Power amplifier with grounded grid

these parameters are significantly affected by the quality of the capacitors in the input circuit: these parameters are significantly affected by the quality of the capacitors in the input circuit? Transistor amplifiers of larger powers are unavailable for us, especially from a price point of view. The cheapest ferrite materials are not even necessary. And so the choice often falls on a PA with a tube.

In the literature and on the Internet, you will find countless instructions, differing in small details. Basically, they are all amplifiers with excitation to the first grid or to the cathode. This article is about the advantages and disadvantages of connection with a grounded grid and excitation to the cathode.

In the previous article dedicated to PA issues, I already mentioned the demands on different voltages of tube PAs excited to g1. If you are lucky and the tube allows you to use a connection with grounded grids, you will only need three voltages: anode, heating and auxiliary (for relays and so on.). Blocking during reception can be solved using Zener diodes.

Benefits of involvement

PA in this connection is characterized by simplicity of connection, high linearity and stability. It also achieves higher efficiency, because the excitation power is added to the output power. The input impedance is inversely dependent on the steepness of the tube and varies in tens of ohms. The apparent disadvantage is the lower gain, but this is usually not a problem, because TCVR provides enough power.

Suitable tubes for a power amplifier with a grounded grid

Not every tube is suitable for this type of connection. Often used GU50 have problems with large grid currents. It is better not to ground g2, but bring a bias to them about 20 – 50V. This will increase quiescent current and linearity. Suitable tubes are, for example: 811, GU72, 3-500WITH, 3CX800, 3CX1200.

Choke in the glow tube

An important part of the PA with cathode excitation is the choke in the glow plug. Cathode system – the filament represents capacity, which is connected in parallel to the input of the amplifier. Unfortunately, the capacity is unstable and therefore it is necessary to take this into account during construction, especially in the area of ​​VHF. We wind the choke bifilarly with a wire with a sufficient cross-section as air or on a suitable ground. We can also use a suitable ferrite material, but it is quite difficult to wind the choke with solid wire on the toroid. It is also convenient to use a pair of blocking capacitors.

Ease of connection

The considerable ease of connection could be an inspiration when building a new PA. My goal was not to provide a specific PA tutorial (it can still be said, that every amateur PA is original in its own way), but to show how it is possible to create a tube PA with maximum simplicity.