A tube shield is a device used to filter out noise causing radio frequency interference (RFI) from vacuum tubes in electronic circuits. Most tube shields are cylindrical covers which slip over a vacuum tube with a ground tag or base that connects to the tube's socket ground point. The tube shield works on the same principle as a Faraday cage, i.e., absorbing and canceling out interference before it is picked up by the tube. Vacuum tube shields are available in a wide selection of designs; some are fairly simple, one piece envelopes and others are more complex, multilayered or mesh covered varieties. The tube shield also often doubles as a heat sink, dissipating thermal energy developed by the tube.
Electromagnetic interference (EMI) or RFI has long been the bane of electronic equipment users, particularly those using amplification equipment. EMI is a major contributor to high noise floor or natural operating noise levels in amplifiers, especially the tube amps which are subject to a huge resurgence in popularity. The latent sources of EMI are also increasing constantly with the radio frequency spectrum becoming more heavily populated every day. Household appliances such as vacuum cleaners, mixers, televisions, cell phones and even bug zappers also add to the EMI soup which causes even the best tube amp to hum, hiss, and howl. There is, however, one simple device which can keep the noise floor of a tube amp as low as possible — the humble tube shield.
A tube shields work on the same principle as a Faraday cage and typically consists of a conductive envelope that covers 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 functions in the same way by stopping EMI-causing signals from ever entering the vacuum tube's internal circuitry. Most of these shields consist of simple metal sleeves which slip over the vacuum tube, although more complex examples with conductive mesh coverings and intricate profiles are also common.
Once in place, the sleeve absorbs most of the hum-inducing EMI signals, thereby reducing the noise floor of the amp. A tube shield is of particular value on tubes which feature the highest gain values such as preamp stage tubes. The use of tube shields on these tubes often serves an additional role as a heat sink. High gain vacuum tubes get really hot, and the tube shield can help to dissipate much of the heat, thus protecting both the tube and the rest of the circuit.
