Making Cathode Ray Tubes at Home

Making Cathode Ray Tubes at Home

It all started with a simple question: is it possible to build a functioning cathode ray tube (CRT) from scratch, at home, without access to industrial equipment? It turns out you can — provided you lower your expectations from commercial-grade vacuum tubes to something more primitive but still genuinely functional.

The tubes described here are cold-cathode CRTs. Unlike commercial tubes that use heated filaments to emit electrons, these use a simple unheated electrode. In essence, these are modified gas-discharge tubes that work with ionized air at a relatively easy-to-achieve vacuum level.

The Operating Principle

These CRTs function at relatively high pressures — several hundred microns — which is far easier for a hobbyist to achieve than the deep vacuum required by commercial tubes. At this pressure range, the residual gas inside the tube becomes ionized and creates a visible glow discharge. By shaping the electrodes properly, you can extract and focus a beam of electrons that strikes a phosphorescent screen.

The Power Supply

The power supply consists of a Variac autotransformer connected to a neon sign transformer, providing 2–5 kV. This voltage range is sufficient to ionize the residual gas and accelerate the electrons toward the screen. Importantly, the author notes that the risk of X-ray radiation only arises at voltages above 15 kV, so experiments in the 2–5 kV range are relatively safe from that standpoint.

The Vacuum System

Instead of expensive diffusion or turbomolecular pumps, the project uses a mechanical refrigeration service pump. These pumps are widely available and can reach the required vacuum levels. Pressure is regulated using a valve on the inlet line — by partially opening the valve, you allow a small amount of air to leak in, maintaining the optimal pressure for the discharge to occur.

Building the Electrodes

The electrodes are made from aluminum welding rods. The cathode is a simple flat or pointed electrode, while the anode features a hole drilled through it to allow the electron beam to pass through. The spacing and alignment of these electrodes determine the quality and focus of the resulting beam.

The Phosphor Screen

The luminescent screen is coated with phosphor extracted from broken fluorescent lamps. The phosphor powder is carefully collected, cleaned, and applied to the inside of the glass envelope at the end opposite the electron gun. When the electron beam strikes this coating, it produces a visible spot of light.

The Startup Process

When high voltage is applied, the electron gun begins to glow. As the vacuum pump lowers the pressure, characteristic striations (glowing layers) appear — a well-known phenomenon in gas discharge physics. At the optimal vacuum level, a bright spot appears on the phosphor screen where the electron beam strikes. This spot can be deflected using an external magnet, confirming that it is indeed an electron beam and not just a general glow.

Beam Deflection

The author implemented magnetic deflection using hand-wound coils positioned around the tube. Moving a magnet near the tube causes the bright spot to shift across the screen. The author also experimented with electrostatic deflection using metal plates inside the tube. An interesting observation was made: when negative voltage was applied to the deflection plates, the beam deflected as expected, but when positive voltage was applied, it actually focused the beam rather than deflecting it in the opposite direction.

Variations in Size

The project includes tubes of various sizes — from a miniature CRT just 3 mm in diameter to larger tubes with a 1-inch diameter. Smaller tubes are interesting because they function at higher pressures, making them even easier to evacuate. However, they are more challenging to construct due to the tiny components involved.

Safety Considerations

The primary safety concern is the high voltage (2–5 kV), which can deliver a dangerous shock. The author emphasizes proper insulation and careful handling. As mentioned earlier, X-ray radiation is not a concern at these voltage levels. The vacuum itself poses a minor implosion risk, but the small glass envelopes used in this project make that unlikely to cause injury.

This project demonstrates that the fundamental principles behind CRT technology — electron emission, acceleration, beam focusing, and phosphor excitation — can be reproduced with surprisingly accessible materials. While the resulting tubes are far from the precision of commercial CRTs, they provide a hands-on understanding of the physics involved and a satisfying glow on the screen to show for it.