Brake grille

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The retarding grid is an electrode in an electron tube . The braking grid (grid 3) in electron tubes is the fourth electrode of tubes with several grids, arranged between the cathode and anode. The anode always follows the braking grid.

On the left the braking grid of an EL84

function

The task of the retarder is to guide secondary electrons knocked out of the anode back to the anode. This is achieved in that the braking grid has the same electrical potential as the cathode . Often the retarding grid is also firmly connected to the cathode within the tube. The primary electron beam, made up of electrons emitted directly from the cathode and then accelerated by the screen grid , is practically not influenced by the retarder grid . Only the much slower secondary electrons, which are knocked out of the anode by the high kinetic energy of the primary electrons, are pushed back to the anode by the braking grid.

The braking grid thus enables operation with low anode voltages. Without a braking grid, the secondary electrons knocked out would fly back to the screen grid at low anode voltages, as can occur with high levels of modulation, reducing the anode current and thereby causing distortions. If the secondary electrons overheat the screen grid by releasing their kinetic energy, the tube can be damaged.

Beam power tetrode

In this tube, the retarding grille acts as a beam guide plate, which - similar to a passepartout in photography - directs the electrons onto a limited area of ​​the anode . The function is otherwise the same. This type of retarder grid is used almost exclusively in power tubes and was developed to circumvent the license fees of the Philips patent on the pentode with conventional screen grid.

In the pictures below you can see two tubes with a very open system construction. The EF91 has a normal brake grille in the usual spiral design, while the PL802 has a slightly darker colored radiant panel. The light sheet with the two connecting bars is the anode.

A retarding grid is only used in a pentode, it is not used in a triode or tetrode .

  • Pentodes

  • Brake grid pentode EF91

  • Beam-Power Pentode PL802

  • EL84 from SEL as a beam power variant: reflective cut-out in the anode plate shows the beam plate.

  • EL84 from SEL as a beam power variant: Only two pairs of bars can be seen, the radiant sheet metal tab covers the cathode.

See also

literature

  • Ludwig Ratheiser: The great tube manual . Franzis-Verlag, Munich 1995, ISBN 3-7723-5064-X .
  • Ludwig Ratheiser: Radio tubes - properties and application . Union Deutsche Verlagsgesellschaft, Berlin 1936.