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Can't see the vacuum tube filament glow?

We sometimes get questions from curious customers that their new tube seems to have burnt out because they can't see the filament glowing, yet the tube seems to be working fine.

We can assure you that the tube hasn't burnt out. On tubes like 12AX7, the filament fits in a small tube. What you see as filament glow is usually the tiny section of the filament that happens to be hanging out. The filament body fits inside a thin tube within the plate box and is well protected.

On some tubes like 12AX7LPS, the filaments don't stick out at all. You don't see any orange filament glow, so it fools you into thinking that the tube has burnt out in these cases. To check, turn off the lights and look at the tube in the dark. You will still see a faint orange glow from the ends of the filament.

Vacuum Tube vs. Light Bulb

The filaments in vacuum tubes do not break or burn out like in light bulbs.

The filament in a light bulb is very flimsy. Light bulb filaments must be very thin to glow brightly at an extremely high temperature. Light bulb filaments are fragile and break easily.

Vacuum tube filament, on the other hand, is nowhere near fragile. The purpose of a vacuum tube filament is to give out heat, not light. A vacuum tube filament heats the cathode to emit electrons and is housed in a thin tube to maximize heat transfer. So the filament doesn't break from shock even when dropped on the floor.

The exception is the DHT (directly heated triode) tubes such as 45, 2A3, and 300B. A filament acts as the cathode in these tubes and is exposed and suspended. You can see the filament glowing if you look down inside the plate box. Even in DHT tubes, the filament is nowhere near flimsy as a light bulb.

A vacuum tube can wear out over time, but the filament is rarely the reason.