You are my first comment for 2021. Happy New Year. Unless one is into 100 pound 20 watt amplifiers and love "big iron" just for its own special beauty, RC coupling is the way to go.
One could conclude that with a transformer as the coupling device that lower frequencies are favored and with circuits using capacitor coupling that higher frequencies are favored. I conclude that this is because the inductive reactance and capacitive reactance are essentially opposing forces (thinking of a passive speaker crossover network). With all things equal in the audio spectrum with nominal size inductors and capacitors...bass/lower frequencies passing through inductors see a smaller impedance (hence less reactance) vs. going through a capacitor and treble/higher frequencies see a smaller impedance going through capacitors (again less reactance). I may get my terms mixed up but I am trying to remember from my old school days in electronics the pages and pages of calculations needed to come up with a final complex impedance in a large circuit. Those were fun days in the lab. Great video!
Wow. This video and the comments on it are like two semesters of electrical engineering. So much left unsaid. I'm bookmarking this in hopes of coming back to it and reproducing the experiment. But I'll have to watch it 100 times over just to figure a bunch of it out.
Thanks for another great video, it's great to see that your skills are not limited to purely practical but include circuit analysis methods. I agree on the capacitors advantages, there is much more of a choice of values in practical sizes to control the phase shift compared to the inductive reactance of a transformer. Also the offending phase shifts are on the opposite ends of the spectrum i.e. a capacitor is unlikely to cause stability issues in the negative feedback ccts with its diminishing reactance and phase shift at increasing feequencies. Great to see your face finally!
I think if your using RF power tubes like 813. You have to run them in class A2. If you have positive grids you saturate RC coupling and must use interstage transformers to make them work single ended? Is this your view? I have a couple 813 and a dozen 815. I’d like to make a single ended guitar amp for fun to get some higher wattage single ended output?
Well done demonstration proving the physics of the coupling method used. The experiment may be slightly prosaic given what we know about iron core transformers, magnetic coupling and known transformer losses. Working in audio frequencies, in my view, is hampered by human sense of hearing. Speakers, while about one of the most common transducers in electrical history, have many limitations. A speaker moving air which is detected by an eardrum is a "lossy" connection. So, here's my point: I'm not sure that in a mixed audio track (music for example) which has so many complex frequencies and harmonics, that a 27 degree phase shift is going to make much difference. A great point offered by another comment suggested that the resurgence of vacuum tube amps for instruments are designed to introduce distortion; to produced a unique "sound." Again, I personally appreciated the methodical approach to demonstrating a challenge using interstage transformer coupling.
Hi Ross, Yes, it does seem some of us do like the artifacts of vacuum tube amplifiers and for sure it is important in the creation of music by guitar amplifiers. It is interesting how we (we, meaning any or all of us at one time or another, in one way or another) seem to have this urge to build things like single ended triode amplifiers using 70 year old tubes and then add in transformer coupling and have this mindset that it is so pure and ethereal. I am not trying to chunk rocks at anyone's audio system but it is like wanting to ride around the streets with a steam engine. And, with that said, I get enjoyment in using the old General Radio equipment to measure capacitors, resistors, impedance, etc. where I have to write Excel programs to put the data in and get a simple expression of complex impedance where otherwise I can connect the component to a $29 Chinese instrument and get the value instantly in digital readout. I guess, to each-our-own... Humans are strange creatures. :-) Thanks for your comments.
That is an idea I never thought of. I am not sure how that would turn out. I did devise a circuit to measure the inductance with an increasing DC bias on the core of the transformer. I simply placed a variable DC supply across the transformer primary, while monitoring the DC current and isolated the inductance meter with large non-polarized capacitors which I also placed across the primary. I could directly measure the (decreasing) inductance with increasing current. It seems to have worked perfectly. I am not sure if I showed that in a video. Even with those 40 pound transformers I have, a steady DC bias of 150 mA would saturate the core and the inductance would drop to only a few mH and 150 mA is the quiescent current of the 833A, so... there was no room left for audio power. I don't want to malign anyone's SE amplifier but it is no secret that SE amplifiers do very poorly in the LF region with more than a watt or so output. I find it interesting how someone can issue a proclamation about something like a SE amplifier with no NFB and proclaim its superiority to PP amps yet show no tangible proof of it, i.e. real measurements and data, yet, these basically nothing but personal opinions, become the gospel and propagate throughout the esoteric society of audiophiles like wildfire and any attempt to disagree with the proclamation is viewed as a conspiracy. . In a nutshell, my short-lived fascination with SE amplifiers is over - I will stick with PP amplifiers and NFB. I really appreciate your thoughts on this subject. This is a great group to share ideas with. I learn more from you guys than you do from me.
I am primarily interested in guitar tube amps and some of your videos are more toward design of electronic circuits that I many never use.With that said, I do not have the knowledge you have acquired and I find videos like this so valuable even to a guy with limited knowledge. I have a degree in electronics but never got into this level of knowledge of audio circuits. I have seen some guitar amps that use a transformer for a phase inverter and wondered if it was a better component than a tube. This did clear up a few questions I had. It was nice to put a face to your voice. By the way you did not look as I had pictured. The mustache was a complete surprise! I will continue to subscribe and watch your great videos. Thanks
I think in the case of amplifiers for the creation of music, like guitar amplifiers, the use of any or all of the methods can be good as the "sound" is what we are looking for. On the other hand, the HiFi or playback amplifier is not supposed to add or subtract anything from what it is trying to reproduce. I have mentioned before that I am almost convinced that we audio people may even like some of the artifacts the vacuum tube HiFi amplifiers adds to the music rather than the sterile, thin sound that - some - solid state amplifiers produce. I have and like SS amps that others would not like but that is just the nature of audio. I try to make my videos with as little opinion as possible and just present the facts as best I can understand them. An amplifier with interstage transformers is not a bad thing but the transformer (inductance) does add characteristics that are different from the RC (capacitance) type of coupling circuit. I think it is up to the individual designer/builder to make whatever they think is best. With that said, we are not always as charmed with our final build as we thought we would be in the beginning so I like to have as much understanding of what might happen as I can so when things don't turn out the way I planned, maybe, I can understand why. That might summarize my approach in many of my videos. Thanks for your comments. This is a great group of people who watch and comment and we all learn from each other.
+ElPaso TubeAmps You are so right! Sound whether it is for guitar or hi-fi is always in the ear of the beholder. I want to build a guitar amp that can do what others don't do. I want it to be both crystal clear and be able to overdrive. Some may argue there are amps that already do that. The ones I have owned or tried just did not do what I am looking for. My thought was using a transformer phase inverter would produce a hi-fi clean sound, and use the preamp as a overdrive element the signal path. There are so many aspects to circuit design, I try to gather any and all information and then make discussions of components to get where I want to get to. Thank you for your reply. It helps. Thanks
all good, all your videos a helpful and full of information i am build a 6V6 Class A hifi amp at the moment. i bought the transformers for China. they have good reviews so hopefully the work well frequency response 20HZ to 30-000 HZ eletrontube42@gmail.com
What does 30 degrees phase shift mean as far as frequency response? Is it not better to check the frequency response with a frequency sweep? How much does it effect output power? Your talking mostly about impedance which effects power. It seems to me that most transformers for interstage are not made with enough primary inductance. It seems counterintuitive but the more primary inductance, it seems that between 2 Luhndal interstage transformers, the one with more primary inductance has lower frequency response. I think that 25 hz is low enough. I see some interstage xformers like UTC A-18 with advertised freq response of 20 - 20 khz. But I dont believe it. Theres unfortunately a lot more than impedance and freq response with transformers also. However, I see more advantages do to a more even distribution of harmomnics and longevity over capcitors.
old video and it's now 2019, but here's my 2-cents, susumu sakuma tube amps, the interstage tranformer coupling in used and in action, and watching his videos and listening to its tube amps, all i can say is wowwww, it's really a big difference from and ordinary tube sounds amp..................
Here must be a mis adjustment of your balance transformer sec. and the resistors. personally I am not in the amp. an advocate of push pull. More single ended and an interstage (well done) is better than R C coupling. you can't just take 1 transformer and say this or that compare, there are more transformers good and poor. we are talking about "Delta" good luck, I like what you do. RC is a filter and flattens, hence a small ideal angle. an interstage example SE runs nicer from A1 to A2, good luck. regards Richard
Hi Richard, I have to admit, I do not have much experience with transformer coupling. Only in coupling a driver stage to something like a high power modulator running class-B. I just never had those nice transformers until now. I have a few but still may not be the best impedance ratios. I have done a little testing with filament transformers at audio frequencies and have found that at the mW or uW level, they all seem to perform really well. I can imagine that they could have a good sound all their own. I think large core vacuum tube output transformers have a sound all their own - seems they have a sound of "power". Maybe it is all in my imagination but some day soon I will have to take a design with the UTC interstage transformers serious and see how it turns out. Thanks for your comments.
Im not sure why doing all this math is necessary. It seems people talk about the negatives of what they dont like and the positives of what they do like. Transformers dont usually go bad do they? If an amp is properly designed wouldnt the transformer last longer than a capacitor? luhndal makes a transformer I am using which is comparable to the ridiculously priced antiques. low freq limit is 25 Hz. And freq response is 25hz - 30khz. unfortunately primary inductance is 42H. Higher inductance increase to 250 H is required for freq response to 20hz. But my speakers only go down to 30 herz. So why do i care then if it doesnt go down to 20HZ? Good caps are not cheap. And bad caps dont sound good do they? Interstage expense is where you spend the money, cap or transformer, or your amp can not sound good.. is it possible to do your test on an Lundahl LL1660 interstage phase splitter? Max current through any section(winding) is 50 ma. max output at 30 hz is 220V rms. What I find interesting is the small size and price is affordable. Along with the fact that I have more accurate balance between halves of the phase splitter, than is possible with a tube splitter. I like the additional benefit, which are many more than caps in my opinion of the ability to increase or decrease voltage, increase, or decrease impedance from stage to stage with turns ratio, as well as control permeability of core, control interwinding capacitance, primary and secondary inductance, Ill stop here , there are many more. Im not opposed to either one myself, just opposed to what sounds worse as compared to what sounds better. But why all the math? isnt it easier to put frequencies in with a audio generator and measure with oscilloscope? i am curious because i believe the accuracy of analog devices. i just dont trust computers. Are the computer programs that you have as trustworhthy and easy to use as the analog stuff you have? Not much info on interstage transformers. Im working on interstage coupled 3 stage amp. i believe luhndals are good quality for good price. I also heard the Hammond Interstage are pretty good. ive heard a lot , but Im into longevity, and believe transformers have that quality. But Im not sure. However , foil in oil caps is my choice, its just that they are as expensive as Luhndal , high quality transformers. Turns ratios are no more than 1:4 . The transformers are made with a trade off considerations of variabes designed for a specific purpose.
