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As a result in a lamp two currents are formed: one — the anode current formed by the electrons which took off from the cathode, and the second — formed by secondary electrons, of the anode, and having an opposite direction. This current of the opposite direction sometimes call dynatron current, as knocking-outs from the anode of secondary electrons under the name of dynatron effect.

Screened lamps were a considerable step forward in comparison with three-electrode. At us tetrodes of SO-12 SB-154, etc. were issued earlier. However practice of use of tetrodes revealed the large shortcoming interfering expansion of their application.

If the coefficient of strengthening of a lamp was more, the equaled 30, anode current would stop at a on a grid — 5 in, and at =100 the termination of an anode would require a tax on a grid of all — 1,5 century.

The way of elimination of unpleasant consequences of dynatron effect is obvious: it is necessary not to allow secondary electrons to to the shielding grid. It is possible to carry out it in a lamp of one more — the third grid.

We noted that the shielding grid being under rather big positive tension (usually the 50 — 70, reports to the electrons forming anode current, additional speed. The electrons flying with very high speed with such force hit a surface of an that other electrons beat out from atoms of metal of the anode. One electron having rather high speed can beat out some electrons from the anode. ­ about the anode an electron it is accepted to call primary, and by it electrons — secondary.

The third grid settles down between the anode and the shielding grid and connects to the cathode. As negative ­ a source of tension it is connected to the cathode, the third grid is loaded negatively concerning the anode. To the secondary electrons which are beaten out from the anode will by this grid back to the anode. At the same time, being rare, this grid does not interfere to fly to the anode to electrons of the main anode current.

It is explained by that on the way of electrons to the anode one more barrier — an additional grid, so the anode decreases by electrons, and the operation of the anode on an electronic stream in comparison with action of an grid is less, the it is more than a coefficient of strengthening.

The coefficient of strengthening of  shows, in how many time the on a grid affects anode current more strongly, than on the anode. If =10, it means that change of a on 1 in acts on a grid the same as change on the anode on 10 Having given on the anode of this lamp tension, for example, 1506, we will create some current in its anode chain. As 1 in operates on a grid of a lamp also as 10 in on the anode, it is obvious that, having given on a grid — 15 in, we will absolutely stop anode current; the action of tension which is pushing away in relation to electrons will counterbalance the attracting operation of the anode.