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

[AMPREPAIRNJ]

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.

If you read the entire thread, you'll see that the condition in order to induce the red plating is extreme, not normal and with newer tubes most likely not in spec with the original 7027's. That extreme setting greatly offsets the voltage across the power tubes, more so than the PI, leaving them vulnerable to red plating at which point the PI circuit most likely just determines which side will constantly red plate due to signal/frequency strength from one side over the other. One side of the PI always has less steps to pass/amplify signal than the other, so there comes a point, in extreme settings, that the PI is not balanced. But the unbalanced PI is not technically what's causing the red plating, but how hard the power tubes are working with such offset voltages. What changing the .33 caps does is take the stress off of all of the power tubes equally, resulting in the tubes not running smashed to the wall trying to amplify unnecessary frequencies, taking enough of a load off the tubes and supply to keep the power tubes from running away. Playing with the phase inverter at these extreme settings might buy you something, but given the 12AU7 and PI circuit does a great job at normal settings, well balanced, is it really worth modifying it to find a middle ground between normal and extreme settings and being happy with both results? I'm not sure. The .33 caps are just letting too much low freq through regardless of the red plating, let alone in extreme settings. I would love to know what results you come up with should you start experimenting though!