Mesa and adjustable bias mod

[camsna]

Nope. They won't. I asked and they 'don't recommend it.' I can understand why, though. They make it as easy on the end-user as possible. But a good tech can always do it. Not too tough of a mod - just replacing a resistor with a pot...

 

[stokes]

Mesa wont do it and doesnt recommend it because if you have an adjustable bias,you wouldnt have to buy tubes from them anymore.It is a fairly easy mod in most Mesa amps,but unfortunately the MKIV isnt as easy.There are issues with the simul class and class A modes,and it involves more than just changing the bias resistor.Also when adjusting the bias in one of these amps,you still have to do a kind of balancing act between the different modes,so you may not get exactly what you want in any given mode.

 

[camsna]

Well. I don't think it's business strategy. I doubt Mesa makes much profit from dealing tubes! I think it saves headache for them AND most Mesa users. Most guys don't know what tubes are or what they do or that they're any different than light bulbs. And they shouldn't have to get their amp biased every time they swap out their tubes. Also - Mesa doesn't have to worry about biasing issues. They just set the bias range and sell tubes that fall in that range. Plug 'n' play.

But for those with a more discerning ear . . . bias is another thing that we like to monkey with

 

[Paul Secondino]

Mesa wont do it and doesnt recommend it because if you have an adjustable bias,you wouldnt have to buy tubes from them anymore.

I think that's lame as sh!t and one of the reasons I'm moving away from Mesa amps...

Mesa isn't really selling amps with a view to forcing you to buy their tubes.they simply know that most guitarists screw up tube changes and then complain to the manufacturer about it. Mesa's goal was to make it simple to change tubes is all.

However, for guys like me that know how to set bias , it's annoying to not have a trim pot inside the amp.

But for anyone that wants to mod the Mark 4, take it to a qualified tech.Simulclass isn't a big consideration in the negative bias setting. But it does mean that you have to find a happy medium for all the tubes as opposed to biasing all 4 power tubes

I believe that the entire bias supply will be governed by only 1 resistor before the voltage is split off to feed the two pairs. So you still only need 1 trim pot. That is unless you want to be really cool and add 1 pot for each pair of tubes.

 

[6L6C]

Well. I don't think it's business strategy. I doubt Mesa makes much profit from dealing tubes! I think it saves headache for them AND most Mesa users. Most guys don't know what tubes are or what they do or that they're any different than light bulbs. And they shouldn't have to get their amp biased every time they swap out their tubes. Also - Mesa doesn't have to worry about biasing issues. They just set the bias range and sell tubes that fall in that range. Plug 'n' play.

But for those with a more discerning ear . . . bias is another thing that we like to monkey with

Kind of funny you got guys out there throwing any tubes in Boogies which are not in the bias range and then you have guys who again just throw any tubes in there amps and do have bias controls but don’t use them.
The fixed bias is kind of user friendly, plug “n” play as you said. The down fall we are locked into tubes within a range in some ways no big deal but it puts a real damper on things if I want to use some NOS tubes. Also I have had some tubes from Boogie which were running pretty dam cold would sure like the ability to warm them up a bit

 

[Lution]

off topic but my thread so what:

I'm reading the manual for the rivera tbr series (out of production).

http://www.rivera.com/manuals_html/tbr-manual.html

very thorough and informative.

there are at least two references to chinese tubes:

Warning: Never ever use Chinese EL-34s, 655Os, or 6L6GCs. They are mechanically unstable and will shorten the life of your amplifier.

never heard that before. so do GT and Mesa not use chinese tubes? only russian then b/c the only other option is JJ's which are slovakian for current production tubes?

that manual must have been written in the era when chinese tubes weren't so good. There was a period of time a decade or two ago when their quality wasn't so good.

They're all good nowadays. I'm running all chinese pre and power tubes in my XTC with no problems at all. sounds great!

 

[ANIMATED SUSPENSION]

I don't understand this obsession with moaning about fixed bias. Just put tubes in that are to Boogie specs !!! What could be simpler !!? All this rubbish about 'running cold' is a ploy by other tube vendors to sell you their tubes that are not in Boogie specs !!! If you don't get the sound you want from a Boogie, don't blame it on the 'fixed bias' or it 'running cold', change to a different amp maker ! :roll: Then you can swap tubes every week and reset the bias to your heart's content. Personally I'd rather play guitar................. :wink:

 

[straitouttahell]

Mesa wont do it and doesnt recommend it because if you have an adjustable bias,you wouldnt have to buy tubes from them anymore.

No, I don't think it that way.

Any monkey with a tube tester can select you a bunch of tubes in a compatible range for any Mesa amp.

I think they make their amp fixed bias because by doing this they can be sure their tubes will work with their amps. Less useless service calls.

-EDIT-

By the way, the thing works well the other way round too: people selling tubes may want to tell you that adjustable bias is better just because they don't have to select them for your amp, which means less hassle and no need to *throw away* tubes

(Don't think they actually throw them away, just keep them for adjustable bias amps)

Who's got the right?

I am going to keep my MkIV the way it left the factory. My quartet of Winged C's are kicking hard even if they were not hand-picked by Mr. Randall Smith himself

 

[stokes]

Mesa's tubes"running cold" is not a ploy by anybody I know of.I dont sell tubes,I repair and build amps,and if you ever checked the bias on Mesa tubes you would see that they are biased cold-very cold in many instances.I have yet to see or hear of a MKIV that has the EL34's biased any higher than 11-24ma's.For optimum performance and tone EL34's in that amp should be about 35-38ma's.If you guys like the Mesa tubes,thats fine,tone is subjective anyway,right?But you are getting zero power tube saturation and all your overdrive is being generated in the preamp.When you do hear how your amp can sound with a little more power tube zip,you will see what we are talking about.For myself the realization came 20 years ago when I first got my Boogie,I got new tubes that were the same grade as the old ones and the amp sounded like crap.When I checked the bias these tubes that supposedly "matched" the older ones biased at about 20ma's,the older ones were 31ma's.I instaalled the bias pot and have used NOS tubes since.I also have a problem paying $45 for a crap pair of Chinese tubes that can be had for $19 elsewhere.The Mesa scheme is beneficial to most players who dont want to be involved with servicing the amp,and are happy with the tone they get from a less than optimal power supply and cheap tubes.Another big reason that Mesa sets the amps bias on the cold side is the fact that there is such a big range in what they consider to be "safe",and safe they are,because if they were to set the bias at optimal bias ranges,they would have to be much tighter with the control over the current draw of each tube.I have seen Mesa tubes in the same grade range 10-15ma's,so if you have the amp biased at 35ma's and you get a pair of mesa tubes that draw 15ma's more you will be too hot and Mesa would have a liability issue.Like I said,if it works for you,then by all means go for it,but there are a lot of better sounding alternatives out there that you just cant experience being locked into cold power tubes.Straitouttahell,you say you are using Winged C's,so you have some idea of what a well biased tube sounds like,since they obviously werent graded by Mesa and I would be willing to bet are biased a lot hotter than stock Mesa tubes are.

 

[stokes]

"I believe that the entire bias supply will be governed by only 1 resistor before the voltage is split off to feed the two pairs. So you still only need 1 trim pot. That is unless you want to be really cool and add 1 pot for each pair of tubes."
If you really want to take full advantage of an adjustable bias supply,there are a couple of resistors on the outer sockets that need to be tweaked.As for using 2 pots,I would prefer to have two seperate bias circuits to split each pair of tubes,and there just isnt room in that chassis.

 

[Nomad]

Just noticed this thread.

I dont know Stokes from Adam but I can tell you guys he knows what he is talking about.

Its not a bad thing that Boogie uses non adjustable Fixed Bias in the amps and sell Tubes to fit the parameters they set up. After all they are a business and need to make money to survive. If they have less warranty issues and sell more tubes they are coming out good. Its not a bad thing.

All that being said because of that Boogies are Biased Cold no doubt. After Biasing Countless Boogies (and other amps) with and without the Boogie Tubes I can attest to this. There isnt any secret what plate dissapation a set of tubes should be running at. Boogie dosnt have some kind of corner on the market on that. That information has been publicly available since about 1946.

In turn its not a bad thing to want your amp running more effeciantly or having adjustable Fixed Bias in your amp be it Boogie or other. First off its your amp.

There are a number of ways you can tackle the Biasing issue.

#1A Use Boogie Tubes. Me no likey.

#1B Have a Tube vender send a set of Tubes matched for your amp (hopefully). Me no likey.

#2 Have a Tech Change the resistor Value to "best" match your Tube choices. This is Great!

#3 Have a Tech Install a Bias Pot and get your own Bias Meter to test your Bias yourself. This is easy enuff but takes some investigation and learning. Even better than Great!

#4 Have a Tech install a Bias Pot and Test Probe Points for setting Bias with a Digital Volt Meter. This makes it quick and easy.

Also for Simuclass amp owners.

The Bias can be set for both Tubes but it takes a little more work (not for the Technically challenged).

Also two Bias Pots Can be added to a Simuclass amp but the biggest problem is (especially in a Mark IV) there aint a lot of room to do this in some of these amps. It can be difficult. Again refer to a Tech.

At my last retube on my Mark III I setteled for a Nice Balancing act with resitor value in Simalclass Mode using a set of =C= 6L6's and a set of JJ EL34's. I had just bought this amp and the tubes that were in it were Biased to **** cold. I brought it up to where it should have been (per Bell Labs Specs not Boogie).

 

[Nomad]

Also ment to clarify for those that dont know.

A Boogie Bias Circuit is a non adjustable Fixed Bias.

A Marshall Bias Circuit is an adjustable Fixed Bias.

A Fender Bias Circuit is an adjustable Fixed Bias.

These all are the same. They all are Fixed Bias.

The difference is most Fenders and Marshalls have a pot in the Circuit that gives adjustment to the Fixed Bias and in Boogies there is no adjustment (there is but you must change a resistor value to adjust the Bias).

The only other type of Bias Circuit (I know of) that is auot adjusting is Cathode Bias. There is no Pot but the Tubes are self Biasing (to a point).

The fact that there is or isnt an adjustable Pot has nothing to do with wether a Bias Circuit is Fixed Bias or not.

 

[stokes]

Well put,Nomad.I was asked by someone about doing a dual bias pot thing for the simul-class amps.I dont think it will be as easy as just adding two bias pots.If you look at where the bias pot would go in the circuit,it is already in parallel with another resistor at what I'd call,for lack of a better term the far end of the circuit,at ground,adding another pot here wont give you what you want.I think,and this is just based on a look at the schem,I havent tried it yet,that you would really need two totally seperate bias supplies.They could come from the same tranny tap,but would have to be seperated there,requiring a full circuit,not just an additional pot.Be kind of tough fitting that in there,neatly anyway.

 

[stokes]

Oh,and there are some older Fender and Marshall amps that have the non-adjustable fixed bias.Back in the day it was necessary to adjust by changing a fixed resistor,until someone came up with the bias pot idea.There is nothing special at all about Mesa's fixed bias scheme,although they would like you to think there is.

 

[Nomad]

Agreed. It could be done in a Simul but would be rather involved and there sure isnt enuffroom .

And yes I know there are Fenders and Marshalls that do no pot and even Cathode (Marshall 18 watter).

I was just using them as a general rule.

I would prefer my Boogies with a Pot but its no big deal to me. I do however love Boogies or at least some of them. I am for sure getting the goods out of my Mark III right now.

Like to see some of your amps though. Might be interesting.

 

[camsna]

Anybody care to add a couple of bias pots and test points to my Mk III?!

 

[Nomad]

I be knowing who can do that for you. Where are you located?

Shoot me a PM.

 

[straitouttahell]

you are getting zero power tube saturation

Thanks, God.

 

[breogan]

you are getting zero power tube saturation

Thanks, God.

You don't know what you are taking about :wink:

(just kidding, I am a 18watter user)

 

[carlosasi]

I just added adjustable bias pots for each tube in my amp wired to a silicon chip in my head so I can think the correct adjustments into my amp while I play. My tone is wonderful but I can't remember where my car keys are.':?:'

edit - just kidding good thread.