Vacuum tube amplifiers require multiple voltage sources - PA with a tetrode: heating, anode, switchable Ug1 (to turn off the tube during RX), Ug2, and usually one more for relay and auxiliary circuits. Enough, right?
Amplifiers with grounded grids to the cathode are popular, which significantly simplifies construction. However, there are few tubes suitable for this mode, but there is a very simple way to simplify a relatively complicated source for the first grid in a very clever way.
According to US patent No. 3129388, it is possible to use a Zener diode (ZD) as a bias voltage source. You can also find an article about it in AR 9/68 by Ing. Viliam Petrík, OK3WAO. One or more ZDs are connected in the cathode of the tube according to the required bias voltage. The cathode current creates a voltage across it (them), which is used as the bias for the first grid.
When selecting diodes, we consider the cathode current and the required bias voltage. To achieve a certain voltage, for example, 50V, it is more appropriate to use 3x16V ZD than 1x50V, because ZDs with lower voltage have much lower dynamic resistance, which means a stiffer bias voltage source.
To overcome the bend in the ZD characteristic, a current of 2 to 4mA is needed in practice, which is important when blocking the PA during reception, because the current flowing through the tube means noise at the resonant frequency of the pi-network. It is possible to power the ZD through a resistor from Ua or Ug2, or another suitable voltage. Diodes should be bypassed with a capacitor. Switching Ug1 is solved with the relay contact, which shorts one or more ZDs during TX.