How to Identify Amplifier Power Transformer Leads

This video is posted in response to a frequently asked question regarding how to identify the leads of an audio amplifier power transformer. Several methods are explained and demonstrated, including: Color of Leads, Twisted Leads, Wire Gauge, Resistance between Leads, Identifying Center Taps, and Voltage Output of Lead Pairs. Rusty and I hope this will enable viewers to accurately identify up to 11 different power transformer leads.
NOTE: Only 2-wire 120VAC primary power transformers are discussed. Diagrams are available at the Triode USA site for transformers with 120/240VAC primaries and four primary leads, which must be connected either in series (240) or parallel (120).

[@mickel7b]

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

[@JimButler1234567890]

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.

[@missingsock18]

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.

[@mixolydian2010]

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

[@ollikivela2804]

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

[@SPWoz]

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!

[@lumpy2080]

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!

[@bullthrush]

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

[@michaeldabrams]

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

[@zaugitude]

This is excellent! So much insight and so well explained.

[@pdimor]

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

[@dougtaylor7724]

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

[@agylub]

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

[@unhinged17]

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!

[@garyadkisson4790]

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

[@rayfisher1564]

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!

[@huberttremaine8343]

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

[@thome1547]

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

[@BenPrevo]

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...

[@Barnekkid]

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