A tetrode adds a screen grid between the control grid and the anode. It shields the grid from the anode (raising μ into the hundreds) and greatly increases the gain, but it brings a side effect. While the anode is more positive than the screen all is well. But when Ua < Ug2, the secondary electrons knocked out of the anode fly off to the more positive screen, and the anode current falls as the anode voltage rises. This is the dynatron effect: a region of negative resistance.
The oscillator is built on it. There is no feedback winding at all: a 1 mH ∥ 1 nF tuned circuit hangs directly on the anode, and the anode supply (120 V) is deliberately set below the screen voltage (250 V) — right in the dip of the characteristic. The valve's negative resistance cancels the tank losses and oscillation starts by itself.
This page is a utility for simulating tetrode: dynatron hf oscillator online with specified initial values.
Using the model. Once the cathode is hot the oscillation builds up out of nothing within a fraction of a second — the scope shows the tank voltage, about 190 V peak-to-peak at f = 1/(2π√LC) ≈ 159 kHz.
Use the slider to raise the anode supply above the screen voltage: the valve leaves the dynatron region, the negative resistance disappears and oscillation stops. Bring it back down and the oscillation returns. That is the clearest way to see that the oscillator lives entirely on secondary emission. Change L or C and the frequency moves with the same square root. Incidentally, this is exactly the effect the third grid removes: swap the tetrode for a pentode and oscillation becomes impossible.
Comments