CORRECTION; I should have said a blue glow for a fraction of a second with the stock AC cord and one/two seconds with various after market AC cords.
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I think you are seeing a brief arc between the cathode and anode inside the rectifier tube. The Cary's operate switch complete's the circuit to ground for the B+ ( high voltage ) and the high tension comes on virtually instantly. This causes the rectifier tube to arc briefly as the output tubes begin to conduct. I believe the "operate" switch "makes" the circuit from the cathodes of the power tubes to ground. The shorter or longer flashes probably have to do with the particular power cord's current carrying capacity. The term "blue glow" refers to the faint blue glow in high vaccum conditions within a tube. It often shows up against the inside of the glass envelope. This is not the same thing as the bluish glow inside a tube that is gassy. The kind of thing you are describing would be a very fast blue/white flash.