Preamplifier Circuit Design
Starting with the preamble of fire going back and look at the pre amplifier here looks so each of these here is a vacuum to gain stage and you know just pull one of these pre amplifier tubes out for staring at so this thing is really just these small little tubes here they're not doing a lot of power work there really just voltage amplifiers and so the basic job of this tube is to take your guitar signal and amplifying just the size of the voltage is not making any real work happen it's not having to make a speaker wiggle it's not having to make a drive a long cable to something it's not trying to do any real hard work it's just a maker bigger it's taking a little signal and making at large um and so realistically you could just have a little guitar signal come in and hit one of these gains stages and it would be large enough to drive the power amplifier and you'd be done at the end of it but there's lots of things you can do by using several games stages and chaining together in a cer...
tain way that make it useful for guitar and um the basic circuitry that is used for a pre amplifier gain stages what's called the common cathode amplifier and the way to think about this thing here is it's that basic trialled circuit that we were talking about earlier you've got this vacuum to scott a cathode it's got a plate it's gotta control grid so the cathode is boiling off a cloud of electrons this plate positively charged is attracting these electrons and your guitar signal is wiggling here on this control grid to dictate how much current spits from this cathode up into the plate through your load and developing this voltage output and is a couple of things to talk about concerning common catholic voltage amplifier one of them is so we talked about bias earlier for an output amplifier and how that is important well we have to adjust the bias for this common cathode voltage amplifiers well but you don't necessarily have to stick a negative voltage on the grid to control that the bias of the amplifier idol there's another way to do it and it's called cathode bias and it's ah it's a self biasing method it's real simple it's it's the oldest way to do this kind of thing basically what happens is current flows from this positive voltage supply towards ground through the tube and as it find its way through the two it drives itself across this catholic resister here and he'll remember the owns law voltage equals current times resistance if you have a current spitting across the resistor that current across a resistor is going to equal a voltage so you'll end up with a voltage here and that positive voltage that ends up cropping up right here with respect to say you have the grid it ground potential for d c meaning that this is essentially zero volts and maybe the current through this resistor brings it up to about two bowls now you've got this amplifiers now biased two volts there that's that's the bias voltage from grid to cathode and that's what makes it work the way it does but it happens automatically I mean you you just end up designing what values of resisters go there but there's no adjustment no tweak for that and that's what makes the whole thing work the way the way you want it to work there's ah another phenomenon of having that cathode resister in place there and that is that once that current spits through that catherine resistor there's two voltages to consider there is the biased voltage that we just talked about that happens when there's no guitar signal feeding in that's just what makes the tube stage this gain stage work the way you want it to work but when you spit your signal in there and your input signal swings positive now the control grid's positive which is going toe amplified the amount of electrons that shoot towards this load and when that happens that's going increase the current that goes through this whole chain from top to bottom and as that current increases so will the voltage that appears here at this point now what happens there is as that voltage increases in response to your signal, that's acting like a negative feedback phenomenon for the tube, meaning that as that was, as this voltage increases, it tends to a want the stage to draw less current so it's, kind of like a fighting what you're trying to d'oh it's, this negative feedback thing and it's without lunch near the equation is a little bit hard to totally breakdown, but the basic thing to think about is that, um, it's fighting your amplification, it's making this gain stage work, but it's fighting the efficiency of the amplification if you'd stick a capacitor across this guy, this capacitor you remember what capacitors are for a high frequency signal? Ah, high frequency signal will see that capacitor is a short, because for high frequencies, a capacitor is a very small resistance. So now, if you had your guitar signal wiggle, this current tries to jump through here. This is a sea current going through in that a sea current it's, wiggling fast, doesn't fight through this resistor, and therefore causing that negative feedback phenomenon toe happen. Instead, it jumps across the capacity to ground, and now you've got the functionality of the game stage that the cathode resister gives you, but your guitar signal doesn't fight, that it jumps across that resister to ground, and so and all effect of that is that you get more gain out of the habits, a game boosting trick, so you don't need it there and a lot and gagne stages don't have them, but you can stick one in there, and if you do, you get more game, um, there's a trick with that as well the value of that capacitor you, khun, very so if you stick a really large capacitor across that resistor, if that's the case, then all audio signals being a basie low note or high troubling signal will find its way through that calf, and it'll boost gain for all frequencies. But if you stick a smaller capacity across that resistor now, low frequencies will still have toe they'll see that cap is a brick wall, and they have to fight through this resistor, doing that negative feedback thing, but if you have a high frequency that will jump through, so now what you ended up doing is effectively equalizing the gain, staged to make it so that while bass frequencies aren't getting through amplified trouble, stuff is getting through its it's, disproportionately amplifying based frequencies versus trouble frequencies, so it makes a brighter it makes the game stage brighter and chinese air, and, uh, we'll talk about it more later, but it makes it a better signal for distortion for jamming these kind of gain stages into one another if you make it a brighter signal but we'll talk more about that a minute one of the really important thing I want to mention about the coming catholic voltage amplifier is that the signal that comes out of the stage is different and then the signal that goes into the stage not only in its size its its larger but it's also one hundred eighty degrees out of phase and what that means is that when your input signal swings up, the output signal swings down when input signal swings down vice versa and that's important because every time you flip the phase you are for example say you want to send these stages into one into another one after another each time a big signal gets distorted by a gain stage it's going to clip one side or the other side of the wave form and that becomes important and dictating how the distortion of the stage sounds um worth keeping in mind for we'll talk more about that in a minute oh and also another thing to keep in mind as well the size of this cathode resistor in its dictating how the stage is biased it will also dictate well, let me say this first so the I'll put it this amplifier will be distorted at some point so say your guitar signal can go in here and it can be amplified cleanly and clearly, but so you have a bigger signal that you sent in there some point this stage can't accurately amplify the signal you're putting in and it'll distort it how it distorts it is important and the basic important thing to think about is that if this is the signal that you're trying to amplify at the output you can have it distort in that gain stage by lopping off this side and not this side in that case you would have some even harmonic distortion and then you could do on the other side say this is not chopped off but rather the signs chopped off again that's also even harmonic distortion if you chop them both off you get off her money distortion now it's important to think about even her money distortion versus auto harmonic distortion because that's a big part of the different sounds of distortion that different amps make and you know, a lot of the benefits people freak out about about two dams will make them so cool is that they have a lot of the stuff that they do. A lot of their failures are prone to a lot more of that even harmony distortion which is associated with musical sounds you know, the timber of beautiful musical instruments they make these sounds automatically, you know, some beautiful sounding woodwinds and, you know, strained instruments just you hit the fundamental note, but the reason that the fundamental note on this instrument sounds better than that instrument is because the way it is made, it actually has a lot more of that even harmonic content um and let's see another way to talk about her monnet content it gets a little bit graphical and nerdy but it kind of paints the picture actually, how useful this gonna be? But I'll just say it anyway so if you if you have something that has a lot of second harmonic distortion content in it, what that means is that you hit a note that's one hundred hertz you're going to hear the hundred hertz come out but you're also going to hear something that's twice that frequency two hundred hertz so that's a multiple of the same frequency it's an active essentially and when you get that active thing happening you know think about what you do when you have an active on your guitar that's a it's a pleasant it's the it's the same note is the same general it's in the same world I guess and so even harmonic distortion could be thought of that ways you're adding distortion that is musical it actually is like a cord I mean it's not quite like a court but it works similar to that and that certain chords are, um there's a word for this uh some are dissonant and some are not so the not dissonant chords sound good even harmonic distortion is associated with that odd harmonic distortion is more brash, abrasive, nonmusical non natural sounding and so that could be really cool but it's just worth knowing what you're doing and how you're going through these stages and distorting them uh I'm gonna get a little ahead of myself there, but at least going into the rest of these devices we can talk about how they affect that phenomenon of distortion so another gain stage not gain stage but active device that vacuum tubes make in the pre amplifier is with called the cathode follower buffer so this is a important use of the vacuum tube and that it takes a signal of input and it spits out the exact same signal to output the same volume same phase relationship so what's the point it's exactly same one thing in the same thing out. So why would you even bother with that? Well there's a couple different reasons one is that you would want the input to be separated from something that's on the output so you want to have what happens here not dictate what happens here you want this side to see a very large and input impedance for example but you want the output to drive a load maybe so you want this thing to drive a really long cable run well it's easier to drive a long cable run with a smaller output impedance and a cathode follower of supplies that smaller output impedance think about a way to make that a little bit simple and useful. So, uh if you had a long cable run or so anything that needed work to be done anything that needed a lot of current to drive it so you know talked about anytime you drag a current across a resistor you get a voltage drop so say you have okay, this is the point where I have to talk about voltage division I guess so if this is year voltage signal going in and this is your voltage signal coming out this is v in the house r one r two so basically, you want to say you want to get the most signal out of this thing that you possibly can but you have this inherent are one here? Well, what happens is that the voters that makes its way out is some divided version of what goes in. So if you had there's a formula but it's just don't get too nervous about it uh basically to think about it is that this is going to think voltage going in is dropped across this whole resistor and this midpoint here, whatever it ends up being is some some components, some partition of what you put into it and the larger this resistor is, and the smaller this resistor is, the less signal is going to find its way out. It's going t take a bunch of way. You're just going to end up with a little bit some fraction of what you put in. So what you want to do in this case is you want to make this a smallest possible so that more of your signal that's coming in find its way to the output and less of it gets mucked up across this resistor. Because any time you spit occurring across the resister, you get a voltage. So all the voltage you want to find its way out here is going to get bogged down in this resistor. So the catholic follower is a good way to drive an output without a lot of loss, because this resistance it comes out of the catholic follower is very, very small, and so you end up getting more signal on down the line. So say, maybe here you had an equalizer circuit a bunch of, uh, resistors and stuff and capacitors, and needed to drive a long cable line. Well, that all those resistors and stuff they are large output, impedance is what you would call it, that you would want to drive that line well, putting this catholic follower in between the two now that q stuff is seeing this brick wall simple tube stage that doesn't load down and just trust me that that's the case I could joyce and stuff the basic idea there is the tubes input is a nearly infinite resistance so this is a brick wall there's the resistance is very very high so all of your signal from this will find its way into this tube but output it is this very very very small output resister they can then find its way to drive ah load easier and you won't lose signal on the way so that's what the catholic followers for I don't think I explain that well oh um move along okay this is kind of what I was just talking about oh was perfect actually so this is the voltage divider this is the idea of taking some of your input voltage and finding its way to the output but beings split between these two resistors so some of the ends of stuck in this resistor and the rest of it find its way to the output well this is a useful phenomenon we can use concerning rc filters rc filters resister capacitor filters and you can see this kind of same shape is what I have drawn over here but instead of just resistors we have a resistance and we have a capacitance so keeping in mind the capacitors are a very small resister for high frequencies and a very low larger sister for low frequencies instead of just shelving away some freak some portion of your voltage that's non frequency dependent such this treats base and trouble all the same way this just take signal away all of it. Where is this of here? If we send some troubling signal through here the troubling signal will see this capacitor as a shore and that's all that content will find its way spitting down through the capacitor so the only thing that gets left is this low frequency stuff. So if you want d c today through and you want the trouble to go away like the gentleman who emailed in saying he was worried about all the high frequency distortion and this happened you wanted toe maybe do something about that show the way some of the bright, brash stuff this would be a simple filter that you could incorporate into a circuit to get some of that high frequency stuff out. This is the most basic filters circuit out there, but it kind of paints the picture using this voltage division idea that you can spit away just the high frequency stuff and keep the low frequency stuff so that's what's called a low pass filter because it's let's little frequencies pass higher frequencies go away little pass filter hi how's filter same phenomenon saying general idea you the division between what's coming in and what's going out between these two devices except this time we've got it flipped around the other way now we have the capacitor here we have the resister here so now high frequencies jumped right through all the way to the output easy low frequencies they hit that capacity like a brick wall and they can't get through and lower frequencies have a harder time making their way through so a low frequency would see this as a very, very large resistor and on lee a little bit of that stuff would find its way to the output whereas high frequencies find a way if you just find high pass filter so high frequencies passed through low frequencies don't and so that's another it's another useful tool if you you know you wanted tio um you know you wanted brighten things up. You wanted tio get low frequencies out to some degree or most importantly in a vacuum tube amplifier to get from each cathode a catholic couple voltage amplifier like that first gain stage we were looking at each of those games stages is goingto have its own d c bias voltages and they're unrelated to each other, so you have to separate them and in doing that you used we called a couple in capacitor and it looks just like this and here's actually an example of three of these common cathode voltage amplifiers one after another and you might have this one biased up a certain way you might have this one by a stub differently in this one by stuff differently as well so you don't want the biased voltages of this game stage to be affected by the biased voltages of this game stage so you need this very important couple in capacity right here and remember that couple in capacitor it sees d c voltages bias voltages as a brick wall they can't get through so that's what separates the two so if you wanted to make that couple in capacitor non frequency dependent you could make it very, very very large capacitor that will let the lowest frequencies through the lowest basie notes through and the troubling stuff um I think it's horrible fires usually you actually choose that component pretty carefully to make it voice the amplifier the way you wanted teo and especially in something like what we're looking at here where you've got gain stage in the game stage in the game stage this is this is a distortion machine right here this is designed to little signal in big signal this can't handle its going to distort and then it's even further going to distort into this next stage and each one of these stages adding high frequency content each one of these distortion stages is really bright brash stuff so you could be playing a low the lowest note a bass guitar can produce and what you get on the outside is gonna have stuff all the way up. Ten. Fifteen k, you know, really bright, bright, nasty stuff, which could be cool, but you can actually do some things to dictate what happens. What comes out and couple of the tricks that air used we talked about before using the cathode bypass capacitor to brighten up a stage so that you shelve away some of the low frequency stuff coming out. You can do that even further with this couple in the past, sir, if you decide to take that couple in capacitor and make it smaller and smaller and smaller, a c thirty is a great example of that. They use a five hundred pico fared cap, which is very, very small for a couple of cap it on lee let's. Very bright signals through. Um, but then you take that very, very bright signal that you just wimped out by sucking out all the low in here and sucking out all alone here. And then he smashed into this distortion stage, and you smash into this other distortion stage and ends up the distortion characteristics and a rebuilding the signal with this totally different version of what you put into in the first place, um another thing that's very important, as faras voicing for this kind of stuff is what's called miller capacitance, so you can see right here between this resistor and this capacitor it's, the same exact picture of what we were looking at. So this situation, a low pass filter here, where you have a resistor in a capacitor, um, to ground that's what we're looking at here in this scenario? Well, this capacitor is not actual physical capacitor that's placed in the circuit, but there is a capacity tense between the control grid going into a tube and the plate at the output of a tube just by the nature of their being two pieces of metal. They have capacitance, it's unavoidable, especially when they're in such close proximity inside of a vacuum tube. And so what you can do is this actually isn't here. So this is just if you're looking at the actual tube stage, it would just be a resistor going directly to control grid. Now, if you have that resister chosen for a certain value, it will form a filter with that inherent capacitance to ground that capacities to ground in this resistor form that that low pass filter and what you can do with that is you can intentionally show the way a bunch of troubling stuff. So here you're shelving away a bunch of basie stuff shelving away a bunch of basic stuff here shelving away a bunch of troubling stuff so you're taking this broad frequencies signal taking away all the low and taking away all the top and you end up with this really wimpy thin, terrible sound and then you distort it distorted to start it take the stuff away distorted take the stuff away distorted take a step away and you end up with what sounds like a high gain guitar, a bullet fired with all that distortion if plenty of bottom plenty of top and you can really dictate exactly what that sounds like and then you can take that signal and you can send it into a tone control and then with this tone control stuff, you can actually actually dictate um even step just having hard fixed filters that are designed to dictate distortion characteristics you can instead take that distorted sound you've already created and actually kind of tune it. Andi that's you know, this is where people get into trouble with fifty knobs on there and you know it will be able to tweak every little thing but some basic circuits that people use for tone controls air what's called the tone stack it's the most common one you see it in almost every guitar amplifier that's out there it looks like this there's not too much to say about it really except for the dictating them the values of these components will make a difference and how it all works I would say that you might just have a look at some other circuits out there what values they're using to get an idea of the basics of what different tone circuits dio and you could tweet from there if you wanted tio there's also some handy tone tone stat calculator software that's easy to find on the internet where you can actually plug in values and kind of see how they affect the frequency response of a tone circuit but usually tone controls like this are the last thing that a preemptive stage would see so it would you take this tone stack filtered signal and then you would send that off to the power amplifier to make it bigger and make it wiggle a speaker um but you know in a similar way that before we were talking about just using the inherent capacitance is and what not to kind of show things away and equalize the circuit like that some makers air they just go about it really brute force they just take one of these tone stacks and instead of having potentially on matters like that this is like your base trouble saw your base control trouble control mid range control instead they just put hard fixed resistors and that same kind of circuitry and they just plug it in between stage is that way. They can actually like on the test bench tweak exactly the way they want it to sound going into the distortion stage. So these tones stack circuits are actually useful as a way to find tune the distortion. That print is making a cz well as shaping the distorted sound that you have made, um, and backs and all circuit is another common e q stage. It looks like that it's, one of the things that's handy about it versus the tone stack is it's it's, not as inner what's the word. They're not as intertwine, you know, with the tone stock it's a bit weird, you know, with the trouble control, you can see pretty clearly as you turn the trouble. Control up. It's. Choosing the output here is choosing mohr of the signal that comes through this tiny little two hundred fifty pico farid cap, which is letting albright signals through. Where is down here? You have these larger point zod point zero two micro fareed caps. This number of might not mean a lot to you, but basically, this is a little cap. This is a big cap, and you're choosing mohr of the bright cap or more of the little cap, and then and one of things that happens because of the way this works is you end up with a little bit of interdependence. As you turn the trouble up, you get less base. You get a kind of a seesaw fact, the backs of those handy because you don't have that interdependence, you turned the base up. Trouble stays where it is. His turn to trouble up base stays where it is like they don't affect one another. There's, no mid range control in this particular backs and all but there's ways to incorporate mid range controls in that kind of circuitry.