What is wrong?
I'm asking respectfully because I'm trying to learn.
That way, I might not have to ask all the questions experienced DIY'ers know the answers to, such as how to select the output transformer for a given tube, about rectifier tubes, then questions about selecting a preamp tube.
I just want to learn how to DIY. So please, rather than just saying the video is wrong, can you guide me to a source that explains the plain ol' wrong right?
I'm asking respectfully because I'm trying to learn.
That way, I might not have to ask all the questions experienced DIY'ers know the answers to, such as how to select the output transformer for a given tube, about rectifier tubes, then questions about selecting a preamp tube.
I just want to learn how to DIY. So please, rather than just saying the video is wrong, can you guide me to a source that explains the plain ol' wrong right?
Yes, and it is not about Williamson, but rather about author's experience, and his understanding.
I learned a lot from the video.
I had never heard of cathode loading before amongst other things.
The video isn't perfect but it is very useful.
It is good you're trying to learn, but it's even better if you learn correctly from the get-go. Among some of the things Tom gets wrong is his confusion concerning transformer loaded push-pull v. single ended. DC magnetization is a problem with SE, not PP. Secondly, he states that the output transformer is designed to match the final's plate resistance to the speaker load. This is incorrect: the OPT transforms the speaker load to the load line resistance the plates of the finals operate into. Plate resistance determines the damping factor, nothing more. Falsely claims tridoes don't give very much power (a PP pair of 811As will get you north of 100W, and PP 845s can do some 300W) and then contradicts himself by stating that the pseudotriode KT66s can do just that: give lotsawatts.
Furthermore, he misunderstands how gNFB works, and points out the normal RC decoupling networks as though these had something to do with rolling off the high frequency response to enhance stability. They don't: RC decoupling is for open loop stability, and low frequency stability. Next, he claims that the capacitor that is paralleled with the gNFB resistor enhances stability by reducing the closed loop high frequency gain. This is incorrect; the capacitor serves to improve the phase margin. It's obvious he knows nothing about Nyquist and his stability criteria. He says that the extreme high frequency cutoff of the original Williamson OPT causes high frequency instability. The opposite is the truth as the original Williamson used such a high gNFB factor that the 100KHz+ high end was needed to improve the phase margin at the upper end of the audio band for stability under unusually high levels of gNFB.
Falsely claims there is no phase shift in a circuit that has two RC coupled stages. These always produce low frequency phase shifts, and explains why the original Williamson didn't include three RC couplings by DC coupling the first pre to the cathodyne splitter. Each RC coupling needs its time constant staggered by a decade if excessive LF phase shift is to be avoided, and that can lead to some very large coupling capacitors.
Misunderstands why pure tetrodes have plate current kinks: makes no mention of secondary emission as the cause. Misunderstands how beam formers reduce plate current kinks.
Misunderstands Ultralinear and cathode local NFB
Nonsensical discussion of the LTP (differential amp)
Tomtektest is another one of those youtubers that "thinks" he knows more than he really does. How he could have been involved since 1960, as he claims, and failed to learn as much as he does is beyond me. You can do much better at Pete Millet's site -- lots of free tech books of all kinds there. If you can handle it: Radiotron Designer's Handbook iV.
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I can tell my story of a Williamson amplifier, but it is not so fascinating though. 
I was a teenager, playing bass guitar in a school band. Also, sometimes we were allowed to play for adults on dances in a local club. Once very old Electron-10 amp got damaged, and the sound engineer took it apart. He knew that I was an amateur designer, and gave me a preamp half with 6N1P and 6N2P tubes, and took himself a power amp part, with 4x6P15P tubes and transformers. So, I decided to build my own guitar amp. I had some parts from Tu-50 amplifier that used 2x807 output tubes. It was a mediocre amp, used mostly with aluminium horns of an awful quality. I took a power transformer as is, it produced 300V rectified, or 600V doubled under the load. One neighbor gave me a pair of Gu-50 tubes. They were never for sale, he brought them from army where he served his draft. I rewound an output transformer for 8 Ohm speakers, calculating it by voltages in primary and secondary. After I breadboarded the amp, I found that the Concertina from that 10 Watt RMS class A amp does not produce enough drive for Gu-50 tubes, so I added one more twin stage between Concertina and output tubes. I did not know then that reinvented Williamson amp. I was young, uneducated, but the amp come out great and I was completely satisfied and proud. However, when later I went to get my professional education, I got a belief that vacuum tubes are obsolete and must die.... ...until in 2002 I heard for the first time the real high end and got bitten by a high end bug. Actually, it was the second time, the first time it was in 1974 in TV studio where I heard Hungarian EAG active biamped monitors with a pair of amps with EL34 tubes then, but I thought that it was a "New Technology, Solid State Hi Fi Amplifier" then.
More about Electron-10: ЭЛЕКТРОН-10 Ламповый комбо-усилитель
I was a teenager, playing bass guitar in a school band. Also, sometimes we were allowed to play for adults on dances in a local club. Once very old Electron-10 amp got damaged, and the sound engineer took it apart. He knew that I was an amateur designer, and gave me a preamp half with 6N1P and 6N2P tubes, and took himself a power amp part, with 4x6P15P tubes and transformers. So, I decided to build my own guitar amp. I had some parts from Tu-50 amplifier that used 2x807 output tubes. It was a mediocre amp, used mostly with aluminium horns of an awful quality. I took a power transformer as is, it produced 300V rectified, or 600V doubled under the load. One neighbor gave me a pair of Gu-50 tubes. They were never for sale, he brought them from army where he served his draft. I rewound an output transformer for 8 Ohm speakers, calculating it by voltages in primary and secondary. After I breadboarded the amp, I found that the Concertina from that 10 Watt RMS class A amp does not produce enough drive for Gu-50 tubes, so I added one more twin stage between Concertina and output tubes. I did not know then that reinvented Williamson amp. I was young, uneducated, but the amp come out great and I was completely satisfied and proud.
However, when later I went to get my professional education, I got a belief that vacuum tubes are obsolete and must die.... ...until in 2002 I heard for the first time the real high end and got bitten by a high end bug. Actually, it was the second time, the first time it was in 1974 in TV studio where I heard Hungarian EAG active biamped monitors with a pair of amps with EL34 tubes then, but I thought that it was a "New Technology, Solid State Hi Fi Amplifier" then. More about Electron-10: ЭЛЕКТРОН-10 Ламповый комбо-усилитель
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Your amplifier is unlikely to sound much like the DTN Williiamson at all. A push pull amplifier transformer can have residual magnetism if the output stages are DC unbalanced. However please revise some basic theory - eg Radiotron Designers Hanbook would be appropriate for these sort of amplifiers.
So you are saying that the video gives accurate explanations as to how the different parts of a tube amplifier works?
Miles has above pointed out some of the issues. The author's presention is quite nice, but not 100% technically accurate. All I was saying is that learning something that is not technically correct is counter productive when trying to aquire knowledge.
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