mesa fixed bias and switching tubes.

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chennie

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As you all know, Mesa uses a fixed bias. That means you just have to put in new tubes and voilá: you're ready to rock. In some amps, like mine rect-o-verb it has even a switch for if you want to use EL34 or 6L6's, zet the switch, put in tubes, rock.

Now the following: the proper bias voltages for EL34's and 6L6's are different, but that is what we have the switch for. But even within the same type and even the same brand there are differenences of proper bias voltage per tube. Pick two 6L6's out of a box of hundred of the same brand or factory, and they won't have the same bias voltages.That's why variable bias settings are in existance with other amps like Marshall.
Mssa has solved this by catogorizing its tubes. The taken the whole range of possible bias voltage for a tube, let's say the 6L6 and divided that in (I thought six groups) and given every group a name of a colour, red, yellow, etc. Now your Mesa amp has a set/fixed bias and that is set to a certain tube with a certain colour code in the factory. Thus if you want to change tubes you have to change it with a tube with the same mesa color code or you have to know with what voltage interval your tube with its specific colour code belongs to.

Now, my question. This story, is this true and complete? I probably think so, so my second question:
I read alot about chaning tubes, about brands en what more, but according to my story it is not possible to just stick in any new pair of tubes, but you have to get some tubes with the same values as your old set.
Now how is this done by you guys ? You just use mesa tubes, or measure your amp settings and pick tubes of any brand you wish ? (how is this done?) Or you just heard this for the first time and you just replaced new tubes ignorantly or you modded it with a potmeter ? or ...?
 
The story is true, there are several ways to get a correct autopolarizate bias circuit, for example the cathode bias.
Nowadays and exactly bias procedure is doesn´t needed only if you wants to get your amp for the NFL. Todays all the amps are class AB that means that every tube is working on a phase has a little current through the tube to avoid the crossover distortion by 0 voltages of the signal.
Well the boogie method is to do a strong circuits that seems to assimilate a great range of current needed to polarizate the tube, this is the reason that there is some colors that allows you to replace the tube like potatoes.
The range of boogie is set to the worst case, the case that a tube needs so much current. Even if you get one of this tubes the circuit will following feeding the tube with the current needed.
By the way I adjust the bias in a lot of other manufactures and the unique brand that needs seriously a good bias point are the marshall amps. Boogie, Carvin, Peavey and just another brands works well even in the worst case that you get the bias so low, without any crossover by zero distorsion.
I think that people wents some paranoic with the bias matter... but the thruth is that great designs has inmunity among quality tubes.
One think that people forget is that replacing tubes needs to be by matched tube, this produces a percepts fx even stronger than a bad bias point.
 
thanks for the reply, but what is NFL ?

And are you practically saying is that it doesn't matter what tube you put in your boogie (but ofcourse with respect to the type, 6L6 and el34 etc), because boogies are made in such a way that they are built with such wide range of tube differences in mind ? But on the condition that they have to be matched.
 
NFL is the high football league in USA. If you want to play your amp with the best quality a fine bias setting is needed. If you are a trash metal player I think that a perfect bias setting is doesn´t needed
 
If you want to play your amp on the very best conditions like a professional a specific bias setting is needed. If you are an amateur don´t worry about the bias matter, because mesa are strong enough to accept several kind of tubes
 
For tubes to sound their best the bias needs to be tweaked.I have a Mark11c+ i have owned since 1989 and had to install a variable bias pot in it.The color scheme gives a very broad range even within the same
"color".Using this system is "safe" because they tend to be biased on the cold side.After trying the color coded tubes I still found the bias to be other than optimal every time.Even in my cathode biased amps I change the cathode resistor with every tube change. Tubes biased to your taste is much more important than having exact matching tubes.In fact a slight mismatch (+ or - 5ma) is actually more musical sounding for guitar amps.Hi-fi units tend to need exact matching,they dont want any distortion in these units.Of course tone is totally subjective and if the color code works for you then by all means go with it but with a simple change of a resistor to a pot you can then use any tubes you want,not locked into using whatever tubes Mesa happened to buy.You cant compare these cheapo Chinese tubes to a good pair of NOS tubes.
 
Tubes biased to your taste is much more important than having exact matching tubes
Sorry to be disappointed. In a push pull configuration the signal is splitted in two phases. The half positive and the half minus of the sinewave. If you doesn´t have matched tubes one of the stages will be more amplified than the other, if it occurs you it will be imposible to you to play a clean tone in your amp.
On the other hand mesa has a good fixed bias systems, ( remember that bias is a DC current that is needed to fix the bias point ) in the case that it wasn´t enough it will occurs an cross by zero distortion that it is softly than a not good push pull balanced tubes.
In my short experience in tubes I only discovers troubles on cross zero in marshall amps. Boogies never do a on my oscilloscope a distort near zero.
you can then use any tubes you want,not locked into using whatever tubes Mesa happened to buy.You cant compare these cheapo Chinese tubes to a good pair of NOS tubes.
Ok with a trimpot you can adjust even the bias on every tube, but did you think that mesa uses chinese power tubes?? please refers to mesa factory and ask them.
The classical NOS matter: Well if you like NOS you have enough money I´m agree with you to buy a NOS valve. But I play on a progressive band that sounds to heavy and I´m not able to hear differences between NOS tubes because my drummer plays so hard. I prefer to buy a sovtek tubes and then give a beatiful gift to my girlfriend. :wink:
Are you using mullard tubes??? well a couple of mullard tubes could be near 150$ with a good matched. So expensive for my taste :(
 
Like I said in the last paragraph tone is totally subjective.If you happen to play so loud that you cant hear the difference then there is no difference.I was just pointing out that with a very easy mod you can open up a whole new world of choices.If you look around you can find NOS tubes at a reasonable price.If you are interested I can tell you where to find NOS Philips 7027A's for under $100 for a matched pair.They sound great in my Boogie,as do Sovetek 5881's-if they are biased properly,which you cant do without an adjustable bias system.Even if you want to use the Mesa system you can get more out of them if you are able to adjust the bias.I dont mean to knock anything about Mesa.I have always been a fan of them and havent heard anything that can come close to the tone I get out of my Mk11c.
 
if you want to change tubes you have to change it with a tube with the same mesa color code

any color mesa tube will work fine in a mesa amp. matching pairs is critical, but only the ones that work together. depends on you amp's power as to how many tubes you can pair, but many combinations are safe...JUST MATCH IN PAIRS THAT WORK TOGETHER
 
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