Adjusting and Understanding Tube Bias
So this is a big one people like to talk about is bias an adjustment of the bias for an amplifier. Um, so what is bias bias is the amount of current that is running through a tube in its idol state, and so it's most applicable when you're talking about an amplifier that is what's called push pull. And so what? That means it's a push pull amplifier, which is most qatar amplifiers and we'll get into a little bit more in the second half, but a push pull. And, um basically you have one tube working the positive side of the cycle and one two working the negative side of the cycle. So the way to think about that is you pluck your string, it wiggles outward and then it wiggles down. Well, the way ends up. Is that when your string is wiggling out that side of the way form so that's what's going in, will you get out of here? Is that this two works the positive side and this two works the negative side. So the way to think about that is that this tube turns on what it's a really, really bad line...
. So this too would turn on for this side of the cycle, and then it would turn off, and then this tube would turn on and then it would be, and then it would turn off. And this was they switch back and forth. So push poll this one's pushing this one's polling. So each two works half of the cycle of your signal. So again, this is reflecting, you know, the back and forth of your of your string wavering over a magnet on your guitar and bias is the idol current each of these tubes is drawing, and how that becomes important with a push pull amplifier is that if you have this to turn completely off at the moment you hit the zero mark, then you can end up with this little hiccup before the other tube has a chance to pick up and start working the other side of the signal. That is what you call crossover distortion, because as you cross over the zero mark, you get that little notch and that's something that sounds gravelly and bad. And so bias is the voltage that you apply at idol, so that if you had the tubes not idling at any voltage, or I'm sorry, any current whatsoever, this is the kind of thing you would see, so, like when you're doing nothing there's, no guitar going in, there would be nothing happening, neither, too, will be working any amount of current, but what you want to have a smoother transition from the positive side to the negative side so if you apply some biased current so that when you hit this zero mark, this tube turns on this tube turns on, but this one is still going a little bit for a little while it can smooth this hour, and so that's what you're adjusting for and the hotter you bias your amplifier, meaning the more current that you have going through each side of the of the signal swing, even idol, the less of that crossover distortion you'll get now, the colder you bias that the less current them or you're going to run into that notch, but the tubes will not be working as hard, and you'll get a little bit more life out of them. So it's a little bit of a balancing act of how much of the gravelly sound you want to avoid and there's some people who like a little bit that cold by a sound. If you want to go really, really hot, then you'll have a much truer representation of the signal you're trying to get but at some point it's to the detriment of the life of the amplifier and your power tubes so that's kind of the balancing acts faras that goes, do all amplifiers need ah bias adjustment? No, absolutely not. A lot of amplifiers use what's called cathode bias, which is kind of a self biasing circuit. It just there's some circuitry within the anthem. We'll have a look at it later that makes the amp just biases itself to a safe place directly. If you if you're dealing with an amp, you've already you own it and say that's catholic bias, you could just pop your tubes and then there's no adjustment of bias whatsoever, but with the amplifier is it is push poll and his fix what's called fixed bias. Then you do have an adjustment to make any? Do you have some say so over how that works? And later on in the second half, we'll talk about exactly how that works, but the basics to consider when you're just from a highly level looking at how this, how this stuff works out is that you just want to adjust the biased voltage, which is kind of like your spigot, deciding how much current the tube is going to draw to a place that is sounds good and it's safe for the amp to be running so there's a couple of different ways to do this, to adjust in and for the correct bias, and I got a little set up here so I can kind of do a little demonstration with a scope. A couple different ways people like to talk about the best way to bias an amplifier one is to just get a signal through the amplifier and onto this scylla scope where you can see the what the signal way form looks like, and if you see that notch on the output, you can just dial it out once the notches gone, then you know, you're the amfa's accurately amplifying the signal that you want, and you're good to go that works pretty well, as long as you know that you are operating the tubes in a safe range that's why a lot of people like to go with this different version it's called percentage of maximum dissipation method, which ceo, I guess which which way to do to do this? First? Here, let's, just do the scope method first, because that kind of shows you exactly what's happening. So I've got my amplifier here hooked up to a dummy load, which is this fixed resister here designed to emulate a speaker but it's handy for doing power maker for doing this kind of thing because it actually doesn't have some of the wave arenas and the frequency response that speaker does. So, um, after we do the biased stuff, we'll talk about power measurements, and this will become more useful for that, I'll explain why, so we'll see, um tampon I've got this little signal generator here that's just putting out a sign you saw it'll signal about the size that you would expect to come out of a guitar and it's feeding the input of the ant, and when you're doing this kind of thing, you want to have the pre amp of amp turned way down so that you're not crunching up or coloring the sound or the the way form of your signal as it works through the amplifier and have the output crying all the way up. That way you get the cleanest signal to the output, which is handy for the purpose of what we're doing here. So amplifiers that air fixed bias push pull amps will have a bias adjustment potentially ometer inside the amps somewhere and it's always a little different, you have to do a little research to find out what it is and where it is, but the idea is that you can adjust with that potentially ometer the amount of voltage they're controlling those output too. So it's, like you're you're controlling the spigot, how much current is going to be flowing through the output tubes? Um so I already know that this thing is set up somewhat safely so we can just go ahead and put a signal through it, and we'll just take a look at what the output wave form looks like so what you're looking at on the screen is going to be the scope probe connected to the output speaker jack of the amplifier and so it's going to tell us voltage versus time if you're looking at the scope uh um graph chart there so as I turn up the signal here feeding into the amplifier we'll see what comes out and then carefully is shocking myself you can see a way for him come up and you can see the notch there you see that notch kind of written across of the zero mark you can see that little hic up that's a little bit of crossover distortion that's not too drastic and that might be very well to sound completely clean if you're playing an aunt that looks like that you might not hear any distortion whatsoever and you're working the tubes pretty cool at that point so that might be a totally safe, reliable way to get the most life out of your too bad if you like the way it sounds but if you do one experiment with a hotter bias and why not? You can turn it back up get that crossover distortion going you can adjust the bias and as I turn up the bias, you can kind of see it smoothing out you're getting a more true representation of the sign decide going back hold again crossover let's cross over so anyway um so you might dial that in tow what you're getting and then dialed into wait till you like the way that it looks and then play guitar through it see what it sounds like see if you like the way it sounds if it sounds good and it looks good then the only thing left to decide is is it safe operating the way that I have it so now I have to do a little bit of math the thing to know is that you would look at your output tubes and say I've got these tubes they happen to be tongue sold sixty five fifties that's the brand on the model number of the tubes you can look up the data sheet for that too and it will tell you what's called the maximum dissipation for that too so so you look at that you know the maximum dissipation is thirty five watts that's how much that tube can handle that's how much he can dissipate before things start to go wrong for starts to fall apart a safe rule thumb is toe bias the amplifier between fifty percent and seventy percent of that value so you figure out what that is I had a calculator somewhere that I owe you are um so you could say because I can write this out too so just do a bunch of stuff in my calculator take my word for it um so say it's maximum dissipation equals thirty five watts well, so you want to go what's what's the hottest aiken go safely with this thing and you say seventy percent is that rule of thumb value so you'd say, uh, zero point seven really get writing times thirty five, which equals twenty four and a half watts so twenty four and a half watts is the most you would want to be dead debating a power tube this particular power tube to be within the safe limits. So now with an on and no single going through it whatsoever, you can take your meter and you need to know if you were called me saying power equals current times voltage, you need to know two things two bits of information the amount of voltage across the power tube from plate to cathode and then you also need to know the amount of current going through the tube once you know those two things multiply them together, and that will tell you how much power the amt is dissipating at idle. Um, so if you see foam own rules, he's the alligator clip and measure the voltage on the plate and again, you need to know the pen out for the tube to know where the plate is to know where you're measuring, but once you measure that value this case six hundred fifty volts tells me so yeah he's the alligator clip and uh so six hundred fifty volts of the bolted show you know um come back to this guy power equals voltage time's current so we just measured the voltage six hundred fifty bowls now we just need to know the current through the tube well there's a couple different ways to come up with that one way would be to disconnect the circuit and stick your meter in siri's with it and major the current I don't recommend that just because if anything goes wrong friar meter and disconnect things is complicated there's some companies that make what's called a bias probe it's a little device that you plug into the tube sock it and then you put the tube in after that and so that goes in between the two and just two little probe lines that come out and it will tell you the amount of current somehow there's I've never dicked around with those things but they're out there I know they are and they will tell you how much current that you're getting the way I like to do current measurements is to stick a one own resistor in between the cathode of the amp and ground and that one over sisters there just because you can measure a voltage across it with the volt meter which is easy and safe to use and that will tell you a voltage across a one own resistor and current equals voltage divided by resistance in this case of one own resister. So the voltage that you're measuring is equal to the current that you would be getting. So if you measure thirty five million volts that tells you that you have thirty five million amps going through the tube, so let's do that now, let's measure that current actually have one home resistors permanently installed within this am so price measure cross that one on one sister. This tells me twenty two million pes so twenty two mil amps point zero to two amps six hundred fifty volts point zero to two amps. Multiply them together and you get fourteen point three watts so you can see this's the number we're getting this is the number you want to stay beneath, so we're in good safe shape thing is running safely if we like the way that sounds like the way that looks that's totally safe. A lot of people just do the scope method and don't bother checking max dissipation. Some people just go for seventy percent max dissipation and don't bother with how it looks. Um, I tend to use all three using the math stuff using the visual stuff and user yearsas well, make sure they actually like what you're getting. So that's that's that business and then we'll have got the setup going I thought we do a quick power measurement to see how much output power we're getting from the amplifier and that is pretty similar setup we send a a nice clean sine wave into the input and we stick the output on a dummy loaded make believe speaker and then the formula for measuring output power sorry this has to get a little bit mathy so see power out equals so what? This is sorry z's impedance so that's your speakers impedance and voltage our mess is the average voltages coming out that I was describing earlier so when we look at the scope it's showing us the actual way for him and showing us the peak well we want to dio is turn up the output voltage of the amplifier and get it as high as it can possibly go before it starts to clip before it starts to square off. And once it starts to square off, you know that's the threshold of voltage that you've reached and so you would measure that peak maximum clean voltage and that is the voltage you would use to solve for this our mess voltage and then divide by the resistance of the speaker after you square that are missed voltage so voltage are a mess you call from earlier equals voltage peak divided by square root of two so let's figure out what that peak voltages with this thing starts to clip so as I turned us up, by the way if you're looking at the screen here and how much of this you can see but there's a scale from the vertical scale here every one of these blocks is equal to twenty volts okay, great turn this up so there were twenty volts peak forty volts peake sixty bullets peak get right up around that seventy volts peak mark there so I think it's probably yeah yeah so take that seventy bowls peak plug it into here in divided by route to and that will tell us the arm esselte we're getting so yeah that measures about forty nine votes are mess forty nine goals so now you take that and plug that into here and could take that value this is c here but the best way to do this forty nine squared divided by our impedance of the quote unquote speakers eight owns that's what I've got for that dummy load is in a dome and that equals three hundred so that's how much power you're getting? How did this amplifiers three hundred watts um math math math was trying to get to matthew but there's no real good way to do that but you can least get an idea for how that works any questions about eyes did does that make any sensors there's really first of all, when you were using the scope where you have the scope probes clip to the output yes the speaker output so you can have it wired have it clipped in parallel to the actual load you have perfect. We've got one from joe jones and two others want to know said have a tube amplifier which has a lot of high frequency distortion above fifteen kilohertz you hear coming through the speakers what could be causing this? Is it the bias? No, um there's nothing about the bias that would make high frequency distortion more prominent but there are tons of other things throughout the amplifier that would distortion is high frequency noise means high frequency it's distortion and you know if you have a fundamental of some low frequency and you distorted what we saw in the scope as I turn this up and you saw that squaring off those hard edges at the top of the way for him those hard edges are distortion to make that happen there's there's a bunch of nerdy math to explain it. But the real easy way to talk about it is that what distortion is if you're looking at it from a purely mathematical perspective is that if this is your fundamental and you chop off the top there like that what that equals one way to look at that is the sum of many different signals some of them the fundamental some of them twice the frequency and some of them like three times the frequency some of them four times the frequency all sign ways that all some together that air just a summation of higher frequencies so really that's complicated not very useful way to talk about the fact that distortion is the addition of high frequencies that's what it's doing is adding higher frequencies and there's lots of things you can do to help make that not sound harsh that we'll get into in the next section great and then maybe one more let's say very mixed says does all this owns thing affect solid state champs it owns and all that stuff absolutely affect sault st ants, but they work very differently with a tube amplifier we're using that output transformer to translate the exact impedance of the speaker to what the amp needs to see solid state amplifiers don't work that way. Salsa amplifiers can drive any resistance you put on output too their maximum extent that they can handle so if you have a solid state amplifier and you stick a sixteen on load on it, you'll get x amount of clean watts if you stick it eight on speaker on that same amplifier, you'll get more power out of it he's taken four on speaker you'll get even more power out of it, you stick a two on speaker, you'll even get more out power out of the thing. But at some point, that solid state amplifier won't be able to produce what you're asking it to do, and it will fail. That's. Why? Most solid state and fires just have a maximum minimum impedance that you can apply before you start to fry things. So two vamps you're matching solid state amps, go as low as you can, safely kind of way to think about it.