Zener diode as a source of bias
Electronic amplifiers need multiple voltage sources – PA with tetrode: glowing, anode, switchable Ug1 (to close the tube at RX), Ug2 and usually one more for relays and auxiliary circuits. Enough, Nope?
Amplifiers with grounded grids to the cathode are popular, which greatly simplifies the construction. However, there are few electronics suitable for this mode, but there is a very simple way to simplify the rather complicated source for the first grid in a very funny way.
According to US patent no. 3129388, it is possible to use a Zener diode (ZD)as a bias source. You can also find an article about it in AR 9/68 by Ing. Viliam Petrík, OK3WAO. In the cathode of the tube, one or more ZDs are connected according to the size of the bias voltage. Through the flow of cathode current on it (them) tension is created, which is used as a bias for the first grid.
When choosing diodes, we are guided by the cathode current and the required bias voltage. To achieve a certain tension, for example 50V, it is more appropriate to use 3x16V ZD than 1x50V, because lower voltage ZDs have much lower dynamic resistance, which means a tougher bias source.
In practice, a current is required to overcome the bend in the ZD characteristic 2 up to 4mA, which is important when blocking PA at reception, because the current flowing through the tube means noise at the resonant frequency of the pi cell. It is possible to supply ZD through a resistor from Ua or Ug2, or another suitable voltage. It is advisable to block the diodes with a capacitor. Switching Ug1 is solved using a relay contact, which short-circuits one or more ZD during TX.
