Vacuum Tube Output Transformer Measurements Determining Turns & Impedance Ratio & Matching To Tubes 

If this guy doesn't teach electronics, he should. I am a retired Electrical Eng., and this was the most basic (yet informative) explanation I've ever heard. Anyone watching it, and then not having a good understanding of " Turns Ratio ", was either not paying attention, or needs to find another hobby.... Jim

A mysterious topic explained so lucidly for a layman. Thank you very much for sharing.

This is just what i needed to brush up on my old electronics knowledge so i can finally build a little SE el84 amp. Thanks so much.

Thank you very much for posting this. Us younger guys feeling our way through the dark with this appreciate all you more experienced guys, your knowledge, and your sharing this with us!

I just wanted to thank you for taking the time to make this video. It's very informative and useful.

This is a great way to both to check a transformer for ratings AND to check if the transformer is blown. Thank you for sharing your knowledge and experience. This was extremely helpful!! Gratitude.

Most my clients are mystified by transformers. I think you covered the subject pretty well and kept it simple in layman terms. Good job.

I actually did this on a Milbert car tube amp to verify the output transformers are the same as the old Berning EA-230 schematic (6.5K). I read the info online but your video was more "real" and very helpful. Thanks!

Thank you for producing this practical tutorial

So that green/white wire hanging under my transformer is a center tap! I never found a schematic for this amp (1956 Ampeg GuitarAmp , according to the serial number). The wire was coiled in a corner unconnected to anything. I connect that to GND thinking it might be a transformer case ground and the amp lost a good amount of its background hum. The rest of the hum disappeared when I ran two 100 ohm resistors from the 6vac filament lines to ground. What a difference! It is a pleasure to play on this amp....so quiet. Thank you for all the great practical information.

The impedance changes with the frequency. I own a digital diplay LCR bridge and a variable AC power supply. The power supply reflects a load from 1 Ohms to 1000 Ohms, @ 1-1000Hz. The bridge will send AC signals from 1Hz up to 100KHz. Some of the audio transformers are impedance rated for ____ Ohms at 100 Hz, 400 Hz, 600Hz (600Hz is the old telephone company standard) or 1000 Hz. A non-inductive DC resistance load is not at all the same as a reactive inductive/capacitive speaker load. That load changes with the frequency. An 8 Ohm speaker can swing from 3 or 4 Ohms up to 13 or 14 Ohms IMPEDANCE.

Nice & simple video that really teachs something practical to the tube and transformer user's community. Congratualtions !!

Great video. I just did this on a used transformer I recently picked up to test. I don't have a variac, so I put a light bulb socket in series with the power transformer and ran the output off the 6.3 filament taps. Worked perfectly.

I used your formula again today on an audio transformer. Thanks again!!!!!!!!!!!!!!!!!

The best video so far!!

Thanks for a great video. I have just started working on Hi-Fi tube gear, and this really helped me a lot [beer-clink]

Thanks for posting this. Very helpful. And I think it's great that you reiterate the safety points. Be well.

Thanks for the advice on safety when working with high voltage. Same rules apply when doing anything critical/dangerous, I would add.

Thanks David. I'm looking at some of your older videos. Good stuff here. 1) I like the fact you twist your wires. I do this with all my wiring except the blue and brown wires on OTs typically used for plate connections on tubes. I think these two should be separated. 2) it's the same safety consideration with soldering. I have burned myself too many times to count. It was usually when I was distracted and not paying strict attention. I had to educate my other half to not disturb me when soldering :-) Also, if your soldering iron slips out of your hand for any reason and lands on a component, that component may be destroyed. I know from experience. Easy to understand explanations in this video. Good job.

Thanks so much for this information on Transformers I think this will really help me in that I consider myself to still be a novice at building repairing and troubleshooting tube amps but I really love this hobby I love music and I think this is truly going to help me. Right now I am working on a silvertone tube amp that came out of an organ from probably the mid to early 60s, I have two of these amplifiers that are identical and they use 6 B Q V output tubes two of them in the output but all the connections are made on the side of the amplifier through a plastic block that couples with another one and of course I don't have a schematic for this amplifier and I'm trying to locate one now but I'm also trying to figure out where the speaker leads would connect and also the volume and tone connections as well. I think this will give me a good idea of what is what and I'm going to try to sort it out from there, thanks so much for the video, great job.

Very informative video! Thanks a lot! It seems I'm not the only one who see similar interesting stuff here and at Uncle Doug's. I'm glad I found both of you! :)

Thank you. I just spent a couple hours looking this up. Reading everything you explained here. I did all the math per what I read and wasn't sure if I did it right....Then I find this video and confirmed it. I have a few more OT's I can go figure out now. Thanks again.

Great! I love practical examples of the theory.

No truer words when working on HV. I was bit by an operating microwave transformer, 2,400V 1000 watts when my shop assistant was yapping in my ear rather than doing his own work. The only reason I am alive is because I followed rule #1, keep 1 hand in your pocket, and wear insulated shoes. It hurt like a son of a bitc- and threw me about 6 feet. The second time I was walloped by the 2nd anode of an operating picture tube because the same assistant didn't clean off the glass, and 2nd anode cup before attaching it. Debris on the glass set up the perfect storm for a bolt of lightning to do a number on my hand, and split my thumb nail in 2. That one felt like hitting my thumb with a hammer. Needless to say my shop assistant was flipping burgers after that. I actually enjoyed firing his ass.

I really appreciate this kind of video... dum dums like me can understand. a lot of videos on youtube that guy is just talking over everybody else's head. but this.. this is my pace.

Easy to follow. Thanks!

Some people think they know ...........YOU SIR are well......... unreal........I truly love your videos and before you I did not think I would ever grasp all this craziness.... I am starting too and I sure appreciate it. If you ever build another kilowatt or whatever huge amp, love too document what it sounds like with a guitar through it. I have guitars and will travel. I love ya brother. I would love too be the first man too play guitar through one of those crazy amps.......I think it would make a lot of noise in the guitar world...get it........Thanks so much.......

Excellent stuff..thanks EPT

Ty, really appreciate your advice - Thank You. The short of it is 1) 30% of my house electrical system has not been updated, and 2) to save on the electrical bill we have all curly Q florescent light bulbs which REALLY DOES save us considerable money. I know both those situations cause havoc on guitars and amps. STORY HAS A NICE ENDING THOUGH - My friend loved the guitar and asked me to sell it to him. I told him to check it out at his house on his amp first to make sure everything was OK, and it was...

Thanks a lot for the video, much appreciated. Take care.

Superb tutorial,sound theory.

very good and informative explanation here ,so good I had to subscribe to your channel ,im a 1st timer but it def wont be the last time ,thanks for your great teaching knowledge uncle doug is a great teacher as well ,between the 2 of you im learning alot ,I have had electrical therory being a commercial electrician ,and your right you dont want any distractions when working with high voltage can def relate to that ,i have installed breakers in 480 volt panels before that couldnt be turned off and you dont want anything or anyone around you

Awesome video and great explanations on how to measure unknown transformers. And for sure you never can warn people enough about the dangers of high voltage! But there was a little mistake with looking up the numbers on the single ended one. You were looking up plate resistance in the book which is not the right value. It must be load resistance. And you were looking up the 6AQ5 in triode connection which needs a different value as well. Plate resistance for the 6AQ5 at 250V Class A is appr. 52kOhms while load resistance is about 5kOhms. For a 6BQ5 or EL84 load resistance is typically 5,2kOhms while with different Bias settings you can also use 4.5kOhms or 7kOhms. So in the end it will fit for both tubes at 8 Ohms secondary.

Fantastic practical video!!!!

Thank you very much for the great videos. I'm just starting out.

THANK YOU SO MUCH! Extremely helpful ! Great Video

This is a great video!

I think it is worthy of mention that because Impedance is the term used for alternating current as opposed to Resistance for Direct Current and both exist on the primary o an Output Transformer and because the impedance of speakers is dynamic - varies with frequency - the actual dynamic impedance of the load on the Primary and therefore on the Plates, varies as well. Also if you notice that the "required" P to P load impedance of a given tube changes with both DC voltage and Class of Operation (Class A needs a higher load Impedance than Class AB or Class B) and what this translates to is the linearity of frequency response under those dynamic conditions, it can be seen that it is safe to use a higher impedance speaker than was originally designed for, especially if you wish to increase supply voltage different than the example in the book or if you'd like to try a different Class of Operation. For example it has long been noted than guitar amplifiers in which some kinds of distortion (non-linear response) are desirable can often benefit by plugging for example an 8 Ohm speaker into the 4 Ohm Jack/Secondary winding. It will likely be necessary to adjust the bias voltage to get the best sound or to get to Class A responsiveness but if you don't go overboard and/or monitor current conditions while playing, you can discover new tonality and distortion characteristics in your amp. YMMV but this is one advantage of Vacuum Tube amps which require Output Transformers as opposed to Solid State most designs of which eliminate the Output Transformer. Output Transformers are one reason Tube amps sound different from Solid State amps... not the main reason, but one reason.

Great Info - THANKS !!

For 08:48, this is a beauty! It is for driving transmitting tubes with grid current:) :) The easiest way to get some serious power out of them is use AB2 with grid current is an interstage transformer....With the grid center tap goes to the bias voltage, or may be ground if you going to power up some 1k or more plate dissipation tube-beasts;)

One of the challenges of building efficient class B audio vacuum tube amplifiers is the fact that the output impedance of the tubes varies with conductance, which varies with screen grid voltage and control grid voltage. It's one of the things that creates nonlinearity in tube amplifiers. Making matters more challenging, imagine you're using MOSFETs to control screen voltage dynamically to respond to program peaks while keeping screen voltage low at idle, so the idle power is low. In reality, high fidelity and efficiency lie on opposite ends of the spectrum. It takes clever engineering to cancel out many of these distortion inducing technologies to make an amplifier that is both efficient and high fidelity.

Nice video! I like the US style of transformers, since here in germany usually transformers have soldersupports points instead of outcommin´ wires.

Thanks for another link sir...i'm a biginer tube audio...thanks again

Thanks . Excellent tips!

you're explaining right at my speed 👍

In every video u explain thing so brief it's so generic thing appreciate your knowledge and experience

That’s exactly what we wanted to know! WHAT IS IT AND WHAT CAN WE DO WITH IT!

formula works great. Thanks!!!!!

Thank you,, great video 💡❤️

very informative..thanks!

Thank you very much it was just what i wanted

Nice info!

Great video sir!Thank you very much! I have a question.My amp is 5watt champ with 4ohm secmndary. What happens when I connect 8ohm speaker to it.The primary is has to be 8kohm.So connecting the 8 ohm speaker I'm actualy rising the primary impedance witch shoud lower my output power?What more goes wrong?:)Does the 6v6 start working beyond his limiths? Thank you very much!

hi i've being looking on the net about how to choose the right PT for instance like with caps to use for the power section and soforth any help would be much appreciated

I built a project ecc83 and 2 el84 amp from an old philips mains transformer the design came from online very low component count but i was too poor to buy output transformers i had 2 matching 230v mains transformers laying around Big ones and worked out if i could use them size of them was overkill for 2x5 watts but the secondary windings worked out perfect for 8 ohms or 4 ohms as it had a centre tap on the secondary I fired it up and it sounded great powerful too considering i used my TDL rtl3 speakers that are no so efficient But then i read that good se transformers have air gaps something to do with the magnetic flux in the core of the transformer. so i looked up how to do that.its air gaps between the E and I metal plates.So i took all the Es and Is out and stuck thin strong sort of stiff cardboard on all and then packed them all back in.And the result was mind blowing.since then 20 years ago i have owned a lot of high end equipment.I love class a so expensive equipment. But i have never heard since the quality of the little amp i made myself.playing a beatles CD you can always hear the difference between john and paul .but with my home made amp the difference was much more like they were singing right in front of you.

May 2nd, exactly on my birthday, nice...

I have three of these beauties now. One measures up to 6KV and the other 5 KV. I "smoked" one not too long ago by measuring an audio voltage out of a modulation transformer that was a bit over 2000 volts AC but it was at 1 KHz. That was a bad idea. It arced the switch and I was finally able to find it and scratch the carbon of with dental tools. I kept a paper schematic inside the meter and it almost caught it on fire. It works proper again now but I don't use it any longer for HV measurements. High voltage at virtually any audio frequency will arc much further than 60 Hz and I knew that but thought it wouldn't happen.

Great Stuff

At around 1:15, you described the configuration with a cap to ground on either side of the choke as a choke input filter. I know you know the difference and that it was a slip, but for those that don’t, it is a capacitor input Pi filter.

I would like to know more about input mic transformers since I'd like to build mc phono step ups. Where's a good source? I wish there were videos like this about them.

Learned more about death, ahmm. I mean power trans vs output trans vs choke than any video on RU-vid. Only 6mins in

i have a 213 magnatone out put sound almost 0 and i see light in all the tubes i think the output tubes look dimmer than the rest could you point me in what direction i should go to repair this.

Hello! Thanks for the very informative video, it's a very simple operation and yet I couldn't find a clear explaination until now. Unfortunately I don't have a variac, could I use multiple low voltage transformers (4.5/6/7/9/12v/24v) and then extrapolate?

good stuffs here, thx.

I had a nice collection of transformers out in California. I had a reasonably good job and a house too. Then the economy turned just enough that I wasn't able to sustain my house. Then I found myself living in an equipment yard in a box truck battling all manner of thieves, tweakers, recyclers and landlords all converging upon me claiming that I was a hoarder. All wishing to help relieve me from my copper. Well now I'm stuck in Michigan where I grew up and there are no surplus houses here and no way for me to rebuild my stock.

I have the same Triplett 630NA..my fave multimeter..

thank you so much

You can use Power Transformer as an output transformer as long a you do not overdrive the magnetic field. I used center tapped (good to have 230V AC mains, i can use 120V two winding or centertapped transfomers) or two times winded Mains transformers a lot to test some pushpull amplifiers without the need of an expensive OPT in the first place;) But this is for experienced "tubetinkers" only;)

Great tutorial. How do you determine the push/pull load on useful tubes where it isn’t noted in the manual? Bogen liked to use 6AV5 and Grommes used 6GT5 which were not specifically designed for audio but for TV horizontal deflect circuits.

How would you suggest I cover the transformer wires inside ( under the bell covers) after I changed the wires. The sticky shellac like paper was covering the wires which I had to remove to change the wires. I'm not sure what to do. Thanks

Good work, neighbor.

Great video! Thanks. I like using old military tubes but sometimes have difficulty finding to A impedance or A - A. Do you know of any books/info that would help with this? Thanks.

How do you determine impedance ratios of small signal mic input or mixer output transformers? I’ve done it for unknown outputs that are used in tube guitar amps. Same way?

Those big gray transformers look big enough to power an SVT and a half!

nice ! , without hollywood "experience", nice presentation about transformers and the way you can deal with,

“I got more transformers than the law allows”... had me laughing

I was wondering whether it wouldn't be safer to avoid the variac/6V TX altogether and use an oscillator like your SG502 or SG505 on 50/60Hz as the voltage source. Is there something against that that says it won't work or is a bad idea? You can get about 5V out of them. I did this once to figure out the secondary phasing on a PTX and it seemed to work OK.

Did any of this come out of Friendship-7? I did not belive about putting my hand in pocket until one day I put both hands on my amp...Keep up the Lessons. Thanks so much...

thank you :)

Do you troubleshoot small 50 watt amps Ive got a pair which are identical builds but do not pass sound I think its jumper related or something's fried the OT on both but I dont think thats it I think im missing jumpers. I get voltages and close readings but no sound. Near Dsllas area... thanks.

so if i am winding a transformer and i want say 6000 ohm primary to 6 ohm speaker how do i know the minimum turns on secondary. i understand the ratio and impediance but you never see a one turn winding, its always multiples

i understand the safety and precaution emphasis but if you wire everything up cold, and then plug in. are you at risk? cause it's leaked onto the case?

Hello sir I need some advice you see over the years my car experienced lot back charge problems I fixed that by using lot diodes on ich device and put on the field wire for my alternator an diode with Puch button to manuali engage it when I turn on engine first I did notice my cheap voltage regulator and ignition module who is an 4pin GM module do not have zener diodes. The problem is I need kinde of system that can out put 2 different paths of electricity.I work lot on engine modifications .So I did in the past try out using ceramic capacitors on my hei ignition coil who is an e core type every thing was fine and good engine starts faster it appears lights are brighter and engine operates nice on centrain fuel types now the down side my alternator cutt in current gets confused in the past it wuld not shut off the charge indicator light unless I rew more my engine so I requare more engine rpm to charge my baterry and make that bulb turn off. So I did eventuali trow all that crap away and put an 10amp diode and push button switch to manuali operate my alternator but still I ques because my voltage regulator on the alternator is lacking an resistor and capacitor like the US version Delco 12si alternator should have I'm confused now how to counter that anoying effect? It's just bunch high voltage ceramic capacitors soldered together 1 side goes to ignition + side the other via toggle switch to ground basicali.I hoppe my starter motor does not have any thing to do with that strange effects because it's an magnet type with s/R starter selenoid .So I hoppe for any advice how can that by solved or if there is any possibility to build an multi center tap transformer with an alternator with frequency stabiliser basicali I want to use charging system form my lead acid baterry and use the second path to charge capacitor banks or an baterry that can not explode because I want my chassis no not get effected by back charge and just make my ignition system more variabel you know just to start better in winter time and tune spark for variuse fuel types if necessary small corrections. Only problem I have so far I don't if those transformers need to by grounded should I make virtual ground with relay not to get shocked true the chassis? I am honestly feed up with my crappy Delco alternator that thing isn't variabel at all and components fail so hoppe you understand my problem and can give my some advice how it culd by possibly achieved multiple independent out puts from an alternator or how to convert that thing possibly.I wasn't Shure but perhaps an external driven alternator amp or voltage booster might work driven by an 2 stroker engine or perhaps AC motor set up .Ya know just some sort of improvised starting asistance in winter time. So I do hoppe my question is not over complicated .I also thought there might by possibility to replace those shity components from Delco alternators to some thing more heavy duty industry parts like bridge rectifiers or voltage regulators who are not sensitive to ground static. So hoppe for your opinion about this topic.

Question: If you apply voltage to the primary side via the 6V transformer/Variac, why not to put 4 or 8 ohm load in the secondary side and then take the volt reading thru that load?

Hi Dave, I have a question about 2 output transformers I have and if they will work in a single ended application. I took them off of a pair of 10" speakers out of a 1966 JVC radio, reel to reel tape, radio console. The unit said Stereo, each transformer on one side has a common, 4 ohm an 8 ohm lead. The primary side has 3 wires, it says 5K and the middle wire is a Center Tap. Could I use them in a single ended amp with a 6V6 or 6AQ5 if I didn't use the center tap? Or will it not work? Im not sure what the center tap is for. Thanks, Tom

if you don't know which of the 3 wires on the primary side is the center tap. how do you determine which it is? thanks.

Hi you, cool channel! Let me first say that I know nothing about electricity or how to build an amp, in future I'll get some techman to recreate an old '66 Marshall JTM45/100 (like the ones used by Hendrix at Monterey or Angus Young in the High-Voltage AC/DC album where he used a 675 Volt Marshall with KT66) by using vintage Partridge transformers instead Drake's. I always seen those Partridges going matched with EL34s. But now I'm wondering if '70s Partridge transformers (those ones used in Hiwatt DR103 amps) could match four KT66 tubes and their high-voltage stuff. Thanks

Thank you for a great and simple video. Are those calculations for both triodes SE and pentodes SE? I find it hard to understand, for example if that load resistance in the tube data sheet would be the same as transformer primary impedance - it would be perfect match, right? But when you take 12AU7 as a power tube for 1W "Micro Champ" built - which has approx 7.7k load resistance, it needs 25k primary.. Why?

I tested my 1959 gibsonette GA-9 O coil using a similar technique and arrived at 27.5:1 With the stock speaker at 3.5OHMS Z= 756.25 OHMS 27.5 ^ 2 so P = 2647OHMS Z * 3.5 I cant find within the data sheets how this single ended dual 6V6 class A should be. The closest I saw was a triode config at 1960 OHMS

Do you repair transforms? I have a large power transformer that feed large 1000watt power amp and pre amp on jukebox unfortunately evertime I connect power supply to it it kicks breaker on power supply. The transformer doesn't smell burnt and sure looks fine. This transformer has many wires and hard to find another is it possible to rebuild or should I just use three separate tranformer instead of one large. Thanks

@El Paso TubeAmps I have pair of Jamo MFB 100 Speakers, both amps working great, but one oft the toroidal transformers is broken, there is no data written on the transformer. I measure 2x23,4 Volt on the intact transformer, with an without load (low volume) Power consumption is (according to the back) 120Watts According to the data sheet it's Nominal power Woofer 44Watts, Mid,23 Watts, Tweeter11Watts ------ Music output Woofer110Watts, midrange 55Watts, Tweeter 40Watts. The toroidal Transformer weights 1150g, 9cm Diameter. Which transformer should i buy 20X20 Volts 22X22Volts and VA?

I love the antique analog multimeter ! whats the brand / model please?

Enjoyable

Got a question You say the push pull 6V6 output transformer is a single ended load but a push pull primary. What would the OT be like for a amp with just one 6V6 instead of two 6V6. Could you make that OT work with just one 6V6 if you left off the center tap wire? Would the impedance be 8100 at 4 ohms without the center tap on the primary if it was just one 6V6? Does the center tap on the primary always mean it is a push pull OT? Thanks

do you have or have you ever had a 70749 power transformer and if so could you please tell me what the wires are and what the voltages are on them?

The plate to Plate impedance is going to be double the plate impedance of a single tube however. A push-pull Transformer with 8000 ohms plate to plate impedance is like two single ended Transformers with 4000 ohms Plate winding in series connected together.

You've got the same calculator and clipboard as Alan Wolke :)

Hi and thanks for the vid!My problem: can I replace the 2 output transformers of my integrated amp Bell 2420 in order to upgrade the sound?...Marked on them : impedance Pri :9500ohms CT, Sec: 16 ohms Tap ,8 ohms ,4 ohms.Many thanks

love watching your videos... question... Mcintosh MC75 uses 2 kt88, but they are able to make the Zout to 600 Ohms..how can i make the same dual kt88 to have 1500 ohm zout? pls help... thanx a million...

Non-electronic question here... very nice video quality, what's the camera?