Mesa/Boogie Bass 400 tube question

[redbeard1206]

I picked up a Mesa/Boogie Bass 400 this weekend, and the shop I purchased it from said that the power tubes may need to be replaced after a few months or so, but the amp could only be tubed with Mesa/Boogie brand 6550s. Anybody know why this is? I would have thought it would be able to accept any 6550 (winged C, sovtek, whatever). I don't really have a problem with it, I like Mesa brand tubes, I was just curious as to why the amp would only accept Mesa 6550s.

[LiquidMidnight]

Mesa amps have a fixed bias, which may not make other tubes that are not matched for the amp optimal. There has been much talk on Talkbass about this practice with a lot of people stating that it's largely a marketing gimmick to get people to buy more of Mesa's rebranded tubes.

You don't necessarily NEED Mesa tubes. Eurotubes sells 400 and 400+ retubing kits with JJs. I've never used one of their kits in my 400+, but everyone I've heard whose used the JJs in their Mesa bass amps have liked them better than Mesa's tubes.

Also, some people have had their Mesas modded with adjustable bias.

[redbeard1206]

Originally Posted by LiquidMidnight

Mesa amps have a fixed bias, which may not make other tubes that are not matched for the amp optimal. There has been much talk on Talkbass about this practice with a lot of people stating that it's largely a marketing gimmick to get people to buy more of Mesa's rebranded tubes.

You don't necessarily NEED Mesa tubes. Eurotubes sells 400 and 400+ retubing kits with JJs. I've never used one of their kits in my 400+, but everyone I've heard whose used the JJs in their Mesa bass amps have liked them better than Mesa's tubes.

Also, some people have had their Mesas modded with adjustable bias.

That was one of my theories as a matter of fact.

As far as fixed bias/adjustable bias goes, are fixed bias amps also "adjustable"? I've read different things about that, and seen different answers. Does fixed bias mean you can just swap tubes and go, or does it still have to be re-biased after changing power tubes?

[scotch]

Originally Posted by redbeard1206

That was one of my theories as a matter of fact.

As far as fixed bias/adjustable bias goes, are fixed bias amps also "adjustable"? I've read different things about that, and seen different answers. Does fixed bias mean you can just swap tubes and go, or does it still have to be re-biased after changing power tubes?

I'm not a tube amp engineer, but I've been learning more about them lately,... Mesa fixes their amps for a pretty middle of the road bias value & sells only tubes that fall within that range. In order to bias the amp for a hotter or colder value, you'd simply need to swap out a resistor. Or, swap out that resistor for a variable resistor (a potentiometer, for instance)- you then can measure & adjust the voltage bias for a new/different set of tubes!

Even easier is to ask any reputable tube reseller for a set tubes that falls within Mesa spec'.

Once I learn a little more, I plan on installing adjustable bias in my Mesa's. It's not crazy complicated, but does require measuring across very high voltages!

So, yeah, your stock Mesa has its bias fixed. Using tubes rated for that fixed value is the easy way to go. The only way to adjust a Mesa's bias is to change the resistor out.

[B-string]

Just to be clear.....? Fixed bias outputs are just that, fixed bias. Most fixed bias amps supply a way to adjust that fixed bias easily, some don't (require component changes). Self biased output design usually don't require any change to the "bias" circuit and you can roll tubes without a trip to a tech. Self biased amps do put out less power to the speakers though.

[TaylorO']

I have a Mesa 400 and when i got it, it had 6l6 tubes in it. Had bias pots installed(one pot for each 3 tubes) and now i run KT88s in it. Normally im not one of those guys that "tube rolls" or can point out the specifics in the"medium highs upper lows" etc etc etc that some people go on about. But i can honestly say with KT88s it has more balls, and ALOT more headroom(but still able to get "grindy" if i want it to), despite being set pretty coldly. As far as how much "output" it has i really dont know. Does that depend on the bias? Anyone know the answer to that? If im biasing each side, 3 tubes to a side, to 120mA does that mean im getting 240 watts? Im electronically retarded compared to alot of guys on here...

But yeah, get 6550 tubes, from somewhere else since MESA doesnt sell them anymore. Those 6l6s are no bueno(in my opinion) as bass amp tubes.

[cheezewiz]

I've had two Mesa 400+'s. I retubed one with Svetlanas that Lord Valve recommended, and one with JJ's from Eurotubes. I liked the Svetlanas better. Any good tube dealer will send you a set for a Mesa 400 that will match up fine.

[N.F.A.]

Subbed to this one. I am getting my 400+ delivered today!

[TaylorO']

Originally Posted by N.F.A.

Subbed to this one. I am getting my 400+ delivered today! Should mention to the OP about the "psycho mods" that got lots of discussion here.

The 400+ is a little different. 12 power tubes. no options in changing tubes, but still a great amp.

[N.F.A.]

Oops, I didn't realize that. Edited my last post.

[Caca de Kick]

Originally Posted by TaylorO'

does that mean im getting 240 watts? Im electronically retarded compared to alot of guys on here...

But yeah, get 6550 tubes, from somewhere else since MESA doesnt sell them anymore. Those 6l6s are no bueno(in my opinion) as bass amp tubes.

240 is about right on with the bigger tubes.
180-190 is about what they are with the 6L6.

[TaylorO']

Originally Posted by Caca de Kick

240 is about right on with the bigger tubes.
180-190 is about what they are with the 6L6.

I havent played with the bias much, but like i said, as it stands its biased at 120mA. What if any changes would i possbily get by raising the bias to 130 or even 140mA? the amp tech who did the work told me to keep it between 120 and 150.

I gotta admit, those giant KT88s in there kinda give me a stiffy, haha..

[chaosMK]

I retubed with JJ's KT-88's and they were excellent. Running the master around 9-10 with some OD-friendly EQ settings dialed (tip- bass at 1-2) was one of my most epic bass experiences.