Audio Output Transformers Demystified - Ask Mark

hey welcome back everyone another ask mark session here at blue glow electronics most of our a smart questions and answers are fairly simplistic in nature of this one's a little more in-depth so you may actually learn something here first up on email here from Tim Bertram he says mark all of your videos I'm a novice engineer getting into restoring radios I purchased a beautiful Grundig 4570 a week ago with a right channel not working he basically says he's traced the problem to a disintegrated output transformer see the photo and the right channel basically output lead from the EO PLL ad output tube to the transformer was disconnected from the attached schematic you can you help me determine if this is a single-ended transformer or a push-pull type transformer how would I determine the correct transformative review to use as a replacement by the way I sent this question to the folks at Hammond the transformer manufacturer and they could not give me an answer so anyway Tim I'm gonna try to give you your answer today okay before we get into answering Tim's question we're gonna break this video up into two segments first I'm gonna answer Tim's question about it being single ended or push-pull output transformer then I'm basically going to go into some depth about output transformers and I think this will be an excellent class today so stay tuned if you want to be a student okay we're gonna pull up this schematic here and Tim was kind enough to provide one I did take the output section and blow it up here just so we can zoom in on it because it's really all we need to be able to figure out Tim's couple of questions here his first question was is this a single ended output transformer or is this a push-pull output transformer well we're lucky enough to have a schematic if this wasn't it was just a transformer laying loose you found at a yard sale or something then we got a different scenario and maybe I'll make a video on that one David this we have the circuit which helps us a lot in identifying the transformer so pretty simple let's let's start talking about known facts that we know okay this is the speaker and we know that this is the right channel and we know this is the left channel it tells you right here right channel we know that this speaker is being fed by this output transformer and we know this speaker is being fed by this output transformer okay we're just walking up the line we know this output transformer is being fed by this tube and we noticed that this output transformer if you notice the plate coming off here feeding into this output transformer this output transformer is being fitted by this tube okay so to have a push-pull configuration guess what you must have driving an output transformer two or more tubes okay so we can deduce from just the fact we have a single output tube here driving the output transformer that this is a single-ended transformer right here and not a push-pull now we can dive in a little deeper and if we wanted to into the actual transformer which i think is worth us doing and I'm gonna redraw this transformer right here cuz it's a little funky everything Philips does with their schematics are a little funky if you notice these little resistors here that's not a very common way of drawing these resistors any rate we're going to draw the two sides of the transformer as ugly as I can here on camera and we know this is feeding the out the loudspeaker and we know this is feeding over here but we've got three wires here coming out of this so we know one of them is a tab somewhere in the middle or potentially off of the middle of this so we've got three wires coming out of this transformer here we've got a green wire black wire we know that's easy on this one we've got a blue up here on one end a red and a yellow oh well the blue is coming off of the plate here which is typically all the way at one end of the transformer however the other side of this there's two wires here and let's take a look at this wire right here that comes down it's yellow it feeds down through a resistor same off the other one they tie together where do they go they come over here and they feed into the screen so all you'd have to do is pull up a picture of this l88 pull up a datasheet for it and you would see that this pin right here is the grid right I mean I'm sorry the screen so tied here to the screen coming off over here tied to the screen we now know that this is the this yellow wire is the screen towel so we can put it right here it's somewhere in the middle between the two ends of this transformer next up we've got the red wire coming off where does the red wire feed it feeds down ties in with the other red wire comes down here ties into what is part of a filter capacitor here it's also tied over here comes around into our full wave rectifier right here and as you can see so we're feeding here with B+ down this line coming ups being filtered some by this electrolytic capacitor it's being fed up it's fed into this output transformer it's being fed up it's fed into this output transformer so we know that the red wire is at the other end of the winding so this represents the length of the windings inside the transformer redzone and 110 blues on the other yellow is somewhere in the middle we know that because it is a screen tap and screen taps or somewhere in between these two now a push-pull transformer has three taps on it typically okay except for in a push-pull configuration this red wire would go to the plate of one tube this blue wire would go to the plate of the other tube in the output push-pull configuration and the B+ would be fed in via the middle wire it's not this scenario we've got going on here okay oh yeah by the way well I've got it this little device right here you may wonder what the heck is that well that's nothing more than a jack an output jack that you could use same here you could use these to bypass the speaker that's what this little push here is and these jacks are on the back of the unit so in other words you could plug in an external speaker and bypass the internal speakers okay one more known fact we're gonna notice while we're here looking at this schematic is this is a 5 ohm speaker so we've got a known quantity right here alright now we've answered the question as a single-ended Paul let's now go to figure out how oh yeah by the way we know this fact ok let's talk about our known facts if you guys remember back in science class or math class if you're trying to solve an equation the first thing you want to know or are the constants or things that you know we know that the output secondary is 5 ohms because that was on the schematic okay we also know from text books on transformers that the windings ratio is directly proportional to the voltage ratio and today for what we're doing what we really need to work with is this voltage ratio and I'll show you why in a minute and that is equal to the voltage on the primary divided by the voltage on the secondary in other words it's the ratio between these two okay then we know from the textbook also that the impedance ratio and that's what we're really chasing today is going to equal to VR squared in other words the voltage ratio squared and one last little unknown fact here I'll leave you with is uncle Doug made a very good video on this topic if you'll search for it I'm sure if you type in uncle Doug transformer will come up to great video ok before we can truly help them figure out which type of transformer he needs we need to learn a little bit about output transformers themselves and so hang tight this might be a 5 or 10 minute lesson but I think it'll be worth it and help make sense of a lot of this ok what a transformer is an output transformer is it's nothing more than a coil of wire and another coil of wire over here okay and these two coils of wire are not connected to each other electrically that is very important okay I'll do these two little lines here this represents the iron core and a transformer we'll talk more about that in a minute but imagine this is a coil of wire and this is a coil of wire okay and what happens is we feed an AC signal into this coil here okay what happens is it induces a AC signal in this coil that feeds back out through its connections purely through magnetic flux okay and so what happens is the closer you get these coils to each other the easier it is for it to induce this voltage across and if you wrap these coils kind of up through the middle and through the middle here and then in between them with an iron core the permeability of that iron core keeps this magnetic flux kind of tightened in and it helps with that transfer from one coil to the other in the most efficient and effective way possible okay and so at the end of the day what we're doing in a pen aid amplifier is on this side we have the power tube right that's doing the amplification taking a little bitty signal in your amplifier along here it turns into a great big signal this great big signal has two things it has high voltage okay associated with it typically anywhere it's - its equal to the plate voltage plus or minus the swing here of this signal in the middle so it could be as much as three four five six hundred volts we're dealing with here on this side also the the power tube here has an extremely high internal impedance to it okay so it's looking for a load that has a high impedance to maximize the power transfer from this tube into this coil here okay so what we're wanting to do here is have a high impedance which we donate with Isaiah here okay what we want to do is transfer this AC signal out the other side and we're wanting to kind of transduce it back into a smaller signal okay smaller voltage over here so if we let's just make this up we had 400 volts going in on this side out on this side we might want to get out 20 volts or so well here's the good news when you induce an AC signal into this side the number of windings in this coil and the number of windings in this coil matter okay so if you fit in a 10 volt signal here and let's say you had 10 windings and on this side you had a hundred windings okay it would come at in volt signal into this 10 the ratio of windings 10 to 100 here all of a sudden on this side you're gonna get out a hundred volts on this side because you've got a 10 times multiplier in your windings ratio here okay which is directly proportional to the voltage ratio here 10 volts in 10 windings or could be 10 volts in hundred windings and then over here it could be a thousand windings either way it's a ten to one ratio you're gonna get 10 times so out instead of 10 volts comes a hundred volts on this side well the inverse happens with current if you were feeding in 10 amps of current here that 10 to 1 ratio there this side gets out 10 times less current so out this side what you would end up with instead of 10 amps that you fit in here you would get 1 amp okay so the 10x multiplier and voltage 10 times divider and current here on this side so what we're gonna gain here is high voltage high current I mean high impedance low current here right out this side what we're gonna end up with is low voltage we're gonna end up with low impedance and we're going to end up with high current here on this side that is the purpose of an output transformer you say well why do I need that why not just connect my speakers up right here first off speaker coil impedance the winding in the coil it's not so typical tube output might be looking for 3000 ohms of impedance okay that it's wanting to see to effectively transfer well speaker output won't be 3000 it's typically what 4 8 16 ohms right so you need something to transfer that over second off you connect your power your speaker leads over here and you put 400 volts on them right and somebody comes along and they're like oh the speaker leads loose on the back of the speaker let me tighten it up a little bit you just introduced an external 400 volts to your amplifier out here on these wires going across the room that your 2 2 year old toddler might walk and pull out of the back of your speakers next thing you know your two year old toddler got zapped with 400 volts it's a bad thing so we want to play with low voltage we need this low impedance because that's what speakers are and we need the high current to drive these feeder speakers effectively so that's what this output transformer does at the end of the day is it transduces all these things from one side to the other alright if you've got that hang tight I'm gonna raise the level of the lesson up just a tad bit more and we'll get to the next step then this is probably the most mysterious part that people really struggle to get their heads around so I'm gonna try to take my time and break it down for you simply here when you go to a website like Hammond or Edie core or you name the output transformer and you look up and you find the specs on a transformer and let's say it says it has an 8 ohm primary and it has I'm sorry as an 8 ohm secondary it's primary is let's just say 3000 ohms okay this unit can handle 10 watts of power how much how much it can transfer from here to here without this transformer overheating and losing efficiency and having problems ok so that's measured in watts and then there might be one others a little speck that shows up and it's called DC resistance keep in mind everything we're talking about is AC feeding an AC signal and getting an AC signal out well if you hook up a DC multimeter a digital multimeter here and measure from here to here there will be the winding resistance from a DC standpoint from one side to the other and I'll just make this up maybe it's a hundred and 50 ohms that does come into play sometimes some transformer manufacturers give you the DC resistance of the windings some of them don't advertise it so easily so I'll talk to you a little later about where that might come into play it okay it's a little well relevant to what I'm talking about right now but let's go back over here to we've got our input we've got our output our primary our secondary got this iron core in the middle if you found a transformer that had 8 ohms on the secondary and 3000 ohms only primary I need you to understand there is nothing about this transformer that is 8 ohms or 3000 ohms it is purely a representation of if you feed ok an 8 ohm load on this side from an AC standpoint this side will see that interpreted because of the windings ratio and the impedance ratio here it will see it interpreted as 3000 there's nothing you can do there's no meter you can hook up to either side and measure and get 8 ohms or 3000 ohms so in reality there's a ratio here if we did the math and divided this out so in reality fired my choice the way transformer manufacturers would list their transformers for sale would not be this and we did the math here 3000 divided by 8 equals 375 that is our impeached impedance ratio 375 to 1 okay for for every 300 75 ohms of impedance over here seen let's just say the load is here it might be better set it this way for everyone own of impedance seen over here this side will see 375 ohms it's all about this reflected impedance as a result of this transfer and kind of the internal for back of letter lack of a better word I'm hesitant to use the word reactance here because I haven't we haven't talked about that but it it is the hindrance that this sees the amount of impedance based upon a 1 ohm if this bumps up to an 8 ohm speaker you multiply that it sees 3000 ohms over here so if I had my choice transformer manufacturers would not do this instead they would say the impedance ratio of this transformer is 375 to 1 it's power handling capabilities is 10 watts okay I might even talk about the bandwidth of this transformer this transformer is good from 10 Hertz to 30 killers that and then maybe the DC are okay the DC resistance of the primary here that's what I'd like to see transformer manufacturers step on their transformers and they used to used to find a lot of military transformers with this kind of stuff on them okay things have gotten simple when people were working on equipment they knew them they knew this impedance they knew the speaker impedance right they can look up in a tube manual what this power tube is looking for and it made it easier to go to label our tubes or transformers this way instead of this way this way it's independent hey if I wanted to run this with a 4 ohm load over here it's 15 hundred ohms over here and you can do that you can take a transfer this that's labeled this then stick it on a 4 ohm load and get a 15 hundred ohm impedance on the other side of it so don't get wrapped around these or specs for the transformer they're more another way of showing using common known loads this impedance ratio that I would rather see them labeled with I hope this makes sense if not ask questions down below we will try to answer this it is the mystery that needs to go away here we are back with our sheet here's the specs that we need to go to look up for Tim okay we know this one 5 ohms ok we don't know what the primary is here right we don't know what the screen tap is we don't know the wattage here ok and we certainly don't know the DC resistance here of this this but what we do know ok we know the blue wire and the red wire or the fatherís apart from each other here because we've studied this schematic a little bit you can also use an old meter to figure that out you will have more resistance from the blue wire to the red wire than you do from the blue wire to the yellow wire or the yellow wire to the red wire okay what you cannot do is use the ratio of resistance here okay to try to figure out your screen percentage we'll talk more about that in a second we do know that the green wire and the black wire here are connected all right if I were you Tim before I went any further on this journey I would get out my plain old multimeter of some sort here something along this lines I would put it on resistance maybe turn on the continuity beeper and I would measure between here and here make sure you've got resistance there if you have zero resistance it means you have an open transformer it's bad okay let's say you find some resistance here on this side then connect from blue to red here okay so if you got resistance if so then see if you got resistance between blue and yellow then make sure you got resistance between yellow and red here okay if so it's possible you've got some shorted leads in hit some windings in here that are shortened you're seeing less resistance than you should thus this transformer is not working effectively it's possible but and more likely that this is a good transformer you need to solve something else in your ample in your radio not the output transformer but if any of these are open you definitely have a bad transformer at this point so once you get that out of the way next I would move here to the datasheet for this l82 okay let's look up here and this is in German so there's a lot of it I can't read but there's some parts of it here at camp this says typical operation this says Class A amplifier which is what the same as a single-ended amplifier then I scroll down through these values voltage voltage ohms milliamps milliamps kill all's voltage voltage percentage W that's wattage well we got here three watts I don't know what this end stands for because I can't read German but basically this is a this is telling me that this tubes plate dissipation is three watts so this is know this this tube won't put out any more than three watts okay so if I were you I would look for something like a 3 watt to 5 watt transformer could you get a 10 watt transformer yes it goes from being this big to this big could you put a 20 or 30 watt transformer yes it starts to give you got to figure out what will fit in your radios but we're gonna come over here and we're just gonna put three to five watt output transformer that we need right here so we're starting to fill in the blanks here starting to answer some of these unknowns that we don't know okay up next I want you to get two devices okay you're gonna need to do this test with you're gonna need some type of signal generator or function generator audio signal generator something that operates in the audio frequency something between one Hertz or 10 Hertz somewhere in that range up to about 20 kilohertz is what you're gonna wall here okay the generator if you find an RF generator won't work that's way up in the megahertz range you need a signal generator you're gonna connect it on the output here why are you connecting on the output because we already know this impedance right here on this side it's kind of our known side the next thing you're gonna get either a digital multimeter analog multimeter or an oscilloscope and you're going to connect it across these same leads what you're gonna do is you're gonna feed a signal in on this side kind of backwards from normal so we're going to kind of reverse induce this signal through this transformer here okay and we're gonna do something like and I'll just make this up a 10 volt AC signal into it and we want to pick something here in the audio range because we know this transformer transduces the audio signal range well so for kicks and giggles we're just gonna use a 1 kilohertz signal it's a good signal that will feed through this transformer quite easily so we're gonna feed 10 volts in and what we're gonna do then once we've make sure that this is on 10 volts AC going in if you have two digital multimeters hook the next one up over here or two oscilloscopes you've got oscilloscope with a second channel put the second channel over here connect one side to the blue the other side of the raft in other words ground and then the tip probe here if for whatever reason you don't have to you meter just move this one over here once you know this has got 10 volts ac coming out and measure what you get out on the other side it's going to be something much larger because there's more windings on this side remember and our picture earlier we talked about from 400 volts down to 10 or 20 or something well if you feed it the opposite way you're gonna go from 10 or 20 up to 400 or so over here measure your output on this side over here for kicks and giggles to make the math easy I'm just going to say we get out hundred volts on this side so we can then take and remember our voltage ratio we talked about earlier is one of our constants up here is equal to V on the primary a hundred divided by V here on the secondary fed in 10 right our voltage ratio is 10 or 10 to one for every hundred volts we feed in over here we get 10 volts out over here for every 10 volts we feed it in over here we get a hundred out over here if it was 40 volts in here 400 out over here 500 in over here it's 50 out of here it's a ten to one ratio okay we're not gonna get into the turns windings today that's not also important but what we do need to know is the impedance ratio of this transformer is equal to remember we talked about it earlier it was the voltage ratio squared well if you say 10 squared and if you do both sides of it 1 squared well 10 squared is 100 and no matter what you do with 1 squared it's still 1 so your impedance ratio is equal to there you go 100 to 1 so let's just say we got the hundred volts out here we did the math we know our meters measures 101 it's probably not gonna be Tim when you do yours it's probably gonna be something like 350 to 1 right but then you know hey we've got a 5 ohm here 5 times 100 500 ohms of impedance on this side is what this transformer reflects back okay remember that's the impedance that's that's when the load looks in here but there's a 5 ohm load on that side what does it feel reflected back it feels this 500 ohm guess what Tim you're gonna need a 500 ohm it's probably not the case in yours you need to do the math you need if this transformer is good measure it if it's not unconnect the wires from right the if the right channel is the bad one and connect the wires from the left channel transformer do all these measurements using the left channel transformer it'll help you figure this out over here okay now next is everybody clear on that alright next we're gonna go one more step deeper into this down this rabbit hole of knowledge here we got to figure out the screen tap well let me tell you how we're gonna do that we're gonna come back over here to the signal generator on this side and you can disconnect this I don't care what voltage you're feeding in you just want to make sure you're meeting reading AC voltage on this side feed a signal in on this side until you get something out on this side that is a multiple of 10 10 volts out that works 100 volts out that works thousand volts out that works okay just get make sure irregardless of what you feed here you want to get a nice even number you can do some math with on this side you wonder why why you would you want that and why do you not care about this well if I feed out feed something in here and I get a hundred volts out on this side if I then move my little lead here all for this and I move it right here okay and I measure and let's say instead of a hundred volts I'm getting thirty five volts here between the red and the yellow I disconnect this one and reconnect this one and I measure between here and here I better get sixty five volts out those two numbers better add up to be a hundred in other words but guess what this voltage in relation to this I just figured out that I need a 35% screen town because this voltage is 35% of the whole and it will be the lower percentage the one below 50% that'll dictate your screen tap over here okay in other words you're feeding in a voltage into this and somewhere along the way you're wanting to take a percentage of that back out and feed it into your screen and that's how it would get measured then all we need to know is the DC resistance here well that's pretty easy okay disconnect this again connect up here connect up here put this on DC resistance right measure from here to here you'll get sure whatever it is and I'll just make this up 120 ohms out you almost have what you need to go by a transformer alright Tim you think you're there right you now know you need a transformer with a 500 on Omen peanuts primary you need a 500 on secondary you need a 35% screen tap you need it three to five watts and if you can find a transformer that lists the DCR it'd be nice to know that it's somewhere around 120 ohms on the primary okay let me show you why that's important okay if you follow the B+ the positive plate voltage here from the rectifier right through around using a filter capacitor here okay so we're feeding up and right and we get to right here we feed in red on this side of the transformer and we feed out through the blue same here we feed in through the red out through the blue here so let me just draw the path here maybe the highlighter here okay come down come across I'm gonna come here it's gonna come over it's gonna go into the red here it's gonna come out of the blue and it's gonna go to the plate of this transformer same voltage is going to come up come in here out of here to the plate of this transformer what is missing well if you said hey where's the plate load resistor hmm I don't see a resistor in that line anywhere and you need one well guess what plays the role of the plate load resistor if you said the windings inside of this transformer right here from a DC standpoint not AC DC you would be correct so the resistance here of the windings otherwise known as DC R or DC resistance so that's where you pick up your play load resistor is the actual windings inside keep in mind its plan this device is now playing an AC purpose and a DC purpose for the plate load resistor here and so you want to try to match that up as close as possible - what's exhibit what's there already so it that's easy to do you have a multimeter with nothing hooked up to this and measure between the red and the blue wire that is your DC R of a good known transformer okay then when you go to buy a replacement try to match it up now if you can't okay and it's going to be off you know twenty ohms hundred ohms two hundred ohms whatever I mean you don't have any other choice you may have to rebuy us this circuit here as a whole to account for that okay and I'm not gonna get into that in today's video it's it's a little outside the scope of this but you could do the same on the other one and by the way if that's the case I would replace both transformers so everything is matched and equal at that point and you can make this unit work again so Tim to wrap up the last piece of this puzzle here's where your problems gonna lie you're gonna go try to find this transformer and five ohms is not a common value for output transformer I'm not saying you can't find one it's possible that some guitar amplifier or something you know has a single ended and uses a 5 ohm speaker and because it's common enough you know somebody's made a replacement transformer for it but highly likely you're not going to find that what you're going to end up finding is a 4 ohm output transformer because that's a common value okay here's the problem if you put it in let's say you matched all the specs except for 4 ohms right here and you can put 4 ohms in that place then you went and bought this and put it in well think about it when you reflect back from the load through the transformer right the windings back out to the other side and you've got your tube here alright if you remember if it sees a 4 ohm load and let's say this up here was 500 you still kept the primary at 500 well what's the ratio here 4 to 500 is ratio of 125 in other words your ir equals 125 or 125 to 1 if you want to write it that way alright and then what are you gonna do you're gonna hook up a 405 ohm speaker to it that's not gonna play out well for you because when it reflects back through five times 125 your tubes gonna see 625 ohms here instead of this 500 that it's looking for because of its reflected impedance so what we really need to do here is figure out what value of transformer that if we put a 5 ohm load on it over here would reflect back a 500 ohm load on the other side but yet that is labeled as 4 ohms well if we're gonna go 5 here to 500 over here right that's a 100 to 1 ratio and if we're gonna put in a 4 ohm load on it let's do the math here with a 100 we're gonna end up with a 400 on primary is what you're gonna want so that way when you drop a 5 ohm load on it Tom's 100 you end up with the 500 that you originally wanted on this side so you would not buy a 5 ohm here you would buy 4 ohm here and you would not buy 500 ohm primary here you would buy 400 on primary but when you put it in a circuit with a 5 ohm load on it that 5 is gonna use the impedance ratio here of 100 it's gonna multiply it's gonna give you out the 500 ohm load on the other side that you were actually looking for to begin with and you're gonna want the 35% screen tap the 3 to 5 watts and if you can match up the DC or as close as possible do that as well that's how you go about finding a replacement transformer for your unit and that's how you go about using a non I mean using a standard load transformer in place of where you might not have a standard load on this unit this same trick can be used let's say you've got a an amplifier and all you have is an 8 ohm speaker I mean I hate ohm secondary and say maybe a 2000 on primary but what you really want is a 4,000 on primary right well you could change that around and use a 16 ohm speaker and end up with this output transformer having your 4000 ohm load on the other side it's all about ratios and what you're trying to get out of it hope this made sense to you guys if not ask some questions below I can try to make some follow-up videos but thought I'd try to demystify this a little bit for you today and thanks for watching so much everyone have a great day

Channel: Blueglow Electronics

Views: 34,757

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Id: DNqkMRwaOLE

Length: 36min 18sec (2178 seconds)

Published: Sun Jun 14 2020