We are often asked ‘How can you tell if a tube is shorted?’ Great question! If the tube is power white at the top like this one it has lost vacuum & it's time to replace the power tubes with a matched set. This is a bit more of a rarity thought it happens. A majority of the time tubes visually appear to be fine & light up though one could be internally shorted. We hope this helps in some small way & have a great day! – Voodoo Amps