Valve transmitters


Valve transmitters

Most high power transmitter amplifiers are of valve construction because of the high power required.

Anode circuits

Since valves are designed to operate with much higher resistive loads than solid state devices, the most common anode circuit is a tuned LC circuit where the anodes are connected at a voltage node. This circuit is often known as the anode tank circuit.

Grid circuits

Active (or tuned grid)

An example of this used at VHF/UHF include the 4CX250B, an example of a twin tetrode would be the QQV06/40A. The tetrode has a screen grid which is between the anode and the first grid, the purpose of the screen grid is to increase the stability of the circuit by reducing the capacitance between the first grid and the anode. In RF pentodes the additional grid is another screen grid which improves the isolation between the first grid and the anode. In audio equipment the third grid in a pentode is used to cure the tetrode kink.

Neutralization is a term used in valved electronics for negative feedback which is used to make the system more stable, this feedback counteracts the positive feedback which exists within a valve. an coupling, they enter a resonant LC circuit. It is possible by the correct choice of the ratio of the turns to obtain a step up in drive voltage which enables this design to have a very high gain. The very high gain is a double-edged sword, as it increases the risk of instability. With this type of stage good circuit layout is vital.

In common with all three basic designs shown here the anode of the valve is connected to a resonant LC circuit which has another inductive coupling which allows the RF output to be taken away

How it works

For a fixed anode voltage the anode current of a triode can be described by the following equation

Ianode = {K1.(Egrid-N1)} + {K2.(Egrid²-N2)} + {K3.(Egrid³-N3)} etc

For a tetrode the equation will be:

Ianode = {K1grid1.(Egrid1-N1grid1)} + {K2grid1.(Egrid1²-N2grid1)} + {K3grid1.(Egrid1³-N3grid1)} etc + {K1grid2.(Egrid2-N1grid2)} + {K2grid2.(Egrid2²-N2grid2)} + {K3grid2.(Egrid2³-N3grid2)} ... etc

Note that the K constants for the second grid are smaller than those of the first grid because the second grid is further away from the cathode.

As the second grid (screen grid) in a tetrode is maintained at a constant potential the equation for the tetrode can be reduced back to that of the triode as long as the screen grid is kept at the same potential.

"In short the anode current is controlled by the electrical potential (voltage) of the first grid." A DC bias is applied to the valve to ensure that the part of the transfer equation which is most suitable to the required application is used.

The input signal is able to perturb (change) the potential of the grid, this in turn will change the anode current. Another term for the anode in a valve is the plate so hence on many designs the anode current is named the plate current.

In the RF designs shown on this page between the anode and the high voltage supply (known by convention as B+) is a tuned circuit. This tuned circuit has been brought to resonance, and in a class A design can be thought of as a resistance. This is because a resistive load is coupled to the tuned circuit. In audio amplifiers the resistive load (loudspeaker is coupled via a transformer to the amplifier. In short the load formed by the loudspeaker driven via the transformer can be thought of as a resistor wired between the valves anode and B+.

As the current flowing through the anode connection is controlled by the grid, then the current flowing through the load is also controlled by the grid.

One of the disadvantages of a tuned grid compared to other RF designs is the neutralization is required.

Passive grid

An example of a passive grid used at VHF/UHF frequencies include the 4CX250B; an example of a twin tetrode would be the QQV06/40A. The tetrode has a screen grid which is between the anode and the first grid, the purpose of the screen grid is to increase the stability of the circuit by reducing the capacitance between the first grid and the anode. The combination of the effects of the screen grid and the damping resistor often allow the use of this design without neutralization.

The signals come into the circuit through a capacitor, they are then applied to the valve's first grid directly. The value of the grid resistor determines the gain of the amplifier stage. The higher the resistor the greater the gain, the lower the damping effect and the greater the risk of instability. With this type of stage good layout is less vital.

Passive grid design is ideal for audio equipment, because audio equipment must be more broadband than RF equipment. A RF device might be required to operate over the range 144 to 146 MHz (1.4% of an octave) while an audio amp might be required to operate over the range 20 Hz to 20 kHz, a range of three orders of magnitude.

Advantages

*Stable, no neutralizing required normally
*Constant load on the exciting stage

Disadvantages

*Low gain, more input power is required
*Less gain than tuned grid
*Less filtering than tuned grid (more broadband), hence the amplification of out of band spurious signals, such as harmonics, from an exciter is greater

Grounded grid

This design uses a triode, the grid current drawn in this system is larger than that required for the other two basic designs. Because of this, valves such as the 4CX250B are not suitable for this circuit. This circuit design has been used at 1296 MHz using disk seal triode valves such as the 2C39A.

The grid is kept at ground, the drive is applied to the cathode through a capacitor. The heater supply must be isolated with great care from the cathodes as unlike the other designs the cathode is not connected to RF ground. The cathodes are also at a DC potential more negative than the grounded grid, and the DC supply for the valve is likely to be more complex than the supply required for the other two designs.

Advantages

*Stable, no neutralizing required normally
*Some of the power from exciting stage appears in the output

Disadvantages

*Very low gain, much more input power is required
*The heater must be isolated with greater care from the valve with chokes


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