Audio Output Transformers Demystified - Ask Mark

[@edwhite7475]

wow....that was GREAT....after 44 years of studying this stuff, half-ass..., i understood about 90% of that. i love how passionate you are when you get deep into it. i still glaze over on this stuff but in the last 5 years im much better.. ..i have a lot of projects piling up and ive GOT to get this down... thank you SO much-

[@ronniepirtlejr2606]

Finally an engineer that can translate into a hobbyist language! Thank you for taking the mystifying BS out of it!👍

[@midmodaudio6576]

As a novice, I have enjoyed watching and learning from your well prepared videos for the past couple of years. I have to say that I had a hard time following this one. The Uncle Doug video you referenced was much easier to follow.

[@reggycorlone2489]

Thank you. Been reading Radiotron handbook trying to understand the relationship between the tube and output transformer and they speaker. Your video made it simple. Thank you 😊

[@mr1enrollment]

1) the DCR is much less important than you suggest. The output tube is a current source and largely cares not about the DCR of the xfmr. 2) in that the impedance of a spkr, and that the xfmr impedance are both very dependent on frequency - the reflected load varies all over the place. for an old tube radio just getting in the 'about right' area is fine - it will work, and it will be much easier to find a xfmr. 3) it should be stressed that since the circuit is SE, the xfmr need be a SE rated unit, and must be able to sustain the DC current. SE xfmr's usually have a reluctance gap. the gap prevents saturation of the xfmr. 4) your use of VR and IR for voltage ratio and impedance ratio is kinda poor nomenclature, easily confused with ohms law calculations. turns ratio would be more correct. 5) your use of the word 'transducer' is also a bit unconventional. usually a transducer converts from one set of units to another, perhaps a thermistor would be a good example which transduces a temperature to a resistance.

[@herbward5240]

Mark, almost all PP OPT will have the B+ center tap wire as red. While this is not carved in stone, 45 years of fixing this stuff leads me to believe that almost all US OEM suppliers of either simple PP OPT, or UL OPT will use a red wire for the B+ center tap. SE OPT do not always follow the same color codes. Especially the european stuff. A great video for the hobbyists, well done!

[@mitokristan4454]

I'm afraid your reasoning about the primary end may be wrong. What you say: 'The center(ish) tap goes to the screen' is debatable. In my opinion, the transformer primary is designed as it is drawn originally, so the red is the B+ supply. There's no doubt what the blue end of the primary connected to the plate does - it feeds the audio signal. However, if you follow the yellow end, it is not only connected to the screen, but it goes on to the next filter capacitor - because it supplies power to the previous stages of the circuit. So, both the red AND the yellow end are bypassed by a filter cap. The power current goes into the CENTER-ISH tap (red) and goes BOTH WAYS. This little part of the winding is actually a built-in choke. It also helps diminish the magnetic flux in the core caused by the DC component of the 'audio-signal' primary, as the DC current in this additional winding is opposed to it. If the capacitors are designed correctly, the AC hum component is greatly diminished because it cancels itself out. As a direct replacement for this transformer would be difficult to come by, one needs to buy a replacement without the in-built choke. To get the best match, it would be advisable to know the DC component as well - you can read it out of the tube specs.

[@neutrodyne]

This was a very nicely done in-depth video on audio transformers. The truth is there is no perfect match on output transformers since the impendence of the transformer will vary with the frequency put into it. The impendence of the speaker on the secondary will also change with frequency resulting in a varying load to the secondary of the transformer. This video however will get you about as closest as you can get.

[@Sparky-Tim]

WOW! Thanks Mark for the great lesson, just what i needed to know to help with my amp build. More in-depth vids like this please.🤓👍

[@MrDoneboy]

Mark, I want to thank you for your practical approach on teaching electronics. I went to a school that seemed to concentrate on a designers way of teaching, which did not help in the troubleshooting aspect of this art. Way too much math involved, man!

[@kharakim]

Superbly well done. You had my undivided attention for the entire video, although it's more likely I'll repair a rocket motor than replace a transformer.

[@rasjase1]

Thank you so much for this video. Anything on OT's is greatly appreciated!

[@Songwriter376]

Very informative and extremely detailed. One of The best I have seen on KZbin. One thing: @20:32 If you have INFIiNITE resistance it is bad or open. I know that is what was meant. Peace out ✌️.

[@1959Berre]

The "N" on the transformer datasheet means: "Nominal", (the same in English). In this case nominal power.

[@richardcline1337]

One little trick that a high school classmate pulled on us when we were in grammar school was to bring an output transformer and a D cell battery to class. He would put the battery on the speaker side of the transformer and have one of us hold the input leads. The second he touched that battery we got a big jolt! It wasn't until years later that I finally understood how a dc voltage could shock you using an ac device. Had he touched the wires to the transformer BEFORE we touched it, nothing would have happened but by doing it the way he did he momentarily created a current flow in the coils and we got zapped! Since he was on the "low" side it meant that he was amplifying the voltage that we got. Modern transformers are too small and don't have the potential that those old tube type transformers did so it wouldn't work otherwise. Some have argued with me about it until I did the same thing and it made believers out of them. The current flow is of very short duration so if you hold on to the leads after the initial jolt, nothing else will happen.

[@ef804stubes8]

Blue and Yellow is the "outside" winding. Red-Yellow is a hum-bucking winding.

[@ef804stubes8]

Yes, it is an additional winding. It is not reverse wound since the current direction is reversed. B+ connected to the tap flowing in one direction to the Anode of the output tube and in the opposite direction toward screen grid.

[@brianwood5220]

Came across this video by accident. Nicely explained Mark. Thanks, i learnt something today. You also got a new subscriber.

[@hernancoronel]

Very nice explanation! Thank you Mark and keep up the great work!

[@gabet3754]

Uncle Doug is the man!! RIP Rusty 😔

[@scratchback2001]

Hey Mark. Andrew from Australia here mate! Rather than use the Omega symbol to express resistance, we are just gonna go for example 8 R. I know that for 100 years the purists will still use colour rings etc but I'm 59 and I cannot remember the values using colour rings! I'm also mathematically dyslexic and apart from everyday basic monetary things like simple maths, I just don't get it! I know that 10 squared is 100 and I understand ratios because I used to be an audio/video engineer and a club DJ, but once you delve into hardcore algebra, my brain just cannot process this information! On the other side of the coin, English and Biology were my best subjects and I often sat at the back of class with a kid who didn't know how to read to teach him! I understand how most electronics works from CRTs to tubes or valves as we call them here but looking at schematic drawings does my head in! In hindsight, I should've gone to technical school but in the 60s it was very much fitting a square peg into a round hole! I love valve amplifiers! I own a Primaluna Dialogue Premium HP Integrated Amplifier push pull and a Leak Point One Stereo system push pull and they sound so much better than a Class A solid state amp that I own! I did learn a lot from this video! Keep up the good work! Andrew from Australia!

[@foureyedchick]

Just say "ohm". I cannot find the omega-symbol on my keyboard.

[@darrenbird2526]

Great informative video, thankyou!

[@NoLandMandi]

Make sure to remove the signal from secondary before measuring DC resistance or your primary or your multimeter might be due for an unplanned upgrade.

[@MrJoejim]

Great video. I learned a lot even after watching Uncle Doug's video on the same topic. Thanks!

[@stevewinwood3674]

so since the OT actually has a RATIO of impedances that is why it is important to make sure you connect the proper impedance speaker load or speaker combination to the output because the it changes the corresponding impedance that the output tubes will "see". and tubes need to see a certain impedance depending on their construction. makes sense.

[@KUDJU]

Mark, really good video, but can you make additional video regarding the following: When making a DIY amp How to choose an receiving tube, regarding the output one; When choosing the output tube how to reed the datasheets and the graphs behind each tube specification; How to choose the working point of the tube; How the working point of the input/output tube reflects on other components on the sercuit and how to choose the components to bias each section (to make it work propperly); When reading the tube datasheets how to choose the plate DC resistance of the primary winding of the output transfomer (on datasheets for kt88 says (Triode connected Ri = 670 Ω, but the winding of Edcor OT you use in the SE KT88 amp says 85Ω), the impedence, the current that has to pass through the primari of the output transformer; when working on a tube amp, how to make an probbing points and where on the schematic to read the tube bias parameter (V and I) so we can manually adjust via potenciometer each section of the amp if needed. I have many other questions but ill save them for another time :>

[@juanjogarcia2956]

Awesome video!!! Just what i needed. Wonderfuly explained!!! Thanks!

[@tongordebeke1355]

Hi Mark, Very thorough explanation. Thanks.

[@PatrickInCayman]

Awesome video, learned a lot about impedance matching.

[@cathodebias]

Thanks. This helped me understand the screen tap for my Marantz 8b output transformer

[@bernhardnizynski4403]

So simply and well explained!

[@danmenke6788]

Mark, forgive me if I missed the announcement, is there a date for the group amp build? I can’t wait to get started!

[@DustinWatts]

Question below, so here we go! Why do you label impedance ratio as Ir instead of Zr? I see I, I think current :)

[@pasixty6510]

N in old school German terms simply means Power (P) in modern notation. Back in the 1960s we started to use P for „power“ to adapt to the international system. Also see the comment about „nominal power“. That might be the explanation why it was „N“

[@foureyedchick]

I wonder how much of the difference in sound of tube versus transistor amps is due to differences between tubes and transistors rather than the fact that tube amps use output transformers and transistor amps don't !

[@pbaemedan]

Mark, great presentation thank you. A recommendation would be to use basic math functions to link the importance between voltage ratio to the current ratio. Again thanks for your time and effort.

[@prahladramesh6562]

Dear Mark, I am trying to build an amp which has a CATHODE FEEDBACK winding on the transformer can you explain this form of feedback and how to cal the winding for this amplifier.. Thanks

[@shodan6401]

"Last known fact: Uncle Doug Video" 😆

[@MrTeamRobert]

This is a realy good video, thanks Mark. Thumbs up 👍

[@MrMannakin]

Impedence ratio written as IR could be confusing!

[@srtamplification]

Zr

[@foureyedchick]

@@srtamplification Right. Because Z is impedance and R is resistance.

[@RobertHallIV]

Not knowing the actual impedance of either, I get your point.

[@RobertHallIV]

I have the same trouble trying to explain why RF cable is either 50, 75 or 300 ohm. It's not electrical resistance. It's RF resistance. How the RF signal sees the feed line. Like a transformer's resistance, the numbers really don't mean anything, at least to a DMM standpoint.

[@Myname549]

Excellent tutorial! Thank you.

[@ZebraRecordsOfficial]

Can an output transformer from an old tube radio wich has the secondary for ex 8 or 16 ohms be converted to some impedance that will match xlr line outs for going into a mixer or soundcard? Thank you!

[@Slugg-O]

Many thanks. Excellent teaching video!

[@W1RMD]

Thanks for the informative video! A word of caution: STAY AWAY FROM 70 volt line transformers! DON'T be seduced by their $15 price tag! I built a 6L6 single ended amp and tried a 25 watt 70 volt pa system transformer. Sounds like crap below 100 hz. It creates horrible distortion. I measured the primary impedance and it measures only 30 ohms dc! I tried a 200 and 100 ohms resistor in line with the plate with no difference at all. Plan on spending $50 for a good Hammond transformer. (if you can find one) Do the job right, go with the sure thing!

[@SheikhN-bible-syndrome]

Good to know

[@dustbowl5897]

Let me ask you a question. What if an output transformer has three: 4,8 and 16 ohm secondary windings, but I want connect 6 ohm speakers to it. Can I do it and if so which winding should I use?

[@Spentelectrons]

The short answer, your speaker should be equal to or greater then the output. If the speaker really is 6ohm then the 4 ohm output is the safest. However, if you put a meter to a speaker an 8ohm speaker will read less then 8ohm, possibly in the range of 6ohm+ and is considered 8ohm but that would be 8 ohm impedance. Impedance and resistance are not 1:1 conversion. A 4 ohm speaker connected to an 8 ohm output would double the current potentially damaging the amp. A 8 ohm speaker connected to a 4 ohm output would half the current putting half the load on the amp. Both of these scenario would to some extent effect the sound as it is an impedance mismatch. You also put an s on speakers. If any of those are parallel or series on a single output that would effect the impedance in total.

[@zxttgg]

I want to know what is ultra-linear and what is screen tap used for?

[@drillgirl718]

I got a question, is an audio output transformer the part of the microphone like a telefunken 251 ela m that gives the vocals that buzzy fuzz sound that sounds so pleasant? And lets say im building a microphone, how can i make it produce more of that transformer sound IF thats what is creating that buzz sound

[@wsscott72]

Mark-Really good video and explanation. I'm a bit of a novice, but am not totally a noobie, and wanted to get your input on an amp that a friend has asked me to check out. It's a 1939 Recording King made by Supro/Valco. It has an unknown OT and I"m trying to confirm what speaker should be used. I tested it in circuit, with the output tubes removed and unplugged. It's PP and running two 6L6GC's, and is cathode biased. I calculated the Voltage Ratio at 48:1 for an Impedance Ratio of 2304:1. Seems kind of high to me, but I'm just a beginner on this transformer stuff. So if I hook up a 4 ohm speaker, it would show a reflected impedance value of 9216 ohms, and if it was an 8 ohm speaker it would be 18,432 ohms, correct? If so, these tubes call for 5,000 ohms, I believe. If that's correct, it seems like this OT is way too "big". Am I missing something? Would this OT harm the tubes or a speaker? Thoughts? Thanks.

[@srtamplification]

All of that you said at the end, is well and good in understanding the ratio, but the speaker load changes constantly with frequency anyway. I highly doubt that a 4 ohm secondary is going to make that much of a difference over one with 5 ohm.

[@christopherhines2718]

ell80 surely thats a double pentode,in one bottle.very rare now.But it is 2xel95 in a single bottle,I hace an old radio amplifier using one,but does not have the centre tap on the output transformers only 2 wires on the primary and two on the secondary.so is it doing ultra linear ? but still class a?

[@jdmccorful]

Great Lesson!

[@CostasLilitsas]

Awesome! 👍

[@Sams911]

The McIntosh Autoformer such a thing? Or are those different?

[@williamsullivan9323]

Excellent Mark

[@luisgama6607]

hello friend congratulations on the content of this channel! Im from Brasil ...it would be very interesting to see a detailed video like this, about diy ribbon microphone and how to think and design a diy ribbon mic. Thank you!

[@VikasG007]

I am from india, and I want to know that can I use step down transformer in place of audio transformer for 5k:8ohm and 6watt audio transformer. If Yes then what is configuration/voltage (Primary: Secondary with Amp) For El84 Singal Ended Tube amps.

[@jimcatanzaro7808]

Great lesson !!!!! I have power transformer 640 cvt 2 at 5v 2 at 6.3 v all at 4 amps I would like to make a pp 6v6 output tube amplifier do you have any recommendations on a schematic I can build off of I have a chassis and a bunch of parts

[@banjoperator]

Hey Mark, awesome video.. explained alot as i was wondering about a ss resceiver i have that handles speaker loads from 2 ohms up to 32 ohms according to the manufactureer.. its a NIKKO unit.. and only used it at 8 ohm loads and it gets pretty hot.. would imagine that changes with varying loads.... your explaination makes sense as to how it can do that.. thanks a bunch..cheers

[@kenhyatt4494]

Great video. Thanks!

[@xprcloud]

Perfect Thank you, Q: why is it bad for certain tube amps if they are on without a load connected(Speaker disconnected)

[@Vidtskuevej9]

Because the transformer is a current transformer. The transformer must have a load or must be shortcut.

[@silasfatchett7380]

If a tube amp attempts to amplify a signal with no load connected, high voltage spikes (1kV or more) are developed across the primary winding of the OPT which can not only damage or destroy the output tubes due to arcing between the electrodes but can also break down the insulation of the primary winding wire causing shorted turns which renders the OPT useless.

[@foureyedchick]

Wow! I knew that there are a bunch of transistor amps out there that would blow the output transistors if there is no load. But tube amps? I always thought that transistors are delicate devices that can get fried easily, but tubes are supposed to be electrically rugged, but mechanically frail because they are made of glass. This was an eye opener for me.

[@jonsnow7092]

@@foureyedchickyou thought this because you don't understand tube amps. tanks are pretty rugged but I doubt they will last long on the surface of the sun. no load means infinite resistance on the secondary of the OT, times the ration, you get infinity on the primary too. This means that the voltage will spike as high as it physically can, usually in the realm of kilo volts (thousands of volts). this makes the electrical current punch through the insulation everywhere it can. it's usually inside the primary of the output transformer as the windings are insulated with a thin coating, but tubes as well as capacitors also don't like those very high voltages, well over what they were designed to see. More than that, you can get arching pretty much anywhere, so the damages can be unpredictable, but in most cases when this happens, it's the OT that dies out first.

[@EmilioGarcia-fr5po]

Have a question please have a Fisher chassis 610- st with two wires coming out of the transformer on the amp. That's not for speakers is it?

[@johnlake6321]

Great explanation!!

[@aharkness5657]

I for one would be very interested in knowing how to identify and characterize loose output transformers.

[@aharkness5657]

That was to the comment of making another video about that a few minutes in. This one was brilliant.

[@shodan6401]

Uh... Holy cow. I gotta leave this to the professionals...

[@stevewinwood3674]

so on the schematic @ 5:20 there are two separate OT transformers?

[@silasfatchett7380]

Yes. One for each channel of this stereo amp.

[@erdnaerramal]

Is the impedance ratio always the same as the DC resistance ratio ?

[@dakata2416]

No

[@Canadian_Eh_I]

Hi just a question: does a step downoutput transformer on a preamp out remove fidelity or just volume on the signal?

[@mikecamps7226]

So in your example you state a 10watt transformer.......turns ratio.....secondary rating reflecting back on the primary. AND manipulating the secondary which then affects the primary via the turns ratio. BUT DOES, the wattage change ????

[@silasfatchett7380]

The power handling of a transformer is determined by its physical construction. If you are asking ‘are the output watts different from the input watts?’, then the answer is no. In a 'stepdown' transformer such as the output transformer, voltage is divided by the winding ratio while current is multiplied by the winding ratio. E.g. A transformer with a winding ratio of 10:1 will transform 100V to 10V and 0.1A to 1A. Power (Watts) is the product of voltage and current. At the primary there is 100V x 0.1A = 10W, and at the secondary there is 10V x 1A = 10W.

[@studio1productions1]

I have a kind of complex question for you it's very important to me is there a way I could contact you ..thanks love your videos

[@rickkennard8027]

Great explanations!

[@50of50]

My 2 cents worth: Findind a rare output transformer can be very time consuming. I usually rewind them . Saves me heaps of time and they will work as new.

[@foureyedchick]

Rewind them you say? 1st things 1st. You have to know how many turns equals how many ohms. Then you have to do a proportion. a/b = c/d. Then c= ((a)(d))/(b). A lot of work, to say the least.

[@50of50]

@@foureyedchick If you think that’s a lot of work rewinding the output transformer compared with finding an output transformer for a telefunken radio with negative feedback winding, think again or don’t rewind. Last transformer, like the one mentioned above took me 4 hours. Let’s make a little experiment. You have 4 hours to find a Tonfunk Violetta w285 output transformer. Please let me know how it went.

[@foureyedchick]

@@50of50 Valid point! I believe 5-ohm and 6-ohm speakers should be banned by law, and manufacturers of them should face fines and imprisonment, with revocation of any patents. This reminds me of a UNIQUE cassette recorder made in the 1980s. There are no other companies that make the tapes. Industry standard speakers are 4, 8 and 16 ohm. All other speaker makers should be shot with a firing squad. Meanwhile, keep busy winding those transformers, lol

[@jonsnow7092]

@@foureyedchick most of the people that designed these circuits and components are long dead, they were probably old men back in the 50s and the 60s. if you know how tube circuits work and you know your math, you know what to look for and you may be able to find a replacement that works. if you don't find anything, you at least know how to rewind the OT or have someone make you a new transformer exactly to the specs.

[@allenwoody5909]

Mark thanks for the well done video. Can you recommend a signal generator, hopefully an affordable one. Newark Electronics offers a "Tenma" model number 72-505 for $39.99. I have several transformers that I want to check. Any advice would be welcomed.

[@sstorholm]

Is there anyway to figure out what range of impedance the SE stage would like to see? Aka. working from the tube data sheet forwards so to speak?

[@grimtale69]

Yes. Look up the output tube. They typically tell the load they are looking for.

[@larryak0z672]

Makes sense, thank you.

[@tze-weichua3143]

Hi Mark. Thank you for another very educational video. I have a question. If I am designing an amp from scratch, how do I know what is the correct primary impedance(transformer) for any particular tube? BTW, I am eagerly awaiting your 300B project to start.

[@acdcjustthebest....8980]

Check the load Resistance or effective load resistance of the the tube in ohms. Look at a 6CA7 data sheet, class A is 2000 ohms class AB is 6500 ohms. Don’t forget to look at the right class for the design of amp for the tube. Check out “uncle Doug” channel. He goes into great detail about O.P transformers in a different way....... Cheers

[@peevonb7070]

Thanks Mark!

[@theguitaramptech]

Another excellent video Mark. Some Push-Pull OT's intentionally (probably financial reasons) have uneven DCR on the 2 halves of the primary. The turns ratio may be OK for AC performance, but I suspect the DC bias will be way out on one of the 2 output tubes. What are your thoughts and can we mitigate this without buying a new OT?

[@Jonathan_Doe_]

Sure you’ve probably worked this one out by now, but for anyone reading: For fixed bias, modify the bias supply so it has a balance pot, or a separate bias pot for each tube. For cathode bias, use individual cathode resistors. Then use the centre tap to plate voltage drop divided by the centre tap to plate resistance (measure this amp warm, but switched off) for each tube to calculate the idle ma for each side, then times that by the centre tap B+ voltage to give the idle watts. So say 1.5v drop/41 ohms = 0.0366 amps. (36mA) 0.036 x 420v CT voltage = 15.366w idle for that tube/side of the push pull.

[@theguitaramptech]

@@Jonathan_Doe_ yes. a balance pot would work. Thank you for your reply!

[@larryak0z672]

Would this also apply to RF transformers?

[@albundy1956]

Great explanation Mark, now I understand little more. Also the SE transformer have to have a gap in the core, or...?

[@Blueglow]

They do. But without getting into the vast variety of physical situations out there, I thought I'd explain it from a schematic POV.

[@williamcaspers5252]

Given the class A with DC current through the primary, the core must be gapped to prevent saturation.

[@Satchmoeddie]

N is for nominal. The Germans spell it "Nominell".

[@cotto887]

it should have been suggested to replace the transformer with the same one or one that was compatible

[@jonsnow7092]

and to get a new identical one, just time travel 50 years in the past. the whole purpouse of this video is to determine the specs of the compatible transformer so that one can try to find a similar one, or, simply have a new one built to the required specs.

[@danielthomas3057]

ELL80 = 6HU8 dual pentode

[@KameraShy]

Coil Windings Matter

[@josepheccles9341]

Edcore will let you spec your own custom transformer if you want to pay for it.

[@borisj4054]

Very messy description. Should break the subject down and design a succinct flow of instruction. Think I will need to make such a video as there seems nothing on Utube that really understands the subject.

[@dakata2416]

Do it then smart ass

[@bmbm2901]

you don't demisify anything, your long woolly story makes the case opaque and messy To demystify, you must first understand the matter yourself

[@nikolaradakovic9482]

Decoding OT is not a job for hobbyst...this clip is not good..