D&A Phantom 10 tube Red Plate

[EchoKilo2488]

I have a D&A Phantom 10 tube 24LQ6 with a 6kv6 pre driver. Looking at it from the top it would be the forth tube in the driver chain, this is the only tube that will cherry within a few seconds of key up. I'm driving it with a Sommercamp TS1000DX 1 watt swinging 15. I've got a small stash of spare tubes and quickly flashed 2 known good ones. I changed the z 14 plate choke, the 100 ohm cathode resistor the 47 ohm parasitic off the plate and a new .01uf decoupling cap on the heater. I'm at a loss on why it's just this tube red plating. I'm open to ideas and it's much appreciated. 73s
Ed