Output Transformers: A Discussion of Basic Principles
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- Опубликовано: 2 окт 2024
- In this video I attempt the unenviable task of trying to explain the design and function of one the most significant, yet misunderstood, components in tube amplifiers: the Output Transformer. Topics include: Purpose of the OPT, Winding Ratios, Demonstration of a Method to Measure the Winding Ratio, Consequences of Using Incorrect-Impedance Speakers, Impedance vs Resistance, Impedance Reflection, and how to select the proper OPT for your amp. Be warned: some math is inevitable, but I try to provide sufficient explanation to (hopefully) make it all palatable. Think of this as a video buffet.....consume it all, or simply pick and choose that which appeals to you :)
If you enjoy videos featuring classic vintage amps, jukeboxes, homemade electromechanical devices, and clear, basic technical presentations, then please subscribe to my channel. You will gain immediate access to over 85 videos, and (if you activate the service) you will be notified each time a new video is posted.
Thanks for watching !!!
This video is excellent. That is the first time in my life that I have ever heard a satisfactory explanation of the difference between resistance and impedance. Thank you so much. I'm learning so much about valve amplifiers just from watching your videos. I'm very grateful that you have taken the time, care, and planning to share your knowledge with us.
Two important points to make:
1. It is important to note that the 1.25 is a rule of thumb which is valid ONLY for the range of frequencies for audio. This difference is a function of the AC frequency, so it's not always true when dealing with radio or other ranges of AC. For more info see: www.electronics-tutorials.ws/accircuits/ac-inductance.html
2. Reactance in AC circuits comes in two flavors: Inductive Reactance which affects the changing electrical CURRENT in an AC circuit, (which Uncle Doug outlines on this video), but there is also Capacitive Reactance in an AC circuit (which Uncle Doug discusses on a different video), it affects the changing A/C VOLTAGE in the circuit: electricalacademia.com/basic-electrical/capacitive-reactance-ac-circuit/
You are 120% better at teaching than any of my EE professors. A+
Wow.....thanks so much, TL.
I'm retired Industrial electrician and a ham radio operator for 40 Yrs but I learn something every time I watch your videos. In a word, they are brilliant. Many thanks.
Thanks so much, Bob. So glad to hear it.
Master class at its very best, impedance, reactance and resistance with AC and DC all explained so much in detail, Uncle Dugg you are the best explaining when it comes to Amplifier and Transformers.
Thanks, SS. I'm glad you enjoyed it :)
I have just found your channel in 2021 and I am having a great time going through the catalog. I have learned a lot and have had it on in the background of the shop for the past week. You are a great teacher!
Thanks so much, Matthew. Welcome to our channel :)
A simple yet complete explanation of what could be a confusing topic! You're a gifted teacher Uncle Doug.
Thanks so much, Aurora. Thanks for watching.
Very good video. I have only one criticism: Please do not apologize for the use of math nor for getting into the numbers involved in calculating these concepts. That is exactly what's wrong with our generation is that people have become afraid of (or at least disinterested in) anything that requires exertion of mental capacity. If someone wishes to become good at audio electronics engineering, they'd better get over any aversion they may have of numbers. So sincerely, please do not apologize.
***** Thanks, Jim. I taught math and science for many years, and agree with you completely. I "apologize" partially in jest and partially as a challenge....that must be met, if one is ever to develop a reasonable level of competency with electronic circuits. Thanks for your input.
***** You are so right!
I agree. And, come on : It's not rocket-science, jiggling a bit with ratios. My opinion is that when you want to be a tech in this field, Ohm's laws must be second nature.
Merjin van Schaik - agreed 100%.
Certainly illiterate people would not be watching this,..
WOW! Uncle Doug is the best teacher. Simply the best explanation of resistance and impedance. Wish I had found him a long time ago. Thanks.
Thanks so much, Red. We're glad you did find us :)
Uncle Doug, you are a national treasure. Thank you!
You are so kind to say so, Gary. Thanks !!!
I second that
He is telling truth.
I’d only add, International treasure.
I second that 😊
Uncle Doug, your contributions to RUclips are priceless. Thank you!
You're quite welcome DW. Thanks !!!
I have to say, Uncle Doug, you are one of, if not the best, teacher I have ever encountered. Your explanations and accompanying diagrams are phenomenal. Thank you so much for all that you do. And please thank Rusty for teaching all of this to you. 👍🏻
Wow, thanks, KK. Please continue to enjoy all our videos. I will ;)
The best explanation ever about the difference between resistance and impedance on RUclips.
Thanks so much, Alvaro :)
Such a well explained article for electronic starters
Thanks, Graham. Glad you liked it. Please note that I have about 110 other videos that you (hopefully) may find equally interesting :)
Uncle Doug As valves were before my teenage life I watch quite a few videos of yours and sometimes several times over a period until some points become understood. I believe your videos to be some of the best and practical tube and electronics tutorials.
This channel is beyond incredible. I took my fair share of EE classes in college as a computer engineer, but never before has someone been able to truly explain what these terms are, and especially well by explaining why they are named as such! Amazing job. I just watched your video on the 5F1 yesterday for the first time as I'd been looking for an in-depth component by component breakdown of how that circuit works, and no one yet has described it better. What an awesome teacher you really are. Thanks Doug.
Wow, thank you so much, M. We're really glad to hear that our videos were helpful.
Your videos are so important for future, and us current, generations. This is invaluable information! Thanks Uncle Doug.
+Fuga Mante You're welcome, FM. It's great to hear that the videos are helpful and appreciated :)
So true. Valves are not going away anytime soon.
all the years of using my Marshall major and my other tube amplifiers I never had the idea of why these negativities could happen here's why in the way you just explained it 🙏
Glad the video was helpful, Charles.
@@UncleDoug thank you Uncle Doug 💜
You explain things well enough that I (a math zero) have a chance, if I watch this over and over, of getting THE MATH! Thanks for your patience!
You're welcome, NYS. Thanks for watching :)
Once again, this is a fantastic video. I busted out my old RCA tube manual and looked up a 6V6 and saw the 6600 impedance and got giddy because I finally understand what that number means. I have a box of transformers, tubes, sockets, caps that are all going to finally become tons of small amps. Thank you so much for all the time you put into these videos.
You're quite welcome, Aaron. Hearing from viewers like you, who actually turn the newly acquired knowledge into practical application, makes it all worthwhile. Best of luck with your projects :)
Uncle Doug thanks! I have an old Gene Leis that is heavily molested and I am currently determining the OT ratio (looks to be replaced). it has a good 8 ohm alnico speaker in it and I plan on building some sort of amp out of it. Gonna play with basic bias values and see what happens.
Uncle Doug you did a nice job covering this topic. There is one other small point that I have never heard anyone mention concerning the speaker impedance and that is this. While the voice coil is the main element involved in the speaker impedance we forget that this coil of wire in also located in a magnetic field. The location of this coil makes for a totally different circuit than if it were just an air coil as the inductance changes. This may explain why some guitar players say they like the sound of certain magnets in their speaker as well as a certain type magnets in the guitar pickups. Since the magnet type can change the sound of something do to the metals used in the magnets and the magnetic energy of the magnets. This could be something you may want to think about doing a video on. Since the magnet type can change the sound of something do to the metals used in the magnets and the magnetic energy of the magnets. Thanks.
This is an interesting concept, Neutro, and one that I have never considered. I will do some more study on this topic.
These are great video's, thanks for them. I'm a retired Electrical Engineer who's hobby is guitars and old amplifiers. I find your video's informative and fun.
That's great to hear, Mack. We really appreciate it :)
Can you make your next video on the mains/power transformer please ?? You are the man!! I love all your videos !!
You are a very good teacher. In the last 3 hrs, minus time away, I learned so much about a subject that was BORING in HS and college plus solid state was the latest thing (if it ain't broke don't fix it!)
Thanks, WS. I'm glad the videos are informative :)
I really appreciate you doing this. I'm watching and doing my best to take in bits in hopes that one day it will all come together and I can say, "Yes, I can build an amp. A dual 6L6'er."
You're welcome, Grady. For your first project, you might want to try a single-ended 6V6 amp, like the venerable Champ. You can learn all the basics without a lot of complexity and expense....and then move on to more challenging circuits. No matter what, best of luck !!!
Thanks' Uncle Doug.I was watching your videos' a few years' back, then stuff came up and I am now getting back to them!Really informative and some info that I wish I had,some years' ago.I hand-wound an output-transformer for a prototype amplifier that a friend of mine was building and we were doing a lot of guess-work,at the time!This explanation video helps' so many people to understand the "black-art" part of tube electronics'!Thanks' again for your generous offerings' and sharing your incredible knowledge with us!
You're welcome, Charles. Another output transformer video is on the way.
@@UncleDoug Thanks' Uncle Doug,will be looking forward to seeing that video!
What a gold mine of information ♥️♥️♥️♥️
Glad it was helpful, Charles :)
Sir, I must declare you a living miracle-worker! You have actually managed to convey knowledge into my logic center which had previously been quite indecipherable!! Thats amazing! This is a great great vid thank you so much!
You're welcome, C. Glad the video was informative :)
Thanks for the lesson. Learned this stuff over 40 years ago, but forgot a lot of it. Btw...don't see em a lot with todays fancy digital meters, but I love the old Simpson 260 meters. Tough as nails and last forever! There's just something better to me to watch the needle than a bunch of numbers. Guess I'm old fashioned! And gettin old too! Lol☺
I agree, 74. Analog meters are way better at checking potentiometers.....you can actually watch the smoothness of response throughout their rotation.
Uncle Doug yep!☺
Greg Clemen yep...you described me perfectly. Hope I always stay young at heart!
Greg Clemen that's for sure! This morning my body feels 90 but my heart and mind...20!
Greg Clemen the "Golden Years" suck! I'm confused all the time! I can't figure out which part of me hurts the worst!
Thanks Doug for a very clear explanation. I looked for this type of clarification elsewhere and couldn't find it. I was amazed however by how willing people are( who had no idea what there talking about) to explain it.
You're welcome......glad you enjoyed the video. The internet is "blessed" with all sorts of self-proclaimed experts. The trick is to separate them from those who actually are.
An excellent transformer treatise. Thank you.
I'm glad you liked it, Andrew :)
Wonderful video! Not only does a great job of discussing the basics of output transformers, Doug also clears up the confusion for me regarding resistance vs. impedance. Thanks Doug!
You're welcome, Tim.
Thank you. Very generous of you to share your knowledge sir.
Have to say Doug you are an amazing teacher. The smoke clears every time you speak. Greetings and best wishes from Yorkshire UK.
Thanks so much, Sean. Likewise to you from the Desert Southwest, USA :)
@@UncleDoug Cheers for the reply Doug. 🎸
Ee by gum lad. God's own country.
Excellent explanation!
Thanks, AB :)
This video is excellent. That is the first time in my life that I have ever heard a satisfactory explanation of the difference between resistance and impedance. Thank you so much. I'm learning so much about valve amplifiers just from watching your videos. I'm very grateful that you have taken the time, care, and planning to share your knowledge with us.
You're welcome, NMK. It's great to hear that the videos are helpful and appreciated :)
THANK YOU SIR. Finally an explanation that is simple and clear. Radiotron Handbook was driving me crazy. 🌠
You're welcome, Reggy.
Thank you Uncle Doug! I wish we had more teachers like you around.
You're welcome, PMM.
If only you had been my math teacher in 7th grade...I would have actually learned something. Thank you for all your great videos.
Thanks :) By coincidence, I was a math teacher in grades 6, 7, and 8 for many years.
Thank you so much for shedding light on an area that as a musician and collector of old amps I am interested in but have always been shy of investigating. Very clear delivery.
+Colms2005 Thanks, Colms. Rusty and I are glad the videos are clear and helpful :)
I watched this the first time a month ago and learned a great deal, but what I have used most since is the phrase "I invite you to try because it's one of life's great disappointments when (insert impossible idea here)". Thank you for all this great content and specifically that phrase. 19:50
You're welcome, CM. I'm glad the video resonated with you :)
Love, love, love your work and teachings... I have a few friends struggling to understand electronics, I am sharing this video (among several others) to help them with the heavy lifting, falling so far out of audio'fool'dom. Doug you are a master teacher... Thank you for the effort!!!
You're quite welcome, QE. I'm glad the videos helpful :)
This lesson is pure gold! Thank you Uncle Doug!
You're quite welcome, Watson, and thanks for your kind words. Rusty and I are glad you enjoy our videos :) ^. .^
This video was extremely helpful - Great Job and thank you so much for the effort and time you put into these video's. I've been studying tube amp circuitry for the past year and the fog is finally starting to lift!
+Kevin King You're welcome, Kevin. I'm glad to hear the video was helpful :)
thank you.
I think the way you explain things makes sense to those who may have not understood math in school very well. I really appreciate the time and effort you put into your lessons. I also appreciate the mathematical explanations as I always found maths complicated. I understand it very well how you explain things here.
Cheers and thank you.
Thanks for your very nice comments, Sonor.
one of the best channels on youtube
Thanks, Luke :)
Thank you for posting this video. You explanation about impedance reflection really helped, as this has been a stumbling block for me. You make it easy to understand. Looking forward to more of your videos
You're welcome, Ernest. We're glad to hear that the video was clear and helpful.
Thanks Doug, this helps me...I am not afraid to tackle my Hot Rod Deluxe...worst case scenario, I strip it down and do a hand wire job using said chassis, speaker, and cabinet. Thanks for all your knowledge.
You're welcome, Dave. Best of luck with your project.
Brilliant explanation Uncle Doug. Very well done.
Thanks so much, John. Thanks for watching :)
You are an excellent teacher providing a valuable resource. Keep up the good work.
Thanks, Fak. I'm glad the videos are helpful :)
wow doug i went through electronics in electrical school ,but it seemed to be just memory rather than acutally seeing how reactance actually works to create the impedance after 28 years i finally got the answer to this ,you are a awesome teacher many many thanks to your videos i know this vid is 5 years old but dosnt matter same results ,you are a true electronics teacher ,wish i had you as a instructor years ago instead of memorizing without understanding what was really the therory wow ,good thing i have a good memory been years but nowe its making sense god bless man you really know how to teach means alot
Thanks so much for your very kind words, Joe. We're glad the videos are helpful.
Great video. I'd also like to add, for the benefit of others, that there is a direct correlation between the impedance of a device and the frequency of the A/C signal applied to it. For example... You have an 8 ohm (impedance) speaker. If you measure the resistance with an ohm meter, your reading will be about 6.2 ohms. The way that a manufacturer determines the impedance is by using an A/C signal of (typically) 1KHz. At 1KHz, the speaker presents a resistance of 8 ohms. With that in mind, using a lower frequency, the resistance will be lower. Using a higher frequency, it will be higher. This is why we measure the resistance of inductive devices (transformers, speakers, chokes, etc.) in terms of impedance. When referring to the 'impedance' of something, we are differentiating that we are not measuring the D/C resistance, but rather the A/C resistance at a specific frequency. I tried to make that as short as possible, so I hope it makes sense. Thanks!
Thanks, WJ, for your nice comment and your excellent explanation of impedance vs resistance.
You're welcome, Uncle Doug! :-)
In fact, the impedance of a speaker varies at different frequencies, but not in a linear fashion as predicted by the formula X = 2πfL. The impedance of a speaker at its resonant frequency can be four to five times higher than its nominal frequency, and its impedance begins to rise above 600Hz or so, again up to four or five times its nominal impedance.
Man thank you so much, im very much a beginner to amp repair and electronic engineering and you make everything as clear as crystal. Im learnin a lot hope to get my hands on an amp soon :)
Glad to hear it, NO. Best of luck with your projects.
Damn I'm super stoked to have found this video..im definitely subscribing...there is so much nonsense on RUclips gold nuggets like this can be tough to uncover
Thanks so much and welcome to our channel, Jon. We hope you become a daily viewer :)
This was a very informative and useful video and I applaud your teaching expertise and am happy to have found you and your extra care in explaining the concepts involved.
Thanks so much, Jeff. Welcome to our channel :)
Thank you so much for that Doug that clears that up so well for me and provides the formulars. I always wondered what the difference was between Resistance and Impedance.
+david dkeole You're welcome, David. I'm glad the video was helpful :)
thanks for your time and passion
just got my first Princeton RI
You're welcome :) Congratulations on your new acquisition......it is one of my all-time favorite amps !!
Such a great review of guitar amp transformers and basic electronics. Very clear examples. A great video for beginner amp builders.
Thanks, PCM :)
I've been struggling (miserably failing) to understand the how clipping, amplification / gain gets incorporated into the input signal to make the output and why it works whilst designing and building my own overdrive / distortion pedal. You have NO IDEAR how much you just helped me by explaining how direct and alternating currents act independently on shared paths in a circuit. I had no idea how to use capacitors now I ve a better practical understanding not only where to place them but why to also how to keep the source signal clean by smoothing out the dc current throughout the different stages, where the dc current needs to be smooth and how to use resistance to make them electrons do as there told by manipulating positive and negative voltages to push and drag those brite lil sparks round my circuit doing whatever i tell em! Essentially I've you to thank for my new found slave Labor force!!! and considering I've a job interview next fortnight for an electrical trade apprentice you hopefully helped prevent the bank from taking their house off me. PAT YOUR SELF ON THE BACK!!! and next time you shout yourself beer and hookers know you deserve nice things compliments karma because you Sir Are a fucking Legend !!!!
Wow....thanks so much, Kerry. It's great to hear that the videos are helpful. Best of luck with your interview.
Excellent. A very good and thorough explanation.
Thanks, Michael :)
That was very illuminating, I learned a great deal, and your teaching style is excellent. Thanks for sharing Doug....
Glad it was helpful, M. Our pleasure :)
I LOVE that old analog Simpson meter! I have an old RCA that was $$$ back in the day and I still use it. You did an excellent explanation of impedance mismatch! Once in a while we see someone tell people to use a higher impedance speaker than the output transformer to get this "great tone." It's not a good idea and counter productive, and you did a great job of telling people why this is BAD. This can only be acceptable if you know what your doing and you have a transformer that has a much lower impedance on the plate side and you need to increase it to match your output tubes. FYI - If you're using "negative feedback" from the speaker output side of the output transformer through a resistor and then tap landed on the cathode of the driver or phase inverter tube, and you experience a loud feedback squeal when the amp is turned on... then reverse the speaker output leads. This should put the negative feedback in the right phase to stop the squeal. I've installed new output transformers or built new amps and followed the color coding and experienced the squeal, so... this means that the transformer manufacturer doesn't always pay attention to the direction that they are winding the various coils.
Thanks, Mark. I found it in a local pawnshop for $15 in filthy, with corroded-battery and no-leads condition. It waxed up to look like new inside and out, and is now a proud addition to my armory of diagnostic tools. Your comments on the proper placement of NFB loops are right on the money. If the connection is to an in-phase winding, you will inadvertently create a PFB siren :)
A siren might sound better than some of this so called music... lol. That was a steal for $15. What a beauty!
I agree, and I pray for the extinction of several un-named musical genres.
A first rate presentation. Thanks Uncle Doug!
Bill Norman You're welcome, Bill. Glad you enjoyed it :)
Brilliant again best on the RUclips platform.
Wow.....thanks so much, TW.
You are an amazing teacher.
Thanks, Jeff :)
Excellent Video.....love your videos....I'm learning so much from you
Glad to hear it, Horst :)
Great Video Doug and Rusty.. I always made these mistakes between impedance and resistance, you have cleared me up once again and now I can go burn up parts with more confidence! Great description with just enough math not to ruin my brain :)
Thanks, Bill. We're glad the video was helpful. As Rusty says (usually after a few shots of tequila) "If you're going to burn parts, at least do it with confidence" ;)
The University of Doug. Such a great explanation using basic principles of electric circuits. As simple as the explanations are however it is still a 200 level course without the math. One must already know the basics to appreciate the simplicity of the lesson. What no calculus?
When I was in college engineers did everything on green graph paper with draftsman quality lettering. Everything was done in pencil and you had to show your work! It looks so familiar, comfortably familiar after 30 something years. Actually a little more math would be more fun IMO.
What my wife doesn't quite understand is how on a Friday night I can drink scotch and watch Uncle Doug instead of some dumb ass television show.
Thanks so much, Bill. Too bad Jack and I can't join you for a drink :)
You did an excellent job.
Only one word: Excellent
Thanks, Fran.
I know this is an old video, not sure if anyone else caught this. At around 9:16 in you're recapping what you've just covered in discussion, but instead of saying 50mA, you say 500mA. Most viewers will probably pick this up if they're looking at the tables you've set up. Great video on a very important concept!
Thanks for the info, Donn. Glad you enjoyed the video.
Well now my eyes and mind are really opening up. The formulas are easy to understand. I was amazed to learn the difference between resistance and impedance. Since i don't have a variac (something tells me I will need to change that fact soon) I will determine an estimate of impedance of the OT I mentioned from the CM23L radio amp using the 1.25 factor comment, and see if I can work backwards to get a winding ratio.
So, looking at the OT from the CM23L from above, envisioning the leads to the speaker at top of OT, the three tube side leads below are yellow (left) red (center) and blue (right). The speaker leads measure a resistance of 0.7ohms (0.875 ohms with the x1.25 factor considered). On the power tube side, including the x1.25 factor, yellow to blue is 201 ohms, blue to red is 173.875 ohms, and yellow to red is 27.625 ohms.
By the formula for determining winding ratios this gives me: (power side wires to speaker side)
yellow to blue = 15.15 to 1
yellow to red = 5.62 to 1
red to blue = 14.09 to 1
Scott Goldsberry Actually, Scott, simply measuring DC resistance is not going to give you a winding ratio. You either have to input a known AC voltage to one side of the transformer and then read the output AC voltage on the other....or know the impedance of both sides of the OPT. Multiplying DC resistance by 1.25 to estimate impedance only works for speaker voice coils, not transformer windings. I am impressed by all your calculations, but the accurate determination of winding ratio requires information other than DC resistance. Good luck :)
Uncle Doug So, it appears I need to acquire and earn to use a variac. I appreciate your patience and help with my questions. On a side note, after watching your videos regarding the Hammond organ amplifiers, I have been scouring the Internet for sources. I have found some, but unfortunately, so far, the costs of shipping a good unit to my location in Canada is prohibitive. It appears that I need to look closer to home, which will be a quest in itself. Cheers, and throw a ball to Rusty for me ;)
A Variac is definitely a useful tool, Scott. They are also rather heavy, so you might want to find one close to home (just like the Hammond amp) to avoid significant shipping expense. Best of luck with your pursuit of knowledge and your projects :)
Thank you for this video! Please keep doing these techie vids. They are amazing!
Also this might even put an end to all those flame wars on forums on wheather or not you can connect a speaker with higher/lower impedance than the amp was designed for.
But now I am starting to wonder why some manufacturers keep making those attenuators with set impedance a.k.a. "one-size-fits-all" - 25Ohms or 30Ohm in some "boutique" attenuators...
You're welcome, AR. Rusty and I intend to keep producing videos as long as viewers continue to enjoy them :) Re the attenuators, I have no experience with them, but perhaps another viewer will share their knowledge with us on this topic. Thanks again !!!
Uncle Doug
Serious attenuators connect some resistors in paralell and series to the output and the speaker to have 1%....10% of the amp´s output at the speaker. But they short the output tube´s life and alter the distortion of the amp as well.
Uncle Doug One last question, if you allow: What determines the impedance of a speaker? Does it mean that, say, 16Ohm speaker has more wire on the voice coil than 8Ohm one?
Apparently, if the speaker impedance is way too high, it causes crazy voltage spikes and fluctuations within the OPT that can cause damage. The ultimate in too much speaker impedance is to operate the amp with no speaker at all (infinite impedance), and we all know this is not a good idea. Perhaps the attenuator causes harm because it does not match (much higher than?) the impedance that the OPT is "comfortable" with.
AnalRectum
No problem, AR. To begin with, the 16ohm Voice Coil has more DC resistance (about 12.8ohms) than the 8ohm VC (about 6.4ohms), probably due to more turns and/or thinner wire. You can easily measure this difference with a DC voltmeter. When AC (the music signal) tries to pass through the 12.8ohm voice coil, the coil reacts to the AC and impedes its passage with an additional 25% of "resistance", for a total of 12.8 + 3.2 = 16ohms of impedance. I hope this makes sense.
Reactance + Resistance = Impedance. It's now so simple! Thanks so much Uncle Doug! I already knew about the winding ratio but finally I now understand impedance. Wonderful :)
Another stellar video!
Thanks, Michael.
@ 19:23: I often do it this way - hooking up the variac at the Lo-imp-side of the OT. This is a good way to test OT´s for shortcuts etc (as you can easily compare voltages between center tap and both plate-taps or also screengrid-taps for their symmetry). What I do in this case: I put another transformer inbetween. It works great this way: variac -> primary of a tube power transformer / 6.3V (or 5V) tap´s of this power-transformer -> secondary tap of the OT / -> primary OT taps -> probes of your multimeter. Then tell Rusty your values, and he´ll tell you what to do with this transformer!
Thanks for the excellent suggestions, Sal. I often use Rusty to perform complex mathematical computations. I give him the problem and then count the number of times he yawns in a minute.....always accurate :)
Hi Uncle Doug, thanks for your wonderful instruction about output transformers. When you are determining the winding ratio of the output transformer, where is the light bulb placed in the circuit? It would be helpful if you included a simple circuit drawing to make everything clear.
Thanks, Curtis
I probably would not use a current limiter when measuring the winding ratio, Curtis. I guess you could use it between the Variac and the 120VAC receptacle.....but it would be optional.
BRAVO!! Being a math idiot, even I understand it. I may not remember it,.. but I understand it! Thank you. :)
Then we have accomplished our goal, Stan. You are quite welcome.
This session raises more interesting questions. How does one determine the maximum allowed primary voltage and current. Would a measure of the weight of the transformer give one a clue on the maximum power-handing capability?
There is really no way to accurately determine the current capability of an unknown transformer, David. Yes, you can base your guess on the gauges of the wire, or on the mass of the transformer itself......but these are just guesses.
thanks for the breakdown.
I've used an LCR meter at 1kHz on the primary and an 8 ohm resistor on the secondary to measure the impedance.
worked well enough, those amps are still alive.
I would assume using the actual speaker instead of a resistor would give a more realistic measurement.
my only gripe with your method is with a variac, the frequency is really low. 50/60hz
the reactance of the transformer will be much higher at 'guitar' frequencies.
I've seen people use your method, only with a frequency generator to test more thoroughly.
You're welcome, B. Thanks for your comments.
Howdy, Uncle Doug, & thanks once again for yet ANOTHER fantastic, informative video! This time, however, I find myself still scratching my head regarding the practice of matching impedances with a transformer; certainly, I can see HOW it's done, I'm just not sure as to WHY. It's been my understanding that the primary purpose of a transformer is to transform voltages and/or currents (hence the name transformer), whereas the impedance matching is more or less a side effect of said transformation. Using your example of the amp owner who wishes to replace his/her 8 ohm speaker with a 4 ohm unit - wouldn't it be simpler just to put a 4 ohm resistor in series with the 4 ohm speaker, in order to present a total of 8 ohms to the amp?
No its not sufficient because the resistor doesn't have or exhibit impedance. In other words its resistance doesn't depend on frequency like the 4 ohm speaker coil does. But your basic line of thought is good, you could put a 4 ohm coil in series with the speaker to get 8 ohms impedance. Or how about another 4 ohm speaker in series with the orig. 4 ohm speaker....
Extraordinario,excelente! Gracias a usted por su sabiduria! Good luck!
Thanks so much, Francisco. Best of luck to you.
Uncle Doug,
Thanks for sharing your knowledge on all maters electronic. I have not looked (not sure how I would do it) but wondered if you had done a similar video tutorial on impedance matching as it relates to microphones and mixer inputs and amplifier inputs and outputs.
What brought this up for me is that I wish to be more knowledgeable regarding what is/are low impedance values for XLR connections and what is/are high impedance values for 1/4" plug guitar connections.
I jam at a church whose setup have wall mounted XLR inputs (assumed to be low impedance) and the behringer mixer they use has an XLR output to feed this connection. I have a smaller version behringer which only has a 1/4" output (probably intended for direct amplifier input) so I dont know how I would match those impedances so I could use my mixer, short of upgrading.
Again, thanks for sharing your knowledge.
You're welcome, Jerry. Although I have discussed impedance frequently in previous videos, I have not made a video addressing your specific request. Bear in mind that my area of interest is strictly vintage tube amps, so interaction with modern sound systems is not part of my knowledge or experience. I will, however, keep your suggestion in mind for the future.
This presentation is absolutely phenomenal. Simple, elegant, best verbal delivery to encourage learning and retention.
Wow, thanks so much, Ronald :)
Very good video.. thanks uncle doug and greetings from Paraguay
Eduardo Acosta You're welcome, Ed. Rusty and I send our regards.....thanks for watching :) ^. .^
Uncle Doug, thank you SO MUCH for sharing your knowledge! I know I have said it before but, you have no idea how much you have helped me. AND, I am VERY MUCH enjoying my new hobby of working on tube Amplifiers!
That's great news, DW. I hope you continue to enjoy and hopefully profit from your new hobby.
I really enjoyed the video. I ran across a Deluxe Reverb Mod blog on fenderguru.com which describes a DR purchased from Linda Ronstadt where they discovered instead of 6V6s 22 watts it was running 6L6s 40 watts but sounded rather shrill and harsh but the bias had been adjusted for the 6L6s I dont recall them saying the Output Trans had been changed. After seeing THIS video the 6L6 tube impedance 8K and the winding ratio one could understand how it would sound shrill and loose tone. But, I don't understand how resettnig the bias would compensate for the tube impedance and regain the tone if it would at all. According to your video 6L6s would probably sound best through a 4 ohm cabinet. I'll watch your setting tube bias video next. Thanks so much.
+barneycarparts You're welcome, Barney. I agree with your observations. An original DR OPT, which is appropriate for two 6V6's @ 22W output, could not do justice to (or survive long with) a pair of 6L6's @ 40W output. Not only would the impedance mismatch be a problem, but I would fear for the "health" of both the OPT and the PT when confronted by the significant increase in power and current demand. Reducing the speaker impedance might partially solve the mismatch issue, but nothing would compensate for increased power and current demands.
Hey Uncle Doug. I am finally getting around to learning tube amps. I just want you to know that you are an amazing teacher. I am a HVAC tech and have an understanding of basic electricity and you explain things in a manner waaaay better than any tech school teacher ever did. I am not sure if you are still making videos or not not, but please keep it up if you are. Thank you thank you thank you! Ryan Wham
You're welcome, Ryan. Thanks so much for your nice comments, and best of luck with your new hobby :)
Doug, Nice layman's terms explanation! Harp players seem to love a mismatch like a 16 ohm speaker on an 8 ohm tap. Putting a 4 ohm speaker on an 8 ohm tap is a great way to burn things up. I have seen a shorted voice coil running at 1.2 ohms on an 8 ohm tap. Thankfully, all I got was a really good light show! I own an old ESI Teague LCR, but the Variac dialed to 1 volts secondary works fairly well, but RMA, NAB etc generally used 1000Hz as their test freq. You will have to use a lower B+ and the 6L6 pair won't deliver much more than 30 watts, but it will be a nice amp. At 5200 ohms you will get 60 watts from a pair of 6L6GCs, 5881s, 7581s, KT66s etc. When I went to college there was a discussion of teaching ohm's law for DC as AC theory with a 0 degree phase angle. Let's face it, going from DC to AC theory was a major learning curve for most people! NEVER say you are sorry for the math! NEVER! That math is what keeps us from screwing up more often than we do already!
Greetings, Seth. Thanks for your kind words and informative input :)
Amazingly good explanation. Thanks!!
You're welcome, OP :)
And I roll my eyes at them eye rollers.
A good tactic, PMD.
Hello very good your explanation and much admire your vídeos.
I have a question .
In cases where 2 stages are used in paralel, can we consider half of the impedance of the stage?
Thank you!
Thanks, Edu. I don't really understand the concept of two stages used in parallel......but if you are asking about the impedance of a single-ended output tube vs double-ended (push-pull) output tubes, the plate resistance of the tubes follows the rules that apply to all resistors, i.e. in parallel, the resistance is half that of a single tube.
@@UncleDoug hello Uncle Doug! !
I ask about for example push pull (2tubes) plate to plate impedance vs push pull (4 tubes ) plate to plate impedance.
Or single ended with 1 vs 2 tubes in paralell.
Tks for your answer! !
Good weekend !!
You're welcome, Edu.
Nenicule multumim si esti meserias !
Bine ați venit........ Mă bucur că v-a plăcut videoclipul.
Excellent video Sir.
Thank you.
Excellent video presentation - all your vids are spot on! thx
We really appreciate your kind appraisal, K. Thanks !!
Very good and clear explanation of what is goin on ;)
My very own insights about OPTs are that a *little* less speaker impedance is ok. Twice as the speaker impedance you may risk your OPT along with the tubes - here is why: take the whole output stage as up and down seeing it from the middle of the OPT - if the Amp is at full scale output one half of a sinusoidal wave is beeing amplified to the upper side. *twice* the voltage will be seen on the bottom side, since the lo-side tube is fully off - the OPT will induce the upper voltage to the down side and ´place´ it on top of the input voltage from the center-tap.
If the sec. resistor connected to it witch is way top high, the primary inductance causes the primary voltage to rise with respect to the winding ratio as well on top of it all - causes the tubes to sparkover from the plate to the next down grid. And this will cause again a sparkover inside the OPT due to its primary inductance. As a drop in current will cause a *large* amout of voltage in the primary - and a little later in the secondary as well, this will blow your speaker.
Here´s why this all happens:
It´s a little bit more complex, as the OPT is not only described by impedance and winding ratio. One huge thing to take a look on is the *primary inductance* since when you miss it out, your amp may have less low frequency response, the OPT will go in saturation very soon and/or the whole thing will be oscillating...
Next time when measuring the OPT as you´ve done at 16:20 you may want also to verify your math like this: feed some exact value in the secondary with a series resistor of 4/8/16 ohm, as you want the speaker to be connentec to your secondary, 5V or so, if this voltage is to low (1V or less), you won´t get serious measurements/readouts.
Now connect a primary resistor you just figured out and see what happen....you may test the whole thing by choose the next higher calculated impedance value to see what the OPT´s reaction is.
If you´re hook it up to a scope you can also see at what level your OPT go into saturation:)
If you hook the sec. to a power amp and feed it with a sinus sweep from 20Hz-20kHz you also get the frequency response.
And if you´ll increase its voltage you´ll get the max. power handling cabability of the OPT and the primary inductance as well. It may take some time to do so, but for a vintage OPT it´ll be may worth it :)
Thanks for your input, TC. I must admit that some of it was over my head, but I really appreciate you sharing your insights with the viewers. I will try the experiment you suggest, and I will amend the video to reflect your input regarding the dangers of too high an increase in speaker impedance.
Uncle Doug
If you tell me where I went ´far off´ I will be gladly explain it to you :)
Your video is great as it is - please do *not* alter it!
You may visit your friend ´ElPasoTubeAmps´ as I think he can wire a breadboard circut to perform the tests :)
Thanks, TC. I think after I have re-read your comment a few times and given it some more thought, it will become clear to me.
TubiCal, not sure this would work very accurately, as a resistor doesn't behave like a speaker! The theory in the measurements/tests makes for the basis of something quite handy, but I think without a load that behaves like a speaker you would be better off with the simple method described in the video. Particularly if, like Uncle Doug, you use an analog meter to read the primary.
Right I was thinking "why not just measure the resistance on each side?" you were like NOPE! and here's why
Nice work. The only thing I would add is that both your DVMs (or AVM in the case of the Simpson) should be good RMS reading DVMs. The example is good because the stimulus is 60hZ and most DVMs will work well with a low frequency sine wave.
Again, well done!
Thanks, AG, for your kind words and input. Glad you liked the video :)
Such amazingly well presented knowledge!
Would be interesting to hear what happens with a OT when transients/dynamics are taken into account. Might be out of scope for what you trying to do with your channel.
But since OT can sound different, even though it's pretty much only a wrapped wire(!!) it would be a very interesting topic to debunk :)
There's obviously more to it then wrapping copper wire around iron x number of turns - given the prices vintage (marshall) drake and radiospare OT's can fetch.
Thanks Doug!
Thanks so much, Eric. We are not convinced that a significant, audible difference exists between OPT's of equal mass and winding ratios, and are in no way supportive of the exorbitant prices charged for so-called boutique transformers. As a result, we're probably not the best man & dog to discuss or defend them :)
Would love to hear a detailed explanation of how to build into the circuit; StepUp/Down fixed or variable boost/drop circuits. For custom builds of course.
We stick strictly with vintage tube amp circuits, James, and have no experience with such modifications. Hopefully there are other sites which do. Good luck.
I saw the difference between DC resistance and AC impedance first had, when I was working at Westinghouse Elevator and someone soldered an AC coil into an DC circuit.
Nice all your videos Uncle Doug, thank you very much for this! If explanation is based on simplicity it becomes really exceptional!
Just one question, you say music arrives to the speaker in AC mode. I supossed it is DC with AC superimposed because central point of the transformer in tube side is polarized in DC.
Sorry for my poor english!
Thanks very much, K. The music signal itself is AC. The DC on the output tube plate(s) is blocked by the output transformer and never reaches the speaker.