Output tube sparked

[fossilshark]

I have 2 6v6s in push pull. Turned on the amp and one of them sparked (inside the tube) and immediately after the tube started glowing hotter and hotter so i shut it down before it grew past cherry red (the heater looked normal though). This amp has been working fine for months with no changes except speaker cab. Any thoughts?

[sluckey]

Replace it.