pairs of my Mk iii. I know there is no way to get a matched quad to run at the same level which is fine if the amps sounds good. But I'm curious if anyone knows how to convert it to the way a 60/100 would run. (Using all 6l6's) is that possible?

 

[morgan138]

I also have been trying to balance good readings between the inner and outer pairs of my Mk iii. I know there is no way to get a matched quad to run at the same level which is fine if the amps sounds good. But I'm curious if anyone knows how to convert it to the way a 60/100 would run. (Using all 6l6's) is that possible?

The manual I downloaded says:

"In the past, MESA/Boogie advertised the "interchangeable power tubes" feature of the Mark III Simul-Class. We told players
that they could use either EL-34's or 6L6's in the outer (Class A) power sockets. However, in recent years we've observed a much
greater reliability factor with the use of EL-34's in these outer sockets, with much fewer incidents of tube failure. Therefore, today we
strongly recommend using EL-34's in the outer sockets and 6L6's in the inner sockets of your Simul-Class Boogie."

Which makes it sound like you can just swap them without any modification, but maybe it'll eat the class A tubes faster than normal?

 

[MrMarkIII]

I also have been trying to balance good readings between the inner and outer pairs of my Mk iii. I know there is no way to get a matched quad to run at the same level which is fine if the amps sounds good. But I'm curious if anyone knows how to convert it to the way a 60/100 would run. (Using all 6l6's) is that possible?

The manual I downloaded says:

"In the past, MESA/Boogie advertised the "interchangeable power tubes" feature of the Mark III Simul-Class. We told players
that they could use either EL-34's or 6L6's in the outer (Class A) power sockets. However, in recent years we've observed a much
greater reliability factor with the use of EL-34's in these outer sockets, with much fewer incidents of tube failure. Therefore, today we
strongly recommend using EL-34's in the outer sockets and 6L6's in the inner sockets of your Simul-Class Boogie."

Which makes it sound like you can just swap them without any modification, but maybe it'll eat the class A tubes faster than normal?

This quote from the manual is old, outdated information.
There's no longer any preference over which tubes to use in the "Class A" sockets, either EL-34's or 6L6's.
I've done both, with no harm.
I'm no expert, but it would appear that the bias trim pots were not installed correctly, as they do not control the inner and outer pairs of tubes, but rather one of each. Of course this is bogus on a Simul-Class, but it wouldn't work on a Twin Reverb, either.

 

[VinylJunkie]

This quote from the manual is old, outdated information.
There's no longer any preference over which tubes to use in the "Class A" sockets, either EL-34's or 6L6's.
I've done both, with no harm.
I'm no expert, but it would appear that the bias trim pots were not installed correctly, as they do not control the inner and outer pairs of tubes, but rather one of each. Of course this is bogus on a Simul-Class, but it wouldn't work on a Twin Reverb, either.

This is pretty much the conclusion I've come to. Even though there is two trim pots, they basically act as one. Since two different tube types are running off the same plate voltage there is no possible way to bias both pairs correctly. This bias mod is really kind of pointless. It doesn't hurt anything, but once the EL34's are biased to, say 70%, the 6L6 bias just is what it is. My main question was how high the EL34's can be biased. If it is true class A, then I could safely bias the EL34's to 45ma @ 505v plate. The 6L6's would still be biased cold, but at least they would be up to 55-60% instead of 40%.

 

[kippiejr]

Try to use your ears and not the 70% rule you've read about. Mesa's gets their gain from preamp cascading gain stages and not from power tube distortion. They run them traditionally colder than let's say a Marshall that needs power tube break up. Mesa's power section is designed to provide huge clean headroom which it does well running colder. But on a meter if there is no cross over distortion and pure power it not running cold. It's running how designed. The bias pots will probably let you put the tubes is lets say C+ range since it ran hotter than a III. But still no where near that 70% which does not apply to Mesa. The fixed bias won't let you get there. Toss out what you've read and let your ears do the rest. 40% might seem cold traditionally but give me that Mesa tone and long tube life any day of the week and twice on Sunday. There not running cold, put your hand on them after 5 minutes of playing. There scorching hot!!!! :lol: