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The issue I see is that of intermodulation distortion on the biasing of the output tube.  It will effectively change HF operating point per supply ripple.  Easy to understand if one imagines a gross ripple of 50V.  Same thing happens with 0.5V ripple, just to a lesser extent.

In my latest ongoing amp project I re-discovered an easy way to deal with some ripple.  The stage runs large bypass cap from B+ to cathode to keep output loop small.  Only thing left is grid.  I drive the grid through an interstage tranny, and set the common mode of the secondary to ground via 50k resistor.  This is for dc bias.  The trick now is to run a 1uF or so cap from B+ to the common mode of the secondary (in my case a center tap).  Thus, any ripple on B+ is also exactly seen on cathode and grid.  The tube has no idea anything is going on, as it only sees a constant dc bias at all terminals.  I got this idea from a WE92A schematic.  It works incredibly well.  My noise floor (on proto) dropped maybe 20dB or more.

jh

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