Power Amplifier Circuit Design
Moving along out pre amplifier now you've got this signal that you've you've distorted however you want to you the cute it however you want tio and you like the way it sounds now is just a matter of getting it to make your speaker wiggle so there's a bunch of ways to do that the most simple way to do that is via a single ended power amplifier I think I mentioned that his auntie and appear earlier was that same single ended amp yeah so this is really basic probably one power to you've been there and small single ended out the transformer it's a totally legit wade amplify a signal it sounds real good. The only thing to consider is that if you're going for real high powered amplifier singling and might not be the way to go you can string together a bunch of vacuum tubes in parallel to one another to drop more current through a load and that's totally a way to get more power out of the single ended topology but there's a couple problems with that um one is that the main one is that the out...
put transformer of a single ended amplifier needs to be very large it needs to be what's called a gap transformer and there's a reason for that that will be easy to explain once I get into push poll amplifiers the's air a little more where there's a lot more like that vintage e old stuff real small simple amplifiers a local builder thd does some really, really great single ended stuff and yeah it's not too much to say about that I guess time being um push pull amplifier so this is like what we were talking about in the first section when you're you've got one to hear that's working the positive side of the signal swing and another tube here working the negative side of the signal swing um interestingly enough this one here is actually not a fixed bias amplifier there's no bias control to be adjusted in this amplifier and the reason that if that's the case and because this is what's called a cathode bias amplifier it's push poll and it still has one to working the positive side want you working the negative side you have a resistor here going between the cathode of both stages on this way to ground exactly like that voltage amplifier we were talking about for the pre empt it works the same way just current spits down through the plate on its way to ground across this resistor developing a voltage that takes place right here at the catheter both of these tubes and what that does is it biases the amplifier too given current and it happens automatically there's no adjustment to be made it's just a matter of knowing the correct resister in a place they're such the tubes will operate safely typically doing cathode bias like this, you end up with a a hotter running amplifier. Usually they buy some pretty hot you can only go so cold with catholic bias because it's all kind of feeding back together to make the thing work so there's not a lot of opportunity for crossover distortion, that sort of thing so it's a simpler kind of a more elegant, truer way to amplify using a push pull method rather than fix bias. But um, it's because it runs a little bit hotter. I think that talking about it before I think between lectures were talking about how why would they even bother switching to a fixed bias approach? It seems more complicated and clunky, but I think it might have just come down to economics. They thought, you know, we can get away with smaller power supply because these tubes and fix bias there off most of the time so they don't really doing very much work. So common ants that use this apology with the cathode bias would be stuff like the box sixty, thirty and matchless stuff and kind of all the ants and that those sort of families, if that sort of gives any sort of association for what sound kind of comes from that amongst a bunch of other things to make them sound the way they do that's one of the parts of the picture um and here's the fix biased push pull amplifier like what this amplifier is and what we were doing the biased measurement stuff here at this point she's got these coupling capacitors that feed the amplifier to separate them from everything before and as the signal find its way through into this area. It is biased at this negative voltage that we just apply here using the little twisty pocket we're dealing with before and then you can see here we've got these power tetro dhs like I was describing the first part of the first lecture that use a screen to help amplify uh more efficiently to these power tubes that's ninety percent of guitar and I'd say something like that you go with the thick spice push pull sent up and when you're doing this kind of setup, if two tubes aren't enough, you know if one pushing and one pulling our enough juice, you can use the same supply voltages but rather have two on each side or three on each side. In the case of this amplifier, having the bench onda more tubes you have the more current they'll draw and you can squeeze more power out of the thing, but you have to make some other changes as well it's not just a matter of adding tubes, you have to have a power transformer that can supply as many tubes is they're using and you have to have an output transformer that can translate your four or eight own speaker to the correct impedance that the tubes you're using want to see and the more tubes use the less impedance you want to see so there's some calculation that has to happen to do that all right this is a popular question about power amplifiers class a class b class a b class d it's a common misconception that class a it's like a class a is number one is the best thing for the good stuff it has nothing to do with the quality or the like mill speck of the wiring or anything like that it's it's completely away to describe how much of the duty cycle of the amplification of away form is happening the a single device so what I mean by that is if you have a tube and it's working the entire time their signals being amplified that is class a so if you had a tube it was on the whole time the single is going that's class a and it's always working it's always running if you have a tube that works this part of the signal and that none of these colors for this so the ad one is working this part the signal and then right in here it turns off that would be a class b amplifier like the push pull amplifiers we're doing because this tube is on and then it's off and then it's on and that's off but it works some part of the negative side of the signal swing uh and why a b will there's class b and class b is this simple off off so class b is a purely just working deposit side and cutting off so when we were getting all that crossover distortion that was approaching class b class b would be a proper stopping between positive sides and negative sides so um and class d to describe that is completely different it's almost like a digital sampling sort of amplification where the devices themselves are only turning on for littlest it's a little like you have like, you know, just like little moments of amplified amplification so it would just be working little like snippets and it's from frequency turns on on on on on on on and then takes that chopped up version of your signal and then it filters out all the like choppiness of it and kind of re creates an actual smooth way for him and that's how they get those little ants you see that air there like a you know, ten thousand watts but they're this big, you know, a little tiny crazy things um it's a different way to do it I don't know tubes don't do it that's that's typically a solid state gig, but so anyway that's that's ah put a d I don't know if that's very helpful in understanding that but basically is always working b is on ly working half the time and a b is working some combination of the two that it's it's working somewhere in the middle of those two extremes um and so some examples of the two class a I can't even think of a classic no can classy amplifiers would be like kurt single into damn all single ended amplifiers are class a you have one two or one bank of tubes that are working the entire signal swing uh and then class b would be like they're used for like cheap uh you know address systems like you know and uh and walk into the airport you hear real crappy sounding announcement recording telling you something or you know I think that's typically class b stuff it's kind of a real cheap efficient wait amplify audio not to sound great but just the audio and class a b is some approximation of the two ideas and that's what's typically used for guitar amplifier is mostly for I think cost effectiveness stuff and getting you know, two bands of heavy they're big transformers and I think going class a b is a way to get a cz much of that benefit without having it being the craziest heaviest gnarliest thing to move around and class d is the extreme uh kind of cost saver energy saver most efficient way to do it
Performing a proper setup for your guitar can seem like a dark art requiring a copper chalice filled with incense. Reading a schematic of a tube amplifier can seem like you’re staring at ancient Sanskrit. Guitar and tube amp masters Kurt Bloch and Ben Verellen are here to help.
In this two-part workshop, Kurt and Ben will show you exactly how to get the best sound out of your gear.
In part-one Kurt will teach the basics of setting up your guitar – you’ll learn about:
- Truss rod adjustment
- Bridge and nut adjustment
- String gauge and playing style
- Guitar maintenance and upkeep
- Guitar electronics and pickups
Part-two is your primer on tube amplifiers. Kurt and Ben will explain how they work and show you how to keep them sounding great. You’ll learn:
- Tube biasing
- Block diagrams and understanding schematics
- Basic amplifier maintenance
- Capacitor, resistor, and transformer replacement
- Speaker repair and power-matching
Kurt Bloch is not only Gibson Guitar’s in-house guitar guru, he’s a legend in the Seattle music scene. He plays in The Fastbacks and Young Fresh Fellows and has a producer credits for his work with The Presidents of the United States of America and Tokyo Dragons. Ben Verellen started Verellen Amplifiers in 2000 and now has a full-time staff churning out hand made custom tube amplifiers for some of the most respected artists in rock and metal. He also fronts Helms Alee, a rock band based in Seattle.
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