Tube shields are used to filter out radio frequency interference (RFI) from vacuum tubes in electronic circuits. They work like a Faraday cage, absorbing and cancelling interference. Tube shields come in various designs and act as a heat sink, dissipating thermal energy. They are particularly useful for high-gain tubes and reduce the amplifier’s noise floor. EMI is a major issue for electronic equipment users, but tube shields can help keep noise levels low.
A tube screen is a device used to filter out noise that causes radio frequency interference (RFI) from vacuum tubes in electronic circuits. Most tube shields are cylindrical covers that slide over a vacuum tube with a ground tag or base that connects to the ground point of the tube outlet. The tube shield works on the same principle as a Faraday cage, i.e. it absorbs and cancels interference before it is picked up by the tube. Vacuum tube screens are available in a large selection of designs; some are fairly simple, one-piece pouches and others are more complex, multi-layered or mesh-covered varieties. The thick tube shield also acts as a heat sink, dissipating the thermal energy developed by the tube.
Electromagnetic interference (EMI) or RFI has long been the bane of users of electronic equipment, especially those using sound reinforcement equipment. EMI is a major contributor to the high noise floor or natural operating noise levels in amplifiers, especially tube amplifiers, which are undergoing a huge upswing in popularity. Latent sources of EMI are also steadily increasing with the radio frequency spectrum becoming more densely populated every day. Appliances like vacuum cleaners, mixers, televisions, cell phones, and even bug zappers add to the EMI soup that makes even the best tube amp buzz, hiss, and howl. There is, however, one simple device that can keep a tube amp’s noise floor as low as possible: the humble tube shield.
Tube shields work on the same principle as a Faraday cage and typically consist of a conductive casing covering a vacuum tube. The Faraday cage is basically a hollow conductor, the walls of which serve to absorb and cancel out errant external signals. The tube shield works the same way by preventing EMI-causing signals from entering the vacuum tube’s internal circuitry. Most of these shields consist of simple metal sleeves that slip over the vacuum hose, although more complex examples with conductive mesh coatings and intricate contours are also common.
Once in place, the sleeve absorbs most hum-inducing EMI signals, thereby reducing the amplifier’s noise floor. A tube shield is of particular value on tubes that exhibit the highest gain values such as preamp stage tubes. The use of pipe shields on these pipes often plays an additional role as a heat sink. High-gain vacuum tubes get very hot, and the shield of the tube can help dissipate much of the heat, thus protecting both the tube and the rest of the circuit.
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