V4B and V4, pair of power tubes, slow red plating

[jjmt]

Hi folks! Sorry, I've been away and so I'm a bit late to this discussion. Since the problem is resolved, I have a question that I'll ask just for academic reasons: I can see how the changing the .33 coupling caps will restrict the passage of low frequencies. What I do not understand is what this implies about the issue as it was initially presented by the OP as red platting on *one pair* of power tubes (one side). Is the implication that the PI is simply imbalanced? Or is the something about the PI that is passing more low frequency on one side than the other? Are we talking about 60hz heater noise here (maybe leaking to the cathode)? If so, wouldn't it be possible to solve the problem by simply using a quieter tube, or maybe a different brand?

I realize that the theory is that this is super-low frequency, well below 60hz. I guess I'm just not sure I understand where that comes from.