Shunt Regulated Push Pull Amp

hi guys this is a video to answer a question that one of my viewers asked he had worked on German radio that had what they called a shunt regulated push-pull preamp in it preamps in it and he wanted to know how these operated a little history I don’t know where this name came from it’s also called a series regulated push-pull it’s based off of an invention in 1943 that the actual name was a balanced amplifier the invention was actually used for or was designed for TV circuits and oscilloscopes using electrostatic picture tubes these are the type of picture tubes that instead of having a vertical and horizontal coils they use electro magnetics to move the electron beam back and forth across the screen to paint a picture they use for deflection they use plates and use electrostatic and the amplifier was designed to drive the verticals and we’ll get into why here in a minute here in a few minutes but it never really gained a lot of popularity wasn’t really used much that I can find although I think there was some Excel scopes that actually used it it was patent really by a designer and working for RCA so we all know how RCA deals with anybody it’s working with them and patents that come out of there anyway so you’ll find different names for totem pole which is what I learned it by mu follower cascade or cast coated cathode follower which is another somewhat popular name although that’s really not an exact name because that’s actually a somewhat different circuit so anyway what really are we talking about here well first of all to kind of understand how the circuit works we’re going to look at something a little different and then we’ll get to the circuit let’s say that we’ve got a voltage source and this thing comes out and feeds and I’ll use well we’ll just draw a box you’re gonna call it load okay now that could be any kind of load but it’s a load all right I could actually put a resistor symbol there because that’s really what a load is it can be impedance which is AC load or AC resistance you may anything but let’s say due to whatever reasons we need to drop the voltage to this load all right well one end is going to be connected directly in order to drop the voltage to it one simple quick way an easy way is put a series resistor in here and that’ll do it just fine you know we can figure out what size that resistor is we know what the load is and what the voltage being supplied and what we can do Ohm’s law get the current going through the circuit and know how much voltage we want to drop and then just do Ohm’s law and figure out the resistance and that’s fine as long as this load is steady no words the load don’t change and one one way one example of this is in your transformer less radios and TVs they tie the heaters or filaments in series and

to drop the voltage from your line across them so you can have you know some you know like 12 B EE 6 or 12 v eh6 and you know various different voltage film tubes as long as the currents all the same on each one of the tubes you know they require the same amount of current this is perfectly feasible and if you’re to count or whatever or or your line voltage if it doesn’t quite fully add up to what your line voltage is you can put a series resistor and drop it law TVs did this in the 50s they went from you know they quit putting Transformers in and they would tie a lot of times usually they had enough tubes they ended up having a tie in a couple parallel Series lines but they would have to put a resistor in because everything didn’t quite fully add up to the line voltage drop so they had to put a resistor in it and that’s a steady load I mean once the tubes heat up they draw X amount of current all the time that they’re on fully heated up they’re gonna stay steady at that current it doesn’t change so we get by with a series resistor just fine nothing changes but in this circuit if that load would change say like say B plus voltage going to tube your B plus going your tube the current net tube starts with fluctuating as you put a signal in and I mean that’s how operates you put a signal into your grid control grid and what it’s actually doing is controlling the current going through the tube so consequently the current then starts fluctuating on our load well when a current starts fluctuating then the voltage drop on this series resistor is going to fluctuate not very stable so how could I do this and change this to fix that problem well there was one quick way of doing that I go ahead and run this straight through well I’m not straight through woops sorry guys come here and I come down through a real simple just to resistor voltage divider I do this I can keep my current relatively stable going through here in fact depending on my choice resistors compared to my load I can keep it extremely stable within a certain reasonable adjustment of this lug current because really basically the power supply is seeing more this here this current loop here than it is seeing here this current will always stay this pretty well stable keeping the voltage drop and keeping the current through this one fairly stable so then the load sees a stable voltage at all times now it’s within reason you can actually go outside you can draw so much current or so little current that you can actually start changing the total circuit current enough that then you will get varying Goldie’s drop but within a certain reason reasonable amount this will stay stable and it’s used conservatively especially when you’re wanting to drop and have different voltages on your B plus maybe you know maybe some of your circuits need say 300 volts maybe some only need a B plus a couple hundred volts some may only need a hundred volts or 125 you can actually just start putting a bunch of different resistors in a voltage divider circuit there and control that to make it a little more stable a lot of times they’ll go ahead and put some filtering caps in there they’ll kind of staple things out too but this here is basically the same thing except we’re going to put tubes in here as this shunt regulated or series regulated push-pull amp or also known as or the true name of a balance amplifier now we’ll draw out the circuit and look at it so basically we have two tubes

now this guy’s going to go up here to be boss all right we’ll come down through here through a resistor to here connect off of this and come up and feed the grid here are the cap come true cathode resistor great resistor and we’ll have our load we’ll call it our L house is a very simplified drawing of this now this basically is something similar to what I had before where I had those two resistors feeding off to my load put that in as a resistor load L kind of similar but in this case we’re using tubes now what is really going on here well we feed a signal in here input to this triode tube and what happens and I’ll probably use a better pointer than that what happens here is course this starts fluctuating the current that’s going through this – as it should now you’ve got an AC signal going here this changes the current through this so we get a change in current coming out and what’s changed is the voltage drop across this resistor well coming off that plate is another line going into this tube hooks its control grid so the change is here affect this which is connected to the B+ through the whole series what that hat what caught what happens there and what that allows to happen as this fluctuates as the input signal fluctuates occurrence in this tube this will counter react in this tube which will keep a steady B plus and a steady current over all through the entire circuit now the way it does that is really kind of relatively simple now there there’s some extensive math to this if you is actually going to design the circuit and I won’t get into this because I’m just basically talking about the basic operation of it but if this signal is going positive at this point then coming out of here will actually be opposite negative going which is what’s feeding this as that becomes more negative less current flows through which allows this thing to be controlled with less current which reduces some of this drop here but keeps the voltage steady as we go the other direction as this becomes more negative we drop the current here that becomes more positive we allow more current to go through now this signal is being seen out here – like this now one of the big advantages of this circuit we’re actually to advantages one we keep things very steady on RB plus the drops across the two resistors here across are loads across the tubes that remember tubes

have internally plate resistance so those have to be tooken in account we keep everything steady as far as B plus the other nice thing about this is if my B+ fluctuates things stay steady also we don’t have to worry about it as much so in cases of not I don’t want to say really TVs because really law TVs don’t use highly regulated power supplies but a lot of your test equipment like oscilloscopes do you want to keep your B+ real steady so that throughout your entire circuitry and especially your sweep circuits are and your deflection circuits everything stays steady so you get a true representation of wave on the tube of what you’re measuring so everything’s got to stay steady the thing it can fluctuate B plus is that heavy loads you know this is going to go ahead and start feeding the rest of your amplifier and in audio sense it’s feeding audio amplifiers which draw a lot of current which fluctuates a B plus some what so this thing also this amp will keep that signal going through very steady even if it B+ changes and any changes internally in in the circuitry here everything stays stable the other advantage is it acts as a cathode follower now what is so nice about cathode followers a regular cathode follower is not much of an amplifier it doesn’t have pi in fact gain is less than 1 but it does allow for very low impedance a very little impedance output now to kind of explain that a little better we need to look at and at some resistance or impedances why do I want a low impedance this is go feed tubes you know the grids of tubes this you know your load will be actually a grid of then of the next stage of the tube which is a high impedance why is that so really vitally important well you need to understand one thing about load and and this sometimes becomes a very confusing thing if I would take two lows let’s just say two resistors again guys I’m not much of an artist so I got one resistor here and one resistor here and this is say a 10 ohm resistor and this is a 1 Meg ohm resistor which do you think is the bigger lugged well think about that for a minute is the 10 ohm a big load or is the 1 Meg a big load well let’s kind of look at this a little bit if I supply the eat both of these either one of these with 10 volts no birds I have a $10 op you know I hook them to a 10 volt battery and I figure out the current because the load heavier loads more current at once if I figure that out well then I just take my 10 volts and I will divide it out and here I have 10 volts that’s going equal 1 amp well what about this guy well that’s going to be a lot less it’s

gonna be about ten milliamps actually less than that be about ten microlabs there that’s better well 10 microamps is an extremely small current you know that’s point zero five zeros and then one that’s one amp so 10 ohms is an extremely heavy load compared to 1 megohm and that that comes back to your that will come back to like your meter loading you know we we talked about this you know ohms per volt meters and stuff like that and you know the digital meter versus the VT VM and stuff you know these digital meters like a 10 ohm 10 mega ohm impedance and stuff and so on okay getting back to this this may be feeding into a grid of a tube and that grid could have you know upwards of 1 Meg input impedance if everything is relative so if I’m looking if I’m feeding a high impedance with a low impedance as far as this circuit is concerned the lower its output impedances the higher this appears as far as its input impedance and less load that it is so the lower I can get this the bigger difference between these two the less current I will draw from this and the more steady this whole circuit becomes and with a because the less current and I can actually drive this tube with a lot more voltage and still keep my current levels real low and that’s what I want to drive an output tube or any be could be even a an input of another preamp stage going before it gets the output to but to drive that if I want to get a lot of power of this I want a pretty decent large swing number it’s a very large swing in my voltage but I don’t want to have to supply a lot of current to that because if I have to supply a lot of current with that amount of voltage it’s a lot of heat a lot of wattage straw which is gonna drag my B+ down which is also gonna work these tubes if I can keep this real low impedance respect to this this becomes a very very low not very heavy load very low load because of the difference between the impedances or resistances and which means then to drive it with this high voltage peak to peak big swings will take near as much current I don’t draw as much current through here B+ stay stable these tubes stay happy everybody’s staying cool and I stay linear meaning that this output going into there will be if it was a hundred percent linear then it will be identical to the input and when it in other words basically the only difference between the two would be just as amplified but otherwise the waveform looks identical other than its larger that’s linearity and in order one way of definitely guaranteeing good linearity is keeping this tube this preamp operating at low current and in order to do that the best ways a Catholic follower but I get the cathode follower here but also get application here normal cathode follower has no application in fact it’s lucky it’s less than one but if I can get application

and the low and output impedance of a Catholic follower then I’ve got something but again the whole that explains what it is the operation of it basically is is you’re running from here to here is your power supply this basically goes to ground you have a current flow through this when this fluctuates this does just the opposite keeping that current steady going through here at all times steady current means for linearity keeps everything operating in a linear fashion within reason and also any fluctuations in B plus will be met with staying stable too because meet the fluctuation of B plus will not affect this circuit because the fact that it’s fed the way it’s fed because this connection in the series fed with the cathode to the plate that’s coming up here controlling this to respect to this tube and it’s called a push poll because that’s what it’s operating as one pushes one polls back and forth back and forth depending on where the waves at what’s going positive what’s going negative there’s just push pull I hope that explains some of it you know I try to keep this in a short short video so much for that I tried to keeping it very simple the big the biggest thing to realize is whatever is going in here is fluctuating right here as this fluctuates the cont through this tube changes and fluctuates which then is the opposite effect up here which keeps the total stable the current flow through the total thing so you know one way to kind of look at it in a perfect world you could say that say this current in this tube is fluctuating by say 10 milliamps then really technically this one is doing the same thing but as this current goes say negative 5 milliamps then this one will go positive 5 le amps making up for it so the overall current stays the same when this swings the other way going to positive 5 milliamps over you know a set current then this one goes negative 5 milliamps again keeping everybody the same across the entire thing and that operates from the simple fact of this grid circuit being hooked the way it is so I hope that answered your question and understanding of it if you have any more questions just post them or anyone else does and I can go over a little more in depth and do a do a little better job of showing waveforms and stuff and everything but that’s really it in the nutshell and how it operates and why it’s got some gained some popularity some and stuff it’s not a perfect circuit by any means and it wasn’t really designed for audio it was actually designed for sweep circuits so but in any case that’s it in a nutshell so I want to thank everybody for watching and I hope this answers your questions and if you have any post them and I’ll make another video to try to answer them so thanks for watching guys and I’ll see you guys in the next video