Your problem may be the same as what I encountered a few years ago with a DIY KT-77 amp. You may have reversed the output tube connections to your output transformer. Try rewiring the output transformer primary leads so that the leads previously connected to the “top” tube are now connected to the “bottom” tube and vice-versa. When the connections to the output tubes are reversed, the amp’s polarity is reversed and the feedback becomes feedforward. That would account for the noise on your video.
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Of course, I am assuming you have carefully traced the wiring to make sure the amp is wired correctly.
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