Hot/Cold Biased
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Guitarzan
Well-known member
Well as I understand it, The bias determines how hard your power tubes will work under playing conditions. The bias sets the "idle speed" if you will. If you set the bias hot you will have less headroom before your power tubes start saturating. Cold bias is usually for those who like the sound of the preamp more than power tube saturation. It's a preference. Personally I like them both they each have their uses, I do lean toward having the bias set hotter than most.
Guitarzan pretty much nailed it,just to add,most Boogies are biased on the cold side,as they are designed for preamp distortion,I do like them a little hotter,but you have to be careful how hot you bias a Boogies output,they tend to get too mushy if you try to get the output tubes to saturate as much as a lower gain Fender type output.I prefer the output tube saturation in most amps,but with high gain preamps you gotta go easy.
fatboy135
Well-known member
That is true, I think that there is a lot of info on the net about the bias but... I will try to explain one more time.
The bias point is the the exactly point where your amp is working, ok? When you refers to a bias point you are refering a voltage and a current, if you don´t say both of them you are not getting the exactly point, ok the bias point is in general the voltage that drops on the tube between the anode and the cathode and the current is the total amount of current that travels from cathode to anode, ok? well, the bias point is EVER a DC parameter, so we fixed the bias point with a continous voltage supply, and we isolate that continuous voltage with coupling and decoupling caps, ok??? A bias point also need to be refered into the graphics of the datasheet tube, you have two kind of graphics, the input refered to the grid and the output refered to the anode and cathode, ok??? so when a dude takes you a few dollars of adjusting the bias I´m totally sure that he isn´t fixing the exactly point, because that point needs a voltage and current measure and that is a good job for an entirely day. If you are adjusting the two parameters you are fixing the maximun power consumption of the tube, in continous the power is rated as P=VxI
So why is important to get a good bias point??? For several reasons:
- First of one only some fets transistors are able to conduct currents without any DC power consumption, all other electron devices needs a bit of DC power to polarizate and begin to work, now imagine that we have a sinusoidal wave, oscillating near 0 volts, if it is very small or even it is higher if we don´t have a DC feeding the tubes, tubes will began to work when the sinusoidal is strong enough to feed the tube to work, so what a nonsense isn´t it? we are trying to amplify our sinus and it is wasted to powered the tube, so the tube need and external feeds independly from the audio, so what happens if our DC is very low or the amp si so cold, well the effect is that DC is not enough to the tube to begins to work by itselfs so it takes part of the AC signal to feed it, so the matter is that low levels sinus or high level ( but when the sinus is getting to zero voltage ) is not correctly amplified, and generate a nasty noise, that issue is called zero crossing distortion and it generates a little square waveform that as I could remember from my university classrooms the fourier transform of that signal is a sinc with all the odd harmonics creating a thd jungle
- The second reason is for a power matter in that reason we could link with the first reason. When you fixed the bias point you are fixing the total amount of wattage that the amp could amplify.
ok now I will try to comment some ideas to check the bias, as I said fixing a bias need to get two parameters, the voltage and the current, for that reason there are several methods based on current and voltage, ok current methods aren´t recomended by me for several reasons, first of all you need to take some external elements and install inside the amp, second you need to measure the current that is hazardous than reading a voltage, ok I usually used two methods, first I take the amp and connect it into a real resistor of 15 ohms/ 50 watts to load the amp, then I connect the scope to the resistor and generate a sinusoidal wave with my function generator then I try to see what is happening on the scope near the 0 volts. With that method I ensure the zero cross distortion. the next test I usually use is to take the circuit and solve the equation of the tube, the equation of the tube is easy to solve you begin on the dc power supply and you end on the grid, so I read some values of voltage on the anode resistor, on the cathode resistor and I get with the equation the anode-cathode voltage drop and the current, next solving the equation and going to manufacturer datasheet I fixed the negative grid voltage, fixing the negative grid voltage you are modding the bias point because all the circuit depends of itself and the grid is one of the terms of the equation so if I move the bias trim the grid voltage changes, well with those two methods that i use I only found one amp that really needs to have a fine tunning of bias point they are Marshall amps.
Usually the manufacturer design and calculates the bias point thinking that a vast of tubes could be mounted without any bias change, so for that reason the isolate the bias matter with a good electronics design a allows the owner to change a bit with a ridicolous trim pot that have a little value compared with the other resistor on the circuit, well marshall is one of the manufacturers that gets a higher value pot.
So there are several class of amplification class A, B, C... through H the class of an amp gives you some ideas about how is designed, the mainly purposes the power, the bias point etc... the class of the amp is one of the mainly things that you need to know when you are buying it, knowing about the classes you could guess what are you trading.
So going now to your question what is hot and what is cold, well, as I tried to explain we have two graphics, now we will talk about the output graphic, so the output graphic rates the power of the amp ( voltage and current through the tube ) with several grid voltages, so now did you imagine which is the next step??? did you real want to learn how to calculate a bias point with that graphics???? ok I propose to everybody who wants to learn to make an easy drawning exercise. I have some datasheets of tubes so I could upload it like an image and you could download, then I will try to solve the easiest as possible a real bias point, with that exercise I´m really sure that you will learn more that another biblic thread, with that exercise you will understand perfectly what is hot and cold. So anyone interested???
Guitarzan
Well-known member
You Go Gordo!
fatboy135
Well-known member
let me a few days to upload the images, if you are the only one interested you could append me to mesenguer and I could explain you directly
A lot of info there,fatboy,great from an engineering standpoint,but for the average tech or hobbyist,there are enough charts out there to get you close enough,as one author/builder here in the states says"there is no absolute "correct bias",whatever gets you the tone you like and doesnt cause the tubes to redplate,is right".Of course this applies to guitar amps,the Hi-fi guys prefer the precision you describe.
fatboy135
Well-known member
thanks stokes think that I spend an entirely year of my life 2 hours per day studing electronics so it is very very difficult try to condensate all about the amplifiers in only one topic, only a few lines and in a foreigner languages, for that reason I recomend to everybody to read the book " Principles of electronics " of Paul Malvino that is the best, complex and easy to read book of electronics I have ever seen.
There are two attitudes, the attitude that people that ask what is a bias and you answer and that is enough and the attitude that people ask, ask again, ask one more time go to the librarie, read books, things about the matter... for that reason I propose that better to explain what is cold or hot to do an easy exercise to learn about calculate a bias point, of course only my great friend tarzan was interested, what a pity an amplifier forum and nobodoy whats to take a calculator a pencil and a paper
There are two attitudes, the attitude that people that ask what is a bias and you answer and that is enough and the attitude that people ask, ask again, ask one more time go to the librarie, read books, things about the matter... for that reason I propose that better to explain what is cold or hot to do an easy exercise to learn about calculate a bias point, of course only my great friend tarzan was interested, what a pity an amplifier forum and nobodoy whats to take a calculator a pencil and a paper
Yeah,there are different ways to approach these things,like I said,for a guitar amp you can find a chart with "acceptable" bias settings and tweak to taste.Of course these charts have to come from someplace,and that is why we need guys like you,who know the formulas and read the in depth,technical stuff,and are kind enough to share it with everybody.I remember the old days,before the internet,this info was much harder to come by,so you had to know somebody like you,or do the math yourself.Even tho it seems that few are interested,it is appreciated by some.
fatboy135
Well-known member
ok if you asked me as a tech I will say to you that the bias point is an strictly point that you fixed thinking how do you want to use your amp, if you asked me as a guitar player I could say to you that everyone has his own tastes, and I try to fixed the bias in a tech mode and then I try to do a fine tunning thinking on the tastes of the client, anyway I think from many years ago that the bias matter is getting crazy some people, they think that they didn´t sound because the bias is not good adjusted, and I always said the same, I have seen Steve Morse five meters away from me, playing with a peavey bandit and believe it, he sounds better than any tube amp, the trick??? very easy a good player and good vibes
For that reason after studing electronics engineer 4 years of my life and after playing guitar 14 years I recomend to people to not to care about the bias, about mods, about bogner.... just get an amp that sounds good and play as a lot as you can, and of course ever try to enjoy it
For that reason after studing electronics engineer 4 years of my life and after playing guitar 14 years I recomend to people to not to care about the bias, about mods, about bogner.... just get an amp that sounds good and play as a lot as you can, and of course ever try to enjoy it
"For that reason after studing electronics engineer 4 years of my life and after playing guitar 14 years I recomend to people to not to care about the bias, about mods, about bogner.... just get an amp that sounds good and play as a lot as you can, and of course ever try to enjoy it"
I have a lot of customers who play jazz,so they use their amps "cleaner" than most rock players.My customers are more particular as to how their amps are biased and can,as I can,hear,or feel a difference in a hotter or colder biased amp.Of course when you crank the amp higher into more overdrive,this becomes less evident.But when playing cleaner there is definately a "sweet spot" in the bias range.It is not as noticeable to the listener,but the player gets a certain "feel"or "response" from a fine tuned bias,like I said earlier,in a high gain amp it is not so critical.But even in my own MKIIC+ I like the bias set just right,more for the clean channel than for the lead channel.The rest of my amps are not high gain,more Fenderish or low powered Ampegs and I find over the life of the power tubes I am re-adjusting the bias more often to get the amp to "give it up"so to speak,even in cathode biased circuits.But like you said,even the best,finely tuned amp isnt going to make someone sound better if they dont have "it" to begin with.Some players,like Steve Morse,as you pointed out,and Stevie Ray Vaughn is another who comes to mind just sound great no matter what they plug into.
I have a lot of customers who play jazz,so they use their amps "cleaner" than most rock players.My customers are more particular as to how their amps are biased and can,as I can,hear,or feel a difference in a hotter or colder biased amp.Of course when you crank the amp higher into more overdrive,this becomes less evident.But when playing cleaner there is definately a "sweet spot" in the bias range.It is not as noticeable to the listener,but the player gets a certain "feel"or "response" from a fine tuned bias,like I said earlier,in a high gain amp it is not so critical.But even in my own MKIIC+ I like the bias set just right,more for the clean channel than for the lead channel.The rest of my amps are not high gain,more Fenderish or low powered Ampegs and I find over the life of the power tubes I am re-adjusting the bias more often to get the amp to "give it up"so to speak,even in cathode biased circuits.But like you said,even the best,finely tuned amp isnt going to make someone sound better if they dont have "it" to begin with.Some players,like Steve Morse,as you pointed out,and Stevie Ray Vaughn is another who comes to mind just sound great no matter what they plug into.
Restless Rocks
Well-known member
fatboy135 said:
Fatboy I hear you loud and clear but unfortunatly your wise words are lost on many. I read somewhere years ago an interview with Eddie Van Halen when asked how he got the "brown sound". I don't remember the quote exactly but it was something like " You first have to have sound in your head and your heart and it will find it's way to your fingers." There's just way too many who think that great tone comes soley from your equipment. Like you say you can always recognise a great guitarist no matter what he plays through.
Restless Rocks
Well-known member
I have a lot of customers who play jazz,so they use their amps "cleaner" than most rock players.My customers are more particular as to how their amps are biased and can,as I can,hear,or feel a difference in a hotter or colder biased amp.Of course when you crank the amp higher into more overdrive,this becomes less evident.But when playing cleaner there is definately a "sweet spot" in the bias range.It is not as noticeable to the listener,but the player gets a certain "feel"or "response" from a fine tuned bias,like I said earlier,in a high gain amp it is not so critical.But even in my own MKIIC+ I like the bias set just right,more for the clean channel than for the lead channel.The rest of my amps are not high gain,more Fenderish or low powered Ampegs and I find over the life of the power tubes I am re-adjusting the bias more often to get the amp to "give it up"so to speak,even in cathode biased circuits.But like you said,even the best,finely tuned amp isnt going to make someone sound better if they dont have "it" to begin with.Some players,like Steve Morse,as you pointed out,and Stevie Ray Vaughn is another who comes to mind just sound great no matter what they plug into.[/quote]
Russ you are so right on too! Hense the great question: " The Sweet Spot" is it art, or science? I hope that science can never quantize and duplicate it, like so many of our digital freinds have tried, and that true artists will always amaze us in finding it where science can't explain why it happens. But I also hope that with science we can continue to improve the tools with which to find it.
Russ you are so right on too! Hense the great question: " The Sweet Spot" is it art, or science? I hope that science can never quantize and duplicate it, like so many of our digital freinds have tried, and that true artists will always amaze us in finding it where science can't explain why it happens. But I also hope that with science we can continue to improve the tools with which to find it.
Restless Rocks
Well-known member
sorry Stokes that was your kudos for real. :?
fatboy135
Well-known member
yes dude, I spend a lot of time studing electronics I spend a lot of time customizing friends amps till I saw steve morse playing a peavey bandit, just in front of me. I couldn´t believe that extraordinary picking and feels that becomes from this little amp, so from this day, I always recomend the same, don´t care much about your amp, try to play and enjoy with it. If you are all the day thinking how to improve your amp you are the slave of amps, and that is just the situation to avoid.
if you think your amp is not in good condition ask a tech and end your thoughts, people that are all the day thinking on mods and customizers and so long are not enjoying theirs amps
if you think your amp is not in good condition ask a tech and end your thoughts, people that are all the day thinking on mods and customizers and so long are not enjoying theirs amps
I dont disagree with what fatboy is saying at all,I agree many players are too busy thinking their amp set-up will make them a Steve Morse or a SRV,if you dont have that "mojo" to begin with some amp mod aint gonna get it.On the other hand if you are an advanced player it can help,but you gotta spend a lot of time developing your technique first.I remember the first time I saw Jimi Hendrix in one of those first documentaries after he died,I'm sure a lot of you have seen this cut,Jimi sitting on a stool playing a 12 string acoustic,no amp,that magic aint coming from some amp mod,its inside him somewhere.As for science/art,when it comes to guitar amps there has to be a balance between them,cant have one without the other.
fatboy135
Well-known member
of course dude, if you are a great guitar player and you need to get 200 mph from your amp I recomend absolutely to take the amp to tech, but if you are a punky dude, drunken and not really care about your amp, you don´t need a tech, you need more time to enjoy your amp with your friends
Guitarzan
Well-known member
fatboy135 said:
How about a punky, drunken frijolero? 8)
