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Problem AN Kit One: Noise on Output Transformers

Posted by beto1 (A) on May 21, 2015 at 10:22:46

I have noise in the output transformers when I play very low frequencies at high volume on my integrated 300B SET amp (AN Kit One, 2004 version).
I have installed a pair of rubber stripes down the output transformers to eliminate any posible mechanical vibration, I have tightened all the screws that compress the laminations and fix the shrouds. I made recently a complete upgrade on the PSU board, replacing all the caps and resistors. But still the noise is there.

One thing caught my attention; when this noise happens, there is a variation on the filament glow, the more the music is high volume/low fq, the more intense is the glow. I was noted this when the problem appears some years ago and now I can confirm as part of the analysis. Varies exactly in the same "intensity" than the music intensity. The noise is like there something inside the tx that is hitting the output tx shrouds. Tiny hits on a rhythmic basis, directly related with the "over saturation" on the music. The frecuency of this tiny hits is the same all the time. The same quantity of hits per time unit.
Since a long time, I am doing a research on the web and the more difficult task is get the appropriate name to this phenomenon to search an appropriate posible solution. Buzzing, motorboating, oscillation, to name a few, but still I don't find the exact match and how to cure it. I believe that the rubber stripes under the output tx helps in a small degree, by the way.
Since I don't know if the variation on the intensity of the filament glowing is normal when the music tends to sound full and complex at high volume, I can't discard that maybe the problem is some kind of gas leaking inside the output tubes or a mechanical problem inside them that is transported to the output transformers, etc.
One explaination I received earlier was "this is that there is some airgaps on lamination and the electromagentic flux generated by the current flow, activate this with vibrations"...but I'm sure that this was not there the 7 years before, so in some point this begins to happen.

At least I can hear music because this noise is only happens with very low frequencies. Those frequencies are not present on orchestral, acoustic instruments music, traditional jazz, rock, blues, folk, etc, and the maybe 90% of the music that I normally hear but there are some electroacoustic music, noise music, experimental music, etc. that can have a huge degree of very low fq.
Any clue will be gratefully received.
Regards

Edits: 05/21/15

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