Phase Inverters....for the Common Man

greetings and welcome to what I think is probably the most ambitious technical video that I have attempted and it's going to be on a topic that I've had several requests to do and that is phase inverters now phase inverters are I think the most complex and least understood part of the amplifier they're also probably the hardest to explain but I think I boiled it down to fairly simple concepts and language here that should make it very clear to you and I all I ask is that you give me a chance to explain it and I think by the end of this video series it may take 2 videos I think you will have to admit you know a whole lot more about phases and burgers than you did before first let's talk about why phase inverters are necessary in single-ended amps there is no need for phase inversion single-ended amps as you all well know have one output tube they are generally Class A amps they operate at a fairly high idle current they produce a lot of heat they're not terribly efficient but they have great tone and we associate them with some of the classic amps of all time like the fender champ but just like a one cylinder engine is nowhere near as powerful as a two cylinder engine once you have two output tubes the output power capacity is much much greater also if you have two output tubes there are several other advantages they tend to cancel out a home they tend to accentuate even order harmonics and also they then operate in an a/b mode which in simple language means that the idle card is much lower so they produce less heat and they are more efficient than a single-ended amp but when you decide to use two output tubes you come up with a little problem and that is that you have to have a center tapped output transformer one end of that transformer connects to the plate of one of the output tubes the other end of the transformer connects with the plate of the second output tube the transformer center-tapped and the B+ or the high voltage the high positive voltage produced by the power supply goes here to the center tap and flows through this winding to this plate and through this winding to this plate now as we learned in the how do amplifiers work video series that I recently posted not only does the B+ flow this direction to the plate but the amplified output signal from the guitar or instrument flows the other way back through the output transformer remember we found that that direct current which is the B plus and alternating current which is the guitar signal can flow through the same wire at the same time without any mixing so we'll use the output transformer to feed the high positive voltage to the plates that makes them into vacuums as you remember for the electrons that are coming from the cathode and also then we're going to send the amplified signal to the output transformer and it is going to change it from a very very high voltage low current signal into a very rather low voltage high current signal that will operate the voice coil of our speaker and make our amplifier work now here's where the problem arises because it's a center tapped output transformer when we feed the signal in from this plate here and from this plate here if you notice we're feeding in at opposite ends of the transformer now think of the center tap as a mirror and it is it's like it's reflecting and where this right here is a mirror image of this down here now what that boils down to is this if we have a cig that looks like this and it's going to rise and fall it's sort of like an S that's tipped over in a counterclockwise direction if we feed this signal in right here to this tube and it comes out to plate and we feed another identical signal to this grid down here and it comes out of this plate the signals then are in-phase let's imagine that this portion of the output transformer is a horse with his head pointed in the plate connection direction down here another horse with his head at the point where the plate connects and the horses are tied together also these horses are specially trained when they receive a positive signal they walk forward when they receive a negative signal they walk backwards so let's send them an in-phase signal both of them then at the same time will receive a walk forward signal a positive signal this one tries to walk forward this one tries to walk forward they're tied together they go nowhere a lot of wasted energy next they receive both negative signals they both try to go backwards dip back into each other they go nowhere again wasted energy nothing the net output of them pulling in different directions or pushing in the same direction the net output is zero now let's send this horse the same signal it's rising it's positive he wants to go forward but let's invert the signal to the lower horse so when this one receives a positive signal and wants to go forward this one receives a negative signal and wants to go backwards okay their joint force their individual force is combined and we have a much greater force moving in one direction then when we on the neck cycle this horse up here will receive the negative signal and want to go backwards this horse down here will receive the positive signal and want to go forward they both go in the same direction and their forces are combined so when the signals are out of phase the output is maximized because both arrows are going in the same direction so now hopefully we see why we have to have a phase inverter if we're going to have two output tubes and if they're connected to the opposite ends of a center-tapped output transformer which is the way they always are connected for such a complex subject the rules are actually quite simple and there's only two of them okay number one rule if you drive the grid in other words if you put the guitar signal into the grid of a tube the cathode will follow and the phase will remain the same we put a rising positive signal in on the grid the signal in the cathode will be exactly the same the cathode follows but if we drive the grid the plate will invert the signal so that the signal that we send into the grid will create a mirror image or out of phase or inverted signal from the plate so let's say it ourselves our mantra for today is if we drive the grid the cathode follows and the plate inverts and now the second rule is if you drive the cathode and this may seem strange but you can actually send the signal the amplified signal into the cathode of a - you don't always just have to drive the grid but if you do send the signal into the cathode and drive it the signal on the plate will be in phase so if you drive the cathode the plate does not end bird okay that's our second mantra if you drive the cathode the plate does not invert okay hopefully that introductory information made sense if not you might go back and look at it one more time but if you feel like you're ready let's continue and look at three specific types of phase inverters which cover almost every type of phase inverter you'll ever find in an amplifier the first one is the simplest it involves a single triode phase inverter it's called the split load or an older term a cathode EIN phase inverter and this should be very simple to understand if you remember that when we drive the grid the cathode follows the plate inverts so let's send our guitar signal in here to the grid of the phase inverter tube in this case it could be one half of a 12ax7 well we said that the cathode follow so look here is the input signal here it goes from the cathode of the tube it looks exactly like the input signal they are the same it's going to go over here to the grid of this output tube and going to be sent to the end of the output transformer from this plate but remember when we drive the grid the plate inverts so the signal that comes in here to the grid of this phase inverter or 12ax7 half comes off this plate and it is the opposite it is inverted it is out of phase with the initial signal so we have exactly what we need here we have one signal that's unchanged and one signal that is inverted or out of phase we send them to the output tubes and they come out here and the horses pull in the same direction now this method of phase inversion is used in the fender Princeton the fender Harvard the Gibson GA 40 and a whole bunch of other amps it's a fairly inexpensive basic way to have phase inversion if you notice here this is the plate resistor and this is the cathode resistor and because we want these signals to be of equal volume or equal amplitude because I want this tube to drive the output transformer exactly that the same power as this tube I want to balance signal then these resistors should be the same we're going to see in some other types of phase inverter x' we're going to change resistance here and there to sort of vary that and balance of the signal now the advantage of this style of phase inverter is it's cheap it's simple very basic but you don't get something for nothing okay you do lose out on the fact that one of your stages here which your phase inverter stage has no gain remember the purpose of an amplifier is to amplify well this type of phase inverter does not increase the volume of the signal well let's face it if you bring in a signal here and split half of it this way and half of it this way there's no way that it's going to get louder if anything it gets a little lower in volume so the drawback of the split load or cathode on phase inverter is that there is no increase in volume in the phase inversion stage now just so you know I'm not making this stuff up let's take a look at a fender Princeton Reverb am now here are the two output tubes there are 66 s and look over here this is the phase inverter it is a split load or katadyn phase inverter as we just discussed look up here the plate drives the grid of the upper six v6 and the cathode drives the grid of the lower 66 it's exactly like the diagram and also as I promised you the plate resistors 56k and the cathode resistor is 56 K also notice the coupling capacitors that are always necessary to keep the direct current on the plate away from the grid of the next tube so we have coupling capacitors here and here and I've included them on my diagrams okay now let's look at the second type of phase inverter and that is the Peres phase phase inverter this is a sort of an old fashioned method it was used in the fender 5z I'm sorry 5c3 deluxe it's used in some other Newcomb amplifiers that you may see in some of my videos okay and we're going to have to try outs so it's more complex than the cathode dine or split load because it's well actually twice as complex there's two trials involved you do get gain in that this stage will not only phase invert but also increase the volume of the signal okay so you do get something for the added complexity let's look at it's very straightforward we bring the signal into the grid of the first of the phase inverter trials and it's remember it's a rising signal here rising to the positive direction and the output because remember when we drive the grid the plate inverts is reversed it's out of phase it's inverted we feed that in through the coupling capacitor to the grid of this output tube now we take the inverted signal send it down here and let's feed it into the grid of the second phase inverter triode and I think you know what's going to happen here when we drive the grid with this inverted signal the plate inverts the inverted signal and we end up with a signal it's back to exactly the way it was in the first place which is in phase with the input signal so now we have the out of phase signal going to this end of the output transformer we have the in phase portion of the signal going to this end of the output transformer the horse pulls that both pull in the same direction and we get a really big strong output signal now let's look at one final little detail here remember I showed you the resistors the 56k resistors on the split load or katadyn phase inverter well let's look at what we have to do here now I've sent my signal through one triode I didn't split it so I do get gain here then I send remember this inverted signal down here to a second triode wouldn't it make sense that this signal would be a lot louder a lot stronger because it's been through two tubes whereas this one's only been through one tube let's put a resistor here and leak some of the signal off from the grid so that we sort of turn the volume down on this signal and balance it so that the output from the second tube is of the same amplitude although opposite phase from the signal that comes out of the first two so as you can see you have to do something when you go through two tubes you have to suppress the volume of that signal or else this one would be driving the phase and the output transformer a great deal this one would not the horses would be pulling in the same direction but at very different amounts of force or strength and it just would not be a coordinated effort that both of the signals are balanced you'll get a lot better result now let's look at a real schematic and see how this actually functions in real life okay this is going to be the early Fender Deluxe uh schematic for the five c3 as I told you this is sort of an old-fashioned method okay just like in the previous diagram the plate up here this can be two triodes there in a6 s c7 a very kind of older style of a pentode tube and like in our diagram the plate of one of the trials is going to drive the grid of this 66 now let's see what happens we're going to bring our signal out here and instead of sending it all to the grid we bring it down here and feed it into the grid of the other triode so remember this signal up here was inverted because when we drive the grid the plate inverts and then we send the inverted signal down here to this grid and when we drive the grid with an inverted signal the plate inverts it back and sends it into this grid of the other six v6 which leads us to the third and final type of phase inverter and it's the most famous and most widely used it's called the long-tailed pair phase inverter used in the Fender Bassman the concert the band master a bunch of marsha laughs all sorts of amplifiers use this method for phase inversion as you can see it utilizes two trials just like the para phase inverter that we just discussed did except they're arranged in a very different manner here's our input signal driving the grid of the first trial by now you should see immediately what's going to happen when we drive the grid the plate in birds look up here here comes our signal and as you can see it's inverted and we sent it in the grid of the output - okay so this is very much like at this point like the pair of phase Drive the grid plate inverts send that signal through the coupling capacitor over here to the output - now instead of sending some of our plate inverted signal down here to the grid of the lower two like we did in the para phase let's connect the cathodes of the two tubes you see that's the fundamental difference between this and the para phase method we're not going from plate down here to grid like we did in para phase we are going to connect the two cathodes remember that the cathode follows the grid so if we have an in phase signal coming into this grid the cathodes in phase looks like this and it will come down here and this cathode will be in phase now remember rule 2 if you drive the cathode the grid does not invert so I'm going to send my cathode followed in phase signal down here to this cathode and when I do it jumps here and stays in phase so my upper 6l6 or whatever type of output tube we have here has a inverted phase signal going to the grid the lower 66 or 6l6 has an in phase signal because we took it from the cathode of the top tube send it to the cathode to the bottom tube and according to rule 2 if you drive the cathode the grid does not invert so here we go and we've got just what we need a signals that are in opposite phase from each other we feed them in to our output tubes they come over here to the output transformer where they now cause the horses if you will to pull in the same direction and give us a huge output signal now since I've talked about resistors on the other tooth this is probably the most interesting of all the resistor analysis that you might see on one of these as if resistor analysis could be interesting but I always wondered when I looked at a longtail pair why is this resistor generally 100 K and this resistor 82 K it would drove me crazy because it looks like look here comes my B+ in here going it's going to be feeding my my tube plates okay and it's like I'm being mean to this tube by putting a larger resistor in line it's it's like you have two kids here and you're given this one up here are more cookies than this one because the lower the resistance 82 K more of the B+ will go to this plate higher resistance we're cheating this plate or so it seems well remember what we had to do in the pair of phase phase inverter because remember here the signal is going through one tube and in this one the signal is going through two tubes so we have to cut back on the B plus the plate current and plate voltage that we apply to this tube because we have to lower its output a little bit because remember this signal is the product of two tubes this signal is a product of one tube so we put a larger resistor here to lower the plate voltage here to bring this signal down to where it's equal to its inverted twin now as we've done with the other types of phase inverter let's look at a real one here this is the Fender Bassman the classic double a 864 Fender Bassman look here see where the cathode are connected together right there as I showed you in the diagram we're feeding the signal in here to the grid and this is a 1287 rather unusual phase inverter but very made famous by Fender we bring the signal in here to the grid of the this half of the 1287 the clade sends the signal right over here to the grid of the 6l6 exactly like the diagram very straightforward but the difference is that we tie the cathodes together and remember that the cathode follows and there is no inversion here so this signal up here is inverted this signal down here is not inverted and sure enough there is the 82 K resistor which brings the B+ up here and gives more plate voltage to the upper triode than the lower tryouts and see it goes through 100 K of resistance so you see here this imbalance is what compensates for the fact that the signal has gone through only one tube up here and gone through two tubes down here well that about does it for this presentation and it appears that it's going to all be in one video hopefully the majority of not all of it make good sense to you if you're still puzzled about parts of it the beauty of having this available to you on YouTube is you can go back and review those parts and hopefully they will become clear for you I know that my analogies and leaving off certain little details will probably upset all the electrical engineers in the crowd but for those of you who like me that want just to have a basic understanding of how these amps work I you're my target audience and that's the way I directed this presentation and I hope that it's taken the same way so I appreciate your time and interest I hope this made sense I hope you know more now than you did before and I hope to see you again in the near future thanks so much

Channel: Uncle Doug

Views: 176,971

Rating: 4.9688473 out of 5

Keywords: Cathodyne, Split Load, Paraphase, Long-Tailed Pair, Phase Inverter, 12AX7, B+, Out of Phase, Inverted Phase, In Phase, Output Transformer, Class A, Class AB, Fender Princeton, Bassman, Concert, Gibson GA-40, Marshall, Fender Deluxe

Id: U6By31V9fDo

Length: 26min 37sec (1597 seconds)

Published: Wed Apr 23 2014