I see the same kind of result in the Counterpoint SA-2 and Aria WV preamp that use the 5651. The 5651 resembles a vacuum tube and yet it really is not. It is simply a voltage reference filled with gas and acts more like a zener diode than a vacuum tube. So if this visually observed behavior of the 5651 is different than the other "tubes", don't worry about it.
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