How to Identify Amplifier Power Transformer Leads 

This is a great video; thanks Uncle Doug! Your videos are still helping people 8 years later!

Love your laid-back tone and sense of humor. I've got a box of mystery transformers that I can now get to work mapping.

I just wanted to say that I have watched several videos online but yours 'clicked' with me. I identified the primary and secondary sides, center taps and which leads were the common, 4ohm & 8ohm on my output transformer! Thank you so much for your great videos!

Excellent video - thank you for posting. A few details that the novice may need to know: 1) After energizing the primary and when taking voltage measurements of each secondary, the voltages measured at each secondary will be higher than you may expect. The reason is because the transformer is not under load. Once the transformer is placed in a circuit and given the prescribed load conditions, the voltages will fall to what you would expect to see. For example, that 370v RMS high voltage, will load down to something closer to 350v when the transformer is placed in service. That 6.3v secondary will probably measure something closer to 7v RMS on the bench, but come down to right around 6.3v RMS under expected load conditions. 2) I will double-emphasize what Uncle Doug said about not getting into a big hurry, and to please use a current limiter.. If you goof and put line voltage on (for example) one of the filiment secondaries (like the 6.3 or the 5v secondary), the transformer will not have enough inductive reactance on that winding to limit the current. At best you will probably throw a circuit breaker in your house, or at worst you may over-heat the transformer and destroy it. 3) If you find an old transformer like this, it may be perfectly good, but it may also have been removed from service for a reason. One such reason might be that it already failed from over-load. When transformers fail from over-load, often times what happens is that either the primary or the high-voltage secondary will develop shorts between the windings. When this happens, it can draw excessive current just by having the primary plugged into the voltages it was designed for, unless the primary already failed open. If this happens, you will get excessive current even when you are doing everything correctly. It never hurts to put an amp meter in series with your primary so that you know what the transformer is doing. Also one caveat and disclaimer is that if you don't know enough about electronics to understand how to connect an amp meter in the primary circuit to measure the current, then you probably don't need to be dabbling with something like a supply transformer anyway, and should leave this kind of activity to people who possess the knowledge and skill to do this kind of work safely.

I have never seen such useful information in dozens of books and hundreds of magazines that I have read. Thank you very much

Thanks again for sharing your expertise in the field - your postings save us individuals (who are new to the field) many hours of frustration - as well as keeping us safe. I did watch and build the current limiter, and it works like a charm... Happy New Year!

Thanks for the video Doug and Rusty. Nice logical layered approach from what is needed by the circuit to confirmation with both resistance and voltage. Great explanation as always. Take care. Colin

This is the clearest explanation i've found, thank you.

Nothing like watching Uncle Doug using his Nicola Tesla channeling techniques. Great information!

Thank you Uncle Doug for all your informative videos. I watched all these before I started to build my first full tube amp.

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 have surplus and junk PTs and OTs, and you really helped me out with identifying the unknowns and turning what were paperweights into useful electronics!

I am a retired electrician with a smidgen of electronic knowledge and am enjoying learning more. Your method of explanation is excellent for novice or experienced person. Thank you for your succinct method of instruction and I am enjoying any of your videos that I can find. When I wrote my qualification exams, I was only allowed to use a slide rule as there were no calculators except 4 bangers at a very high cost.Whereas I have had an opportunity to study modern electronics, the basics are usually lost, especially tube workings. Thank you again.

Finally... a teacher who knows how to unlock these electronics mysteries, for us newbies. Thank you Uncle Doug for these many tutorials - the quality of which I have been unable to find elsewhere. In terms of your invitation for further topic requests - and related to this particular video on amp power transformers - I'd like to see another angle on this topic. That is where there is a need to replace an amp's power transformer with a new one, but the schematic [where available] does not specify windings values. I'm confronted with this exact scenario just now, whereby I would like to convert my 120V amp to 240V - and the current transformer does not carry a spare set of windings for the purpose. Exactly how do I confirm the power transformer specifications for my existing amp to a potential PT supplier? A topic for a further session, perhaps? Again, heartfelt thanks for these invaluable tutorials. Uncle Doug, you rock - and you are pretty handy with that ax! :)

As always another great video! They will erect a statue of you and Rusty somewhere someday in remembrance of your undying posting of "the best" informational videos of all times. What a great pup! Thanks

Great instructional videos! Thank you. I wish you had been the instructor in my Freshman AC classes! To this day (many years later) I still have to use reference material for some very "basic" troubleshooting. I'm glad I found your videos -- and kept my old textbooks! Thanks again!

good to hear you again -- as you know with a 6.3 v winding with c/t some tube rectifiers can be used -- 6X5 EZ81 etc...

This is excellent! So much insight and so well explained. I paused at about 2/3 to check and was glad to see you are still making videos; subscribed.

Possibly the most helpful video Ive watched on youtube. Thank you Uncle Doug

A very clear and easy to follow explanation. Superb hand acting as usual, and good camera work from Rusty. Thank you.

Thanks Doug, I actually stripped the radio down to a bare chassis, and never considered attempting to use the circuit for an amplifier. The chassis is being set aside for a future amp build.I did a bit of reading about the heterodynes, and am aware of the dangers of power possibly crossing to the chassis. I will watch your output transformer video, and test the OT to verify readings of the wires (ohms/continuity). Thanks again

Thank you for the info. I never bought second-hand transformers because of the wireing. All the spare ones I had, I removed myself carefully and labled them as you do.

I was just in the dining room (my electrical shop) inspecting a power transformer id bought. Your video came up and was very informative. Thanks for posting it.

Hi Uncle Doug, great video, good explanation and, as usual, solid Rusty camera work.Thanks for sharing.

another great video Doug! I have a pile of old transformers I've been hording for many years and always just plugged in the blacks and measured... good to be safer and more methodical!

Thanks Uncle Doug!! My buddy just dropped off a nice GA23P from an old Gibson...your Rosetta Stone is magical!!

Hey uncle doug! stumbled across your channel recently because im a young person thats self taught in electronics, and i saw this video on how to identify transformer leads, Very informative and makes identifying the leads easier, i have a bunch of salavged power transformers and i thank you for uploding this vid!

This was a big help to me with a couple of 1947 transformers. Thanks much.

Always look forward to your videos. Full of good information. Thank You, and Rusty whose steady hand on the camera as always is commendable.

Great instructional video and easy to follow. You mentioned hum is reduced by twisting the associated secondary leads around each other. I would appreciate you expanding on this. In addition, is there a simple method for establishing the max current ratings for the secondary windings? Thanks again for sharing. Don

That was just great. I'm an analog amp nut and I love your information.

Excellent video, Doug. Serves as a great transformer primer, as well. Thanks!

very informative and well thought out delivery. using size of wire, colors, number of wires ohms, to have a broad way to confirm that's what. not sure why anyone would give the thumbs down like i see, nor why folks that do that dont post WHY they dislike.... whats not to like about Ole' Rusty! thanks for posting, will look for more of your stuff. It was easy for me to understand, and that Sir, is no small feat.

Great video as always, I am new to your channel and enjoy your videos.

Uncle Doug, thanks so much for making your videos. I know so much more about about guitar amplification electronics. What a fun introduction to electronics. I really appreciate your teaching style and sense of humer as well. Your workbis a great contribution. Thanks so much!

Great info, thanks a bunch. I have a late 60's tube amp that I have been waiting to turn into a nice guitar amp. The tube layout are 2 - 6v6's, a 6x5 rectifier tube, a 12ax7 and 12at7. I'm a 5e3 Tweed board would be nice. The output trans has 4,8,16 ohm speaker connections. Time to get it going.

Thanks Uncle Doug. This was very helpful.

Again, i find another one of your videos extremely helpful. You are a great teacher. Thank you very much! All the best to you!

Thank you Uncle Doug. I needed to know that and it helped a lot.

Uncle Doug, I don't remember if I were one of those asking but I will defiantly appreciate your knowledge! Busy testing an old phonograph type and was not sure about the power cable, as all the cable were open without there male or female plugs. Yes I could have tested the wire before I connect to the power, I track the wire at the back on the inside at the Transformer, but that wasn't the power cable, it was the speaker wire. So a little bit of smoke and there I made a big mistake with this 50's Phonograph with a Receiver. I would like to send you pic's, yes! SO after knowing about the awful mistake I took the other double wire and connecting it to the power after tracing it, I had to dismantle the whole system. Luckily, Luckily it is still power on, the only problem is that there aint any sound any more... I know now why and would of coarse love to fix it. Regards

Thank you Doug, your videos are priceless !

Thanks VERY much for this video!!! I work on tube radios as a hobby and the transformer wires always mystified me

Uncle Doug, you really are amazing. Thanks for all your educational and informative videos, and thanks for the wonderful dry humour (sorry, I'm Scottish and need the extra "u" in humor...). I hope you, Rusty and Jack are all well. Thanks again, Coll.

UD I am restoring a 1980 Marshall 100w lead, 2203 circuit. I had to use this video today to id the primary and secondary leads of the PT...This amp was a mess, no twisted pairs, all wires different colors and was a bit more complicated than your simple one here but Anyway this video helped a great deal. thank you!

Absolutely brilliant video Unckle Buck!!

I can't believe Rusty is 18 years old! That's quite a long run for a dog. I was also surprised when you said that you were 78. I believe that the key to growing older and keeping your mind sharp is to have hobbies, interests and things to challenge yourself. I became interested in electronics when I was 9 years old and I'm 48 now. I have had a very rewarding career as an electronics technician/engineer. It saddens me when I see young people that don't even know which way to turn a screw. It's up to people like you and me to get younger people excited about electronics. You do it through your videos. I have plans to start my own RU-vid channel at some point but, for right now, I have been teaching electronics to other people in person. I have nephews and sons of friends who play music. They often bring guitars, amps, pedals, etc. to me to have repaired. I always tell them that if they want it done for free, they have to "help" me. When they leave with a working device, they know more about electronics and have also learned the process of troubleshooting. Thanks for what you do here. Take care of yourself and I hope you have many more great years helping others understand electronics.

Thank You uncle Doug!!!! I usually watch watch your video's a few times I have to take notes hahaha, I'm just a old guitar player and started trying to build my own amps or at least make the ones I have better. I really like your videos no bull, just knowledge thanks for helping the little people lol!!!!!

What a wealth of information. Thanks uncle Doug

Thanks This video has been very helpful!

As always great video and a good lesson for us tube-o-holics :)

Thank you for an easy to follow video.

Nice presentation! Thanks.

Doug, Great video and excellent tutorial.

I really enjoy your videos and have learned so much from you , thank you. A few months ago I had just built my first amp , out of a old tape deck. These things seem to reproduce now there are 5 and they make me buy test equipment. I'm not addicted to the smell of old tape recorders , I can quit anytime I want. When I hooked up a power transformer to the current limiter , without a load on the transformer I got nothing. Using a small Craftsman digital VOM. Then I hooked up another transformer I knew was good and nothing. It's good I didn't put my tongue on it. lol When I hooked it up direct to AC ..... I got power?

Loved this video. Keep up the good work

Great Video! Thank you.

That was fascinating! And helpful!

Thank you, that was a very logical and useful explanation.

thank you, sir! this is really shed a light to my sight :)

Very thoroughly explained. Thank you.

Excellent video! very informative.

Excellent info, expertly explained. Thank you, helped me out.

great information doug, i've quite a few unknown transformer's, now i can figure out what goes where, p.s. say "hi" to rusty,

Thank you so much! Very helpful.

thank you sir, helpful as always

Best videos on the WEB! Thanks Doug....

you are back to making at leat the occasional video that is awesome sir

Very helpful thanks!

This video was playing while I was asleep and it gave me one strange dream!!

Hi Uncle Doug, I hope you, Rusty and the fam had a wonderful holiday season! I've been sitting on my hands, eagerly awaiting your next excellent video. In the meantime, I've carved out a new work space, installed some functional lighting and a permanent, wall mounted current limiter, using one of several vintage mogul base 300 watt bulbs I got from an old hardware store that recently gave up the ghost. Here's my question: I'm rebuilding a 1957 Les Paul Jr. GA-5 circuit from scratch and the original PT is blown. I have obtained a vintage PT absolutely free, so I'd like to use it. Trouble is, it seems a little big. There's tons of literature about running amps with small PT's, but it's hard to find info for those of us blessed with too many volts and plenty of current handling. My free transformer measures out at 375-CT-375 (750V); 3.8-CT-3.8 (7.6V) and 5.8V. The original circuit was for some sort of antenna array, which used a type 80 and a type 83 rectifier. Those tubes are supposed to have 5.0 volts, same as a 5Y3. So, I'd be interested in your opinion, can I run 7.6v instead of 6.3v on the 12ax7 and 6v6? Is 5.8v instead of 5.0v is OK for a rectifier such as a 5y3? And if not, would I reduce the voltages with power resistors in series? Thank you.

Thank you.

I like your videos because you explain things very thoroughly. I thank you again for your efforts. have a great day.

Thanks Uncle Doug, another great informative video plus....a grand appearance by Rusty! He's probably a little upset about being on the "end" of the vid! LOL!

Just wanted to say thanks. I'm sitting here with a tangle of transformer wires, think I've got it sorted now.

Thanks a lot Uncle !

Thanks very much for excellent video's explaining both the "power" transformer and "output" transformer funtion in a tube amplifier. Very helpful information. Will need to rewatch videos a few more times before passing any test but great information I failed to find in other sorces. This the type of information I need to learn and really understand how a tube amp funtions. My eventual goal being to (diagnose problems) and repair tube amps for personal pleasure and hobby. I have more questions as well as answers now. Good thing you made more videos covering other tube amp components and their funtion. One question however, can any other tool perform the funtion of a variac? Seems I am going to need one. Thanks again for all.

CKHC VD: Your inquiry had no Reply capability. Hopefully, you will see this. Testing the transformer is rather simple. Once you identify the primary and secondary leads, you can determine if the leads pairs have measurable resistance. If a winding is open, the resistance will be infinite. Finally, you can connect the primary to a current limiter and (very carefully) measure the voltage across the matched output leads.

very informative I just ran across your Channel, I only have one thing to add , next time please can you show us the actual readings and the identification of each pair with the multimeter, it would greatly help my learning curve thank you.

This video is gold

thank you so much for your kindness--

Great video! I'd like to add, look for a short to the iron core from any of the winding's, I've seen this problem a few times, and it will make for a nasty shock!

Thank you very very much for posting this :-)

Thank You!

Echo, I cannot reply directly to your inquiry. Hopefully, you will see this response. If your amp is operating properly at the moment, I honestly would not recommend changing to a Mercury power transformer. Others may disagree, but I really don't think the benefits would be worth the (substantial) cost.

Very easy the way he presented it.❤

Great Video

great video

Thanks for the great video! I have a question for you. Trying to repair a solid state guitar amp which has a noise (a grounding noise), I came across this : - The ground from the ac socket is connected to the amp chassis. - A black wire coming out of the transformer is connected to the ground of the pcb ( where the sleeves of the jacks are also connected) But, there is no connection between the sleeves and the chassis. Does this mean that the transformer is faulty? And is it ok to connect these two grounds (the jack sleeves & the chassis) with a cable? Any help would be much appreciated!

I have tape decks that don't have a primary line fuse so I put one in is that a good idea I put half amp in cassette decks and CD players

Hello Doug! Are you planning to do a video about the basic safety tips associated with tube amps? Your videos are such a fun and easy way to learn! Thanks, András

Okay that makes sense. All resistance readings across primary and secondary windings on both the PT and OT pass inspection. I may have to move on to more advanced testing for both units. My main reason for testing these is that I feel the guitar amplifier should be much louder. I feel that I shouldn't be able to set both the channel volume and master volume to full on a 100 watt guitar amp and driving only one 12" 65 watt 8 ohm speaker. In comparison, my friend brought over his 60 watt Fender amp and we could only get that Master Volume to 2 before it was shaking the house. My amp does sound great and is not weak sounding at all. It has 4 new matched output tubes , New preamp tubes and filter caps.

While I was watching this video, when you mentioned about the low resistance of the primary, 5 V and 6 V windings and how hard it would be to measure the low resistance, I immediately thought about using the ESR Blue to test the windings. That's what the meter was designed for, accurately measuring low resistance. Of course you would have to have a Blue Meter. What do you think Uncle Doug? Would it work? I'd try it myself but I don't currently have an extra power transformer.

Nice 1 doug , so how do we go about building this ultra low noise amp then ?

Fantastic video! Could you make a similar episode covering output transformers? For example, how to identify output impedances and wattage?

Got a quick question for the all mighty uncle Doug, lol I got a power transformer, it's got 120v input and green filiments which measure 7.8v which is high, then it's got yellow and red wires for the 6x4 rectifier, with a red and yellow center tap, but then the confusing part it's got two blue wires and a blue and yellow center tap, when I measure them to the center tap I get 15vac apeice 30v together any idea what these could be for, the transformer came out a old Ampex recorder, it ran 10 12ax7/at/au preamp tubes and the 6x4 rectifier, I want to use it on a small build but don't know what the blue wires are for. Thanks any help would be appreciated. Your pupil

Great video. I fear electricity greatly! Just as a PSA I guess (Public Service Announcement) I have been working as an Industrial Maintenance Technician for about 22 years. Granted we are only talking about 120 VAC input here and single phase BUT the output of this transformer is well over 300 volts...PLEASE PLEASE be EXTEARMLY careful in dealing with transformers of this type (300 plus volts output voltage) I have seen the result a dead short in action and when a transformer blows up from a dead short it is not a good day. Please listen to Uncle Doug when he says use a current limiting device to do your tests...the life you save could be yours!! Thanks for the video!!

This is great and you are ACE :-)

Excellent! I'm going to firm this knowledge in my brain...hoping it will stay there. Am I correct in assuming that you have created a similar video regarding Output Xformers?

Hi Uncle Doug. I have got myself a princeton reverb reissue circuit that has been gutted out of a new princeton reverb. I have purchased upgraded transformers for it and at the stage of starting to wire it up. The Power transformer I got is a Hammond 290BEX. On the diagram it is showing an extra 50V blue tap in the cetre of the 330v secondary. I take it that for a princeton reverb type amp I will not be using that and just isolate it. Thanks for reading.

Hi Doug! Your video's is such a great source of information, really appreciate it! I've got some questions which I hope u don't mind... I'm building a bass preamp and are researching about some things,I will use a hybrid tube/ss rectifier and as I understand there should be a limiting resistor between the ss rectifier and the tube, your thoughts? and I'm searching for a gain/sensitivity control which appaerntly should have been used in some of the matchless amps, do you have any tips for me there? Keep up the good work and give my best to ollie and Jack!/Peder J

Fantastic.