Mesa boogie 50/50

[brewu22]

If I am asking a dumb question, sorry in advance. Why did Mesa build a amp with no bias adjustment? With no adjustment, what tubes can I replace the stock tubes with?

[stokes]

The bias on a Mesa amp can be changed it just doesnt have an adjustable bias pot,but like many other amps you need to either change a resistor or replace it with a pot.Mesa's scheme is that if you use their tubes you dont need to rebias the tubes are pre tested,sorted and coded.If you contact them they will get you the tubes that will be safe in that amp.If you want to use other than Mesa or GT's you have to have the bias checked,just like many Marshall and Fender users do all the time.

[Tom Phillips]

Decided to jump into this...


Mesa Boogie has their own philosophy about bias adjustment or, more accurately, the lack thereof. You can read Randall Smith’s view at http://www.mesaboogie.com/US/Smith/biasadjust.html

In the paper he says “The short answer is that during my 12 years of repairing Fenders, one of the most frequent problems I saw was bias controls that were either set wrong or that had wandered out of adjustment due to vibration.”
Interesting take. After my 41 years of experience repairing Fenders I say that the most frequent cause of incorrectly set bias is replacement of the tubes without a re-bias. Yes, you can now buy replacements that are graded the same as your old tubes so that you can replace them without re-biasing. This assumes that the supplier is still around and has not changed their grading methods. In the old days, tubes were not supplied with any grading numbers and matched sets were a special order. However, I think that there was much less variation in characteristics between individual tubes of the same type.

Concerning the ” wandered out of adjustment due to vibration” statement. I have never seen any significant evidence of that. If an amp with a decent adjustment pot had been vibrated that severely I say it’s time for a look inside the chassis anyway.

If we apply the above two statements to guitars we could make the case that the tuners should be removed to prevent mis-adjustment by the user and changes due to vibration.

Another part of the white paper Mr. Smith says “It only takes a moment and a volt meter: The Fender diagram shows how: Adjust this trim pot for - 52 volts. That's it. Nothing more”. He is saying tongue in cheek that if you always set the bias voltage to the same value anyway then you don’t need an adjustable circuit. Just design the bias circuit so that it always produced the same voltage.
To this I say HOGWASH. I have never seen a Fender schematic that said "Adjust this trim pot for - 52 volts." There is a reference to –52 volts but that’s just a nominal value. A starting point if you will. Those in the know have been setting the bias current by adjusting the bias voltage for decades. As it turns out, -52 volts worked pretty good for most tubes available in the ‘60s but it hardly ever is a good setting for today’s tubes. An adjustable bias lets you set up the amp correctly and since there is an acceptable range it allows you to customize the sound to your needs. Yes – you do need to know what you are doing.

MB says that if you buy their tubes, you can just pop them in the amp and they will be biased correctly. Does this mean that they reject a high percentage of the tubes they buy because they fall outside their acceptable parameters? The article says they do but I doubt if the percentage is very high. In fact, when you actually get their tubes they come graded with colored dots. The instruction sheet with the tubes says to use all the same color dotted tubes in your amp or put one of each color on each push-pull side to maintain balance. In reality MB amps are biased so cold that you would be hard pressed to get into trouble by ending up with any power tube biased too hot. IMHO the MB sound does not come from the power amp section anyway so the biasing isn’t as critical to the overall sound as it is for a classic Fender or Marshall.

As far as I’m concerned Mesa Boogie can design their amps any way they want. Some features of their products are very good and I like their customer service. However, it pisses me off when they bull shit me with the stuff discussed above. To me this really makes it hard to keep reading the article even though there is a fair amount of correct information.

I think I’ll stop now. Probably ranted too much already.

Cheers,
Tom

[stokes]

Couldnt agree with you more Tom.Its a great way for them to sell tubes-lots of them.Most players just want to plug and play and when they here about the Mesa deal of course they will.Its a great marketing scheme for them.Dont think you ranted too much,or ranted at all.Its just the facts.

[stokes]

Tom,I had never read that "white paper",mainly because I never bought into the Mesa bias scheme,either.I just read it and cannot believe the BS he is throwing out there.It's amazing.I own a couple of boogies and I installed bias pots when I changed the first set of tubes in each.It is the only way to go.

[brewu22]

question

Tom,I had never read that "white paper",mainly because I never bought into the Mesa bias scheme,either.I just read it and cannot believe the BS he is throwing out there.It's amazing.I own a couple of boogies and I installed bias pots when I changed the first set of tubes in each.It is the only way to go.

How hard is it to install bias pots in a 50/50? I am a novice, but I work with electricians that would be willing to help. Thanks agian for helping with my question.

[stokes]

Putting the pot in is not hard,it is just a matter of replacing a resistor with a small cermet pot.If you can read a schematic,there are 2 resistors in that bias supplly in series,one is 47k the other is 68k.The 68k gets replaced with a 100k cermet pot.Get the multi turn pot if you can otherwise the regular cermet will do.Make sure your friend knows what he is doing,as well as setting the bias once the pot is installed.You can find the schem at schematic heaven.com-be aware that there is dangerous,possibly lethal voltages in an amp, if you or your friend do not know where these voltages are in an amp it might be better to get someone who does to do the job.

[Enzo]

Instead of replacing that resistor with a pot, might I suggest replacing it with a pot and resistor in series. That way no one can turn the pot to zero and kill the bias altogether.

So instead of 100k pot for 68k, make it a 50k resistor and a 50k pot. That way the minimum resistance is 50k (or whatever value you decide makes a reasonable minimum voltage) with the pot at zero, and then the whole range of the pot is more useful.

[stokes]

But if you come across a pair of tubes that need less than the 50k fixed, you are back at square one and have to replace or remove the resistor.If you mount the pot inside there is little chance that someone will turn the pot to zero.I do not in any way say that what Enzo said is not sound advice,it is a safety measure and you cant go wrong being safe.I could be wrong but I think the 47k resistor in series with the pot would be a good enough safety net.If you turn the pot to zero you will have the full 47k.

[jak0lantern01]

So I had my Mesa at an authorized repair shop and the two techs there asked me why I use the Mesa tubes. My reply basically reflected the Mesa line that their tubes are guaranteed and properly biased, yada, yada, yada. The two guys said that it was NOT necessary to use the Mesa tubes, there are others that can be used as well and Mesa is milking my bank account. What's going on here? I mean, can I really just throw some Groove TUbes in or any other brand? Judging by everyone's respone here, I'd say NO. Then again, why would two authorized Mesa repair techs tell me I'm wasting my $$$ on Mesa tubes? I'm a little creeped out by this one, I don't need another catastrophic breakdown of the amp.

[drewl]

You're wasting $$$ because Mesa charges alot for tubes.
You CAN use any brand, but as stated here they need to be biased.

Some people change tires and randomly inflate them without checking tire pressure, but we all know a properly inflated tire will last longer, perform better and safer....same with properly biased tubes.

[jak0lantern01]

But that's my point, or perhaps I wasn't clear enough. It was indicated that I didn't have to worry about the biasing. That's the part that scares me in their statements. These are the guys who fixed my amp. They didn't replace the tubes with Mesa's, and they sure as all hell didn't do anything to alter the bias. This leads me to a question: what are the negative effects of installing tubes without correctly setting the bias? I do have to admit, the EL34's they installed for me sound damn good, though.

[Tom Phillips]

My current approach to MB tube replacement:

I keep tubes in stock with a decent range of pre-tested and graded bias requirements. I can then select a set that works as well (or better) than the Mesa tubes. I still check the bias and make any required repairs, such as replacing burnt screen & hum balance resistors, before I button the amp up. This is the best solution for me and the customer. This doen't take me any more time because all my customers want the amp bench checked anyway. Sometimes they buy an additional spare set of tubes matched to those I install.
Customer gets a properly biased amp. No circuit mods required so no-one can ever complain that I voided their warranty, screwed up their amp, reduced the resale value etc.

Customers have brought in MB tubes. They don't all fit in a narrow bias range. See my earlier comments about the color coding.

Tom

[stokes]

As long as your plates arent glowing red or orange you are safe.Dont confuse the normal glow of the heaters with a red plate.The plate is the large gray structure in the tube,it will look like someone hit it with a blow torch if it is running too hot.If it is have the bias set properly or the tube will fry,and may take other components with it.Groove Tubes can safely be used in a Mesa if they are in the #4 to 7 range.If that is what they put in there you should be okay.I dont care for this system,but it is safe.Like Drewl, I like to set the bias for what I think is the best performance.

[jak0lantern01]

What about pre-amp tubes?