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G2 voltage regulator.
A G2 voltage regulator is a very important part of tetrode PAs. It must work as a parallel regulator, since the current can flow not only in, but also out of the G2 especially at higher drive. This current must be absorbed in the regulator without changing the G2 voltage. 

 

 Why can the current flow out of the G2? There’s following effect that occurs at higher drive. Electrons emitted from cathode flying along the G2 are attracted more by the anode, since it’s on a higher potential then G2. A small part of them is captured by G2 and causes normal G2 current flowing into the grid. The electrons that land on the anode can strike other electrons within it. This effect is called secondary emission. Normally these electrons land back on the anode due to its higher positive potential. However, when the drive exceeds certain level, i.e. the amplitude of the AC anode voltage increases, the instantaneous anode potential can get under the G2. At this moment the secondary emitted electrons start to strafe G2 and the G2 current flows in opposite direction then usual. This effect is known as dynatron effect and it creates a negative internal resistance, a “hump” on the tube characteristics. This problem can be solved by adding of another (third) grid very loose winding between G2 and anode whose role is to capture secondary electrons (the resulting tube is the pentode). The third grid must be on ground or cathode potential in order to work properly. Unfortunately, this increases the capacity cathode-anode. For this reason there are no pentods on the frequencies over 100 MHz.

The voltage regulator described in this article can (except of  the usual voltage regulation) also absorb the current flowing out of the G2 and keep the voltage stable at all circumstances.  The voltage is determined exclusively by the junction voltage of the Zener diods or transistors. These transistors have relatively low amplifying factor (10-15), which eliminates any oscilations. All circuits with operational amplifiers or many transistors with the high gain in the regulation loop (sometimes over 1000!)  (as we may see in some articles) are potential source of trouble. Be aware that 144 MHz can get everywhere!

The regulator is a simple classical circuit, the Zener diodes are blocked with capacitors to avoid their noise. The value of the resistor should be calculated according to the voltage of the transformer for the current about 100 mA.  Except of the basic regulator there are some additional protection circuits in the design. A varistor/resistor combination is connected to the output of the regulator, direct to the G2  and a fuse (about 200mA) is put between the G2 and the regulator in case a discharge occurs for some reason in the the tube (typical for tetrodes). If a discharge occurs the fuse will blow and split the regulator from G2. The varistor then cuts off  the voltage peak on G2 while the resistor limits the discharge current. Without the protection the huge current caused by the charge of the capacitors in the anode power supply would flow through the regulator and destroy the transistors and/or Zener diodes. Another protection circuit  switches off the G2 voltage in case the anode voltage gets permanetly lower under the G2 voltage. This is a very dangerous state for the tube, because G2 becomes an “anode” then and takes over the whole anode current resulting usually in the blowing of the G2 and destroying of the expensive tube. To avoid this a relay controlled by anode voltage can cut off the G2 from the power supply

 

This is the circuit for the relay control. Note that there’s a high voltage on the resitors and capacitors. They must be sized accordingly. !

 

 

A characterictics of the G2 grid current depending on the G1 and anode voltage for the tube GU78b. Pay attention to the spot where the current is zero!

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