Tube biasing and such
run_into_the_rain
Posts: 93
I recently bought a B-52 and I'm pretty happy with it. I've been able to get a
pretty good variety of tone out of it. I started thinking a ways down the road
and I was wondering what you guys could tell me about replacing tubes. any
info regarding this topic would be helpful
pretty good variety of tone out of it. I started thinking a ways down the road
and I was wondering what you guys could tell me about replacing tubes. any
info regarding this topic would be helpful
7/02/06 Denver
12/02/06 Honolulu
Post edited by Unknown User on
0
Comments
If that's the case you can't swap out other kinds of power tubes in place for what it uses, you have to use the correct tube or you'll hurt the amp.
Other amps have auto biasing and allow you to replace the power tubes with other kinds of tubes.
And yet other amps have a switch that allows you to select the kind of tube you use - it all depends on the amp.
If you want to use another kind of tube you need to know if you CAN.
Most of the time an amp needs to be biased by an amp tech or certified tech.
Messing around with an amp can literally KILL you (even if it's unplugged) so watch yourself - they hold a lot of voltage.
Cheers . . .
- Ian C.T. vom Saal
<b><font color="red">CONTACT ME HERE</font>: www.myspace.com/ianvomsaal</b>
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No such thing as auto- bias, however, there is such a thing as fixed bias, and I know that Mesa and Peavey have that, not sure about B52, but either the store guy should know. I just checked their web-site for you, but nada info there.
There are four types of power tube 6L6, whihc comes in a low ower version of 6V6, and EL34 which comes in a low power version, EL84. 6L6 and EL34 use the same socket, and 6V6 and EL84 use the same socket, so those are interchangable, but the bias needs to be adjusted one way or another when you do that. Your amp will be optimised to run one or the other. The bias can be adjusted either by a bias set switch, or by a tech.
There are various brands of tubes, and people talk a lot about teh virtues of JJ Tesla over Sovtek or Electro-HArmionix etc, but I'm not convinced that that is not a lot of bench-racing. That is probably what you are referring to in your question. People also tend to claim superiorority for biasing hotter or colder from factory dettings, but this is again claiming that they know something the amp designer doesn't, which is a bit doubtful.
You can safely change tubes without fear of electrocution so long as you don't open the amp chassis, which you don't need to. If you want more info on this, there are several excellent articles on the Mesa Boogie web-site. There is a specific section on changing tubes. Another myth is that you can't touch tubes with bare hands. Xenon bulbs get very hot and will crack if there is sweat or grease on them, but amp valves run much cooler, and don't suffer teh same problem. Good ventilation at the back of your amp is a good idea. You don't want too much heat in behind there. My Lonestar actually has a fan in it to keep it cool. Most amps are prety open at the back to help that too.
Amps don't hold voltage either, but the capacitors do hold a charge, which is different. Alowwing the guitar to keep playing for after the amp is turned off will drain the caps though, you will hear it fizzle out after a few seconds.
Unless your amp is a floor model that has been flogged, you won't really need to change tubes for a while anyway. They should last a year or so before the power tubes need changing. Pre-amp tubes don't usually need attention, but teh driver tube can be the problem if the performance is failing.
autobiasing exists.
Desktop Reference of Hip Vintage Guitar Amps states "self bias" (highlighted):
http://books.google.com/books?id=fvUVBP3wW1gC&pg=PA102&lpg=PA102&dq=guitar+amp+%22self+bias%22&source=web&ots=0vowg5CXXH&sig=iP4MnGtFIz9Yi74Y6KuqlHAj03A
Cathode bias is also called "self" bias.
Here are a few amps that state Auto Bias or Cathode bias (aka Self Bias)
http://www.mojaveampworks.com/index.php?id=11,0,0,1,0,0
http://www.zzounds.com/item--MSHSTACK12
This one actually says AUTO BIAS: http://www.zzounds.com/item--ASDPM60
http://www.ampaholics.org.uk/vox_ac4_blonde.htm
Here's a bunch of stuff on auto bias:
http://hhscott.com/cc/self_auto_bias.htm
http://www.tubecad.com/2005/May/blog0046.htm
http://www.tubecad.com/2005/April/blog0044.htm
http://www.tubecad.com/2005/April/blog0043.htm
Cheers . . .
- Ian C.T. vom Saal
<b><font color="red">CONTACT ME HERE</font>: www.myspace.com/ianvomsaal</b>
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OK, good link, interesting article, but for the purpose of discussion about guitara amps, when people say "auto-bias", they are really talking about "fixed bias" systems, where the bias is set, and tubes are tested to ensure they are within the bias tolerances for the amps they are sold for. The point being that you can change them without having bias adjusted by a tech.
Wihtout reading the whole link, I gahtered that it was referring to hi-fi systems rather than guitar amps.
Desktop Reference of Hip Vintage Guitar Amps states "self bias" (highlighted):
http://books.google.com/books?id=fvUVBP3wW1gC&pg=PA102&lpg=PA102&dq=guitar+amp+%22self+bias%22&source=web&ots=0vowg5CXXH&sig=iP4MnGtFIz9Yi74Y6KuqlHAj03A
Cathode bias is also called "self" bias.
Here are a few amps that state Auto Bias or Cathode bias (aka Self Bias)
http://www.mojaveampworks.com/index.php?id=11,0,0,1,0,0
http://www.zzounds.com/item--MSHSTACK12
This one actually says AUTO BIAS: http://www.zzounds.com/item--ASDPM60
http://www.ampaholics.org.uk/vox_ac4_blonde.htm
Here's a bunch of stuff on auto bias:
http://hhscott.com/cc/self_auto_bias.htm
http://www.tubecad.com/2005/May/blog0046.htm
http://www.tubecad.com/2005/April/blog0044.htm
http://www.tubecad.com/2005/April/blog0043.htm
Cheers . . .
- Ian C.T. vom Saal
<b><font color="red">CONTACT ME HERE</font>: www.myspace.com/ianvomsaal</b>
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i don't think that's correct usage of the terms intuitively but i'm far from an amp expert.
check out the gain chart at http://www.thetubestore.com it's a nice reference. 5751s will work too.
http://www.thetubestore.com/gainfactor.html
The power output tubes are the hard workers, and the bias sets the voltage that runs the tubes. The bias is the voltage at the tubes when there is no signal, (idle). More voltage and the tubes are hotter. Higher voltage to the tubes is a higher "idle" so the tubes will break up and distort faster than when the bias voltage is lower. (haha,,,, break up and distortion is a feature, not a problem in tube amps.) NOT good in a car!
The power output tubes, say, 6L6's will have different voltages because they are infinitely different inside, so a good factory or reseller will test them and try to match and rate them at their voltages.
A "matched set" is a set of tubes that are about identical in voltage to each other. When you buy a matched set, though, they will be identical to each other but probably not identical to the ones in your amp.
That's where the hardness rating comes in. Some sets of tubes will break up and give you that distortion earlier than others. Hard tubes can play clean louder, and soft ones break up at lower volumes.
So in your first tube change, you would put some new tubes in and get the amp rebiased to the voltage of the new tubes.
Tubes also wear, and you can rebias your amp as they age and get the best out of them.
Fixed bias means two things:
Some amps have the fixed bias preset in the chain of mysterious capacitors and resistors. To reset that bias, a tech will change resistors and caps to match the voltage that's optimum for the new tubes. You can change tubes yourself as long as they are matched and equal to the old set.
That gets tricky because you really don't know unless you buy a quantity of them. Otherwise, you have a tech check it when you replace tubes.
Some have fixed bias which is actually adjustable. It's kind of an odd term for an easily adjustable bias. There's a little knob/ pot/ dial/ or screw that works like a voltage regulator that adjusts the voltage.
Same deal with the tubes, though. You're still best off getting that done by a tech, and it's easy and can get the best out of your amp that way.
Like Ian said, auto or self bias is a self regulating system that usually has a cathode and capacitors and more mysterious things:) that keep a level voltage. A lot of old tweed Fenders, Champs, I think Vox 15's and 30's are self biasing. Matchless,,,, man, I don't know past that, but it should be in your amp literature, if you have it. Otherwise, check with the company or an amp book like Ian said up there.
If you have an auto or self biasing 6L6 amp, you can put any set of 6L6 tubes in as long as they are a matched set, and you shouldn't have to reset the bias. If you want to make a big change, like go from 6L6 to EL tubes, then you should get a tech in there to make sure it's right.
Regarding a B-52,,,, I say play that sucker and get your sound and not worry too much about tubes yet. If you like the sound, then you're fine. If you want it to break up at lower volumes, then you can try to put in some "softer" output tubes. THis would only be useful if you're playing the amp loud, though. If you're using pedals or preamp distortion, then changing output tubes won't make that much difference. If you're running the whole thing loud,,, then you CAN get some different sounds by changing tubes.
>>>>>>>>(((((The MAIN thing about playing and getting the best out of your tube amp is to learn the dynamics of volume and technique with your fingers, no matter what the tube is. The beauty of tubes is the way you can make them sing with your fingers. When you find the sweet spot, it's the best! Especially if the neighbors are cool!
Don't be mankind. ~Captain Beefheart
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7/02/06 Denver
12/02/06 Honolulu
Man, I'd just be playing that sucker and not worrying about changing good tubes. Amps are made fixed bias to keep you from needing to spend good money on techs.
Out of bias amps still work anyway, just the power tubes wear out a bit quicker. Just crank it up and have fun !!
However, the 12AX7 you want to keep track of is the one in the V1 slot.
The V1 gets the brunt of the hit first, so if that tube starts going bad it'll amplify and send it down the line.
The V1 is usually the culprit of most of your amp problems.
Cheers . . .
- Ian C.T. vom Saal
<b><font color="red">CONTACT ME HERE</font>: www.myspace.com/ianvomsaal</b>
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The squealer!! When you plug your guitar in and turn it up to eleven,,, and you get that high pitched squeal that has dogs and cats , rabbits and mice running into the woods, women gathering their kids up off the street,,,,, that's usually the v1!
IDgotI: You can interchange 12ax7 and y7 and u7's if you want.
Generally the 12ax7 has the highest gain. So if you want your amp to be cleaner at high volumes, 12au7 that has less gain, and 12ay7 has even less gain than the au7.
Regarding fixed bias,,, one last time
Fixed bias means that you DO adjust the bias. The term means that you "fix" the bias.
Think of it as an old car. The power tubes are like the spark plugs.
You can drive your car with a bad tune up and blowing smoke and bucking and hesitating and overheating.
Or you can tune it up to get the right power to the spark plugs and have it run smooth and purr and enjoy the experience.
The bias setting is that final thing that you do to make the power tubes run to their optimum.
If the bias is too hot, you run through tubes too much. If the amp plays clean when you first turn it on, then breaks up after ten minutes or so, then it's probably biased too high.
If it's running too cool, the tubes last longer, but you can burn out capacitors faster, so you want to bias to meet the specs of the amp.
Auto bias is like a flow regulator to keep the DC voltage more constant to the power tubes.
So what type B-52 do you have, run into the rain?
Don't be mankind. ~Captain Beefheart
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its an at212. It has a voice sort of similar to a mesa. i have found it real flexible. The second gain is heavy, the od is realy variable through the volume knob and it has a real nice clean. So far I'm pretty happy with. As far as tubes go I had no idea what I was getting into, thank god for techs and all of you great advice
7/02/06 Denver
12/02/06 Honolulu
MESA ENGINEERING
WHITE PAPER
TOPIC: BIAS ADJUSTMENT
September 25, 1996
Here's a question we often hear:
"Why doesn't Mesa put bias adjustments in their amplifiers?"
Well, there's a short answer and a long answer.
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. As any honest tech will tell you, there's lot's of easy money to be made by sprinkling "holy water" on amplifiers ... uh, what I meant to say is "Your amp needed biasing." See what I mean? What customer is going to argue with that?
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.
Now don't be fooled into thinking that tubes "draw" more or less bias, they don't. The way a bias supply is connected to a tube is akin to a dead end road, it just trails off to nowhere without really completing a circuit. It's a static voltage and regardless of what tube is in the socket -- or even if the tubes aren't plugged in at all, it doesn't change the bias voltage a bit.
So the end of the short answer is this: Since a bias supply needs to put out the right voltage and never vary, I wanted to build amplifiers that were individually hard wired to the correct values and NEVER needed adjustment. And for 25 years, that's how Mesa/Boogies have been built.
Time to change tubes? Just plug our tubes into any one of our amps and you're DONE. No tech needed. NO bills and no BS about biasing. And most important: The bias is RIGHT because it can't change!
Now, you want the long answer? Here's more information on how our hard-wired bias avoids trouble. Please read on.
But first, let's make an important distinction. Our business is designing and building high performance amplifiers. And for this we need tubes whose variance is within a narrow range. Our warehouse is full of rejects ... oh, they work -- they just don't perform within our tolerance range. We have a very sophisticated computer - based tube testing system (nicknamed "Robotube") that matches and measures tubes over seven important parameters. It can even predict which tubes are likely to have a shortened lifetime -- even though they work perfectly during the test.
Because our business is building quality amps, we can afford to reject a lot of wayward tubes. The guys you hear complaining because Boogies don't have bias adjusters are primarily in the business of selling tubes - not amps. They don't want to throw away 30 percent of their inventory, so they promote the idea that tubes outside our parameters can be used to "customize" amplifiers and they criticize us because our amps can't be adjusted to accomodate their out-of-Mesa tolerance tubes.
Now you might be thinking, "But I thought you just said that tubes don't "draw" bias, therefore they don't effect the bias supply and thus it doesn't need to be adjustable."
And that's right. Tubes don't effect the bias setting, but the bias setting does effect how the tubes work. But HOW it effects the tubes is difficult to measure.
When you set the bias (whether it's by selecting the right resistors, as we do, or adjusting a trimmer -- which is quicker) what you are doing is establishing the correct amount of idle CURRENT that flows through the power tubes. But you can't adjust the current directly, you can only change it by adjusting the amount of bias VOLTAGE that goes onto the tubes' control grids.
Voltage and current are NOT the same. Current is the AMOUNT of electricity, the "quantity" -- and is measured in amperes. Voltage is the degree of electric charge -- like the "pressure" to use the old water analogy. Let me illustrate how different voltage and current are:
When you scrape your feet across a carpetted floor in dry, wintery conditions, your body can become charged with 50,000 to 100,000 volts of static electricity. And when you reach for the door knob, a spark jumps and you feel it! The voltage is super high but the current (measured in micro-amps) is tiny - otherwise you would die from electrocution.
Contrast this with your car battery, which puts out a mere 12 volts. You can lay your hands right across the terminals and not feel a thing. Yet the amount of current available can run to several hundred amperes .. enough to turn over a cold engine and get it started.
So current and voltage are two totally separate electrical parameters -- though when you multiply them together, you get POWER, which is measured in watts.
When you set the bias of an amplifier, you are adjusting the static VOLTAGE at the control grid of the tube in order to produce a desired amount of idle CURRENT flowing to the tube's plate. A small change in grid voltage, produces a large change in the amount of current flowing -- and that's basically how a tube works. Say that again because it's super important: A small change in voltage at the grid causes a large change in current flowing to the plate. See, that's the essence of amplification: A small change causing a large change. And here it's a small voltage change causing a large current change.
The bias conditions are what determines how much current flows through the big power tubes when you're not playing. And what drives your speakers is fluctuations in that current flow when you are ARE playing. If the amount of current increases and decreases 440 times per second, then you'll hear an A note. If the fluctions in current flow are large and still at 440 per second, you'll hear an A that is LOUD!
But for purposes of biasing, it's the amount of "plate current" flowing with no signal applied that's important. Unfortunately current is hard to measure because the circuit must be interuppted -- as in "cut the wire" -- and the meter spliced "in series" with the broken circuit. But measuring VOLTAGE is easy. It is not necessary to interrupt the circuit because a voltage reading can be taken in PARALLEL with the circuit intact.
Thus, as a matter of convenience, most bias settings are given in volts at the grid ... even though current through the plate is the important factor. In fact plate current is so inconvenient (and dangerous) to measure that Fender doesn't even state what the correct value should be. They only give the grid voltage that will produce that current. (That's the minus 52.) But that only happens if the tubes being used are "in spec."
As long as the tubes ARE "in spec", the right bias voltage will always give the correct plate "CURRENT" -- but then there's no need for the bias voltage to be adjustable!
If the tubes are NOT in spec, then the only proper way to re-set the bias is to cut the circuit and measure the current while adjusting the bias ... but no manufacturer I know even STATES the desired current value! Be that as it may, when the original bias voltage is altered far enough, it will compensate for the tube's abnormal performance and the correct amount of idle current flow may then be restored. Clearly this is something most repair techs should not attempt.
Some newer amps have LED indicators connected to the circuit which will turn on when the right threshold of current flow has been reached. This is an improvement, and almost worthy if you're willing to except resistors and lights added into your amplifier's audio path -- which we aren't. The other "advantage" of this system is that it allows some amp manufacturers to avoid matching their power tubes. The thinking is that adjusting the bias to each tube separately eradicates the inherent differences between the tubes by insuring that the same current flows through each one.
Again, this has some merit .. but it's still not as good as using tubes that are matched in the first place because compensating for the mis-match causes the push-pull circuit itself to become unbalanced. Two wrongs don't really make a right.
Some of the other recommended biasing, "methods" -- such as -".. tubes running red hot, increase the bias .. sounds harsh and runs too cool, turn it down ..." are guesswork at best. Luckily, one of the great things about tube amps is that they can usually stand some abuse without causing any real harm ... at least not immediately.
But don't these alterations imply that you are second-guessing the amp designer and that there's a better set of operating conditions that the designer missed but the tube sellers have discovered?
Now some players may like the sound of their amp altered by tubes with extreme characteristics and with the bias set to help compensate. But often it is the mere novelty of change that they're really responding to and when the amp goes back to the proper original way, we've seen them be far happier still!
Because every part in every one of our designs has been meticulously evaluated, compared and stressed over -- no matter how seemingly insignificant it might be. And with every design we look for a "sweet spot" where all the parameters -- including the bias -- come together to give the best sonic performance, consistently and reliably. Every part and voltage is important -- yet no one complains that these other parameters aren't available for tinkering.
Consider our patented Simul-Class circuitry where there are two different bias voltages used for separate pairs of power tubes ... and changing one voltage also changes the other. Great care goes into getting this just right and we think we'd be asking for trouble to have it adjustable for the world to play with ... unless you like paying to have your amp messed up. Sorry, I meant to say, "Uh, ... your amp needed biasing."
If that doesn't appeal to you, then merely plug a matched set of Mesa tubes into one of our amps and you're ready for tone. Guaranteed. You'd be amazed at the number of service calls we field every day that lead to a diagnosis of out-of-tolerance, non-spec tube problems. To think these would be prevented by including a bias adjustment is something of an insult to you and us. If you put the wrong size tires on your car, do you think changing the pressure will make them right?
Please, don't think this is a blanket indictment of the other guys selling tubes -- it isn't. And their tubes aren't all bad either. It just doesn't make sense to pay more of your hard earned cash for tubes that were probably made in the same Russian or Chinese factory and which have the possibility of being outside the performance window we select for your amp. And it pains us to hear the hype and mystique built up around biasing when twenty-five years of evidence affirms our decision to make bias circuits that "never need adjustment". How much money and trouble that has saved Mesa/Boogie players you couldn't estimate.
Our rigorously tested and hand selected tubes are available at your nearest Mesa/Boogie Pro Center or from us directly. Nobody offers better price, quality or warranty than we do ... so why swerve?
Next time we'll talk about our part in developing the great Sylvania STR 415 type 6L6 and how we're on the verge of seeing something fairly close reappear on the market. Remember, we still have some of these super rugged mondo-bottles available for older amps -- Boogies only please! Until then, Relax, Breathe and Nourish your soul!
Cheers!
Mesa/Boogie Ltd.
Randall Smith
Designer & President
© 2004,Mesa Engineering Mesa Home / Articles / Artists / Products / Dealers / Contact Us / Pricing Policy / Registered Trademarks
To understand bias further, There is also an article on Class A and what it means, on the Mesa website.
So "fixed bias may mean different things to different people, but in regard to Mesa amps, it means "fixed, can't be adjusted".
7/02/06 Denver
12/02/06 Honolulu
NO drama !! After reading this article, it apins me to hear people talking bias mythology as well. There are endless bench racers out there who claim they can effortlessly improve the performance of one product or another with this mod or that, and sometimes they can, but there is alweays a pay-off. With motor-bike engines, is means they become illegally loud and lose mid-range. Guitar amps won't be different.
And before any starts talking about Keeley or Analogueman, I would say they are knowledgeable experts who have made significant design changes to achieve what they do.
No drama on this end either.
I wouldn't go so far as to say that bias is "mythology". It's just different on different amps.
Randall Smith is certainly a genius. I used to live right up the road fom him when I lived in Northern California, when Mesa was just becoming the amp to have. He deserves all the success he has because he's a top notch guy!
In 1982 Randall Smith made two 6 X 6L6 tube Mark II amps for a friend of mine, and for me who was on this board once. I needed more power for more clean headroom for me, for a band I was travelling with.
In 1983 he put bias pots in those amps because they were running a little off!
Back then the stock tubes weren't available and changing and more unreliable, so when players were on the road, it was hard to find close matches. I still have that amp and it's a beast! It's in your thread with all the gear pics.
When Randall says his amps are fixed, it means that it doesn't need to be changed as long as you stay with Mesa Brand tubes. You can change the bias resistors if you drastically change the tube style, though. With Mesas it's never really needed for the most part because those amps are so well done.
Not every amp is like that, though, so there is no harm in getting the bias checked, and if there is a tube change,especially to something different than stock, or if the amp is older it's not a bad idea.
It's like a tune up in a car.
You can drive across country in a sputtery car that misses and smokes and eventually get there,,,, or you can get it tuned up and enjoy the ride!
I've been playing electric guitar for about 40 years now. The first amps I played were Fenders that had no need for much adjustment,,, and really I was just wailing in the basement or in garage bands. Later in the 70's, though, when I was playing professionally, we all started taking note of everything that could make the sound better, and more reliable.
Don't be mankind. ~Captain Beefheart
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I was referring to fixed bias amps, vs amps with adjustable bias, and I know that you need to use tubes that are specced to be good for the bias you have fixed.
I am not disputing the need for appropriate maintainence and care of amps, only stuff which is not needed, but common belief would have people think is needed. I favour the fixed bias system because I used to live 400km from the nearest amp tech, so packing an amp and risking it in travel, not to mention not having it for a month was a major hassle, and I ended up just plugging new tubes in and accepting that their lifesspan, would be halved, which it was. STill cheaper and easier than trnasoprting it. I got it sorted as soon as I could.
I suspect that when Randall Smoith installed those bias pots, he also taught the guys how to do the adjustment themselves.
I got that letter when I got my Mesa amp last year and it kind of went against everything I had ever been told, mostly by guys at the guitar shop. I haven't changed the tubes yet, but plan to get a matched set from Mesa when I do and I'm not worrying about the biasing b.s. anymore.