Testing method of electron tube
1. Observe that the color of the top of the tube is normal. The color of the top is silver or black. If the top has turned milky white or light black, the tube is leaking or aging.
2. Observe if there are any debris in the tube, shake it gently or flick the glass bulb with your finger, and then carefully observe whether there are debris, white oxide, broken mica and other debris inside. If there is debris in the electron tube, it means that the tube has undergone strong vibration in the middle, and the possibility of a short circuit between its poles is greater. 
Testing with a multimeter
1. To measure the filament voltage, use a multimeter R × 1 to measure the resistance of the two filament pins of the tube. The normal value is only a few ohms. If the measured resistance is infinite, then the filament of the tube is broken.
2. Detecting whether the tube is aging By measuring the emission capability of the tube cathode with a multimeter, you can determine whether the tube is aging. During the test, the working voltage of the filament of the tube can be provided separately (the rest of the voltages are not added). Preheat for about 2 minutes. Use a multimeter R × 100. The V battery is equivalent to adding a positive bias grid voltage to the electron tube, and measuring the resistance between the grid and the cathode. For a normal electron tube, the resistance between the grid and the cathode should be less than 3kΩ. If the measured resistance between the grid and the cathode of the electron tube is greater than 3 kΩ, it indicates that the electron tube has aged. The larger the resistance value, the more severe the aging of the tube.