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Technical review

Polarization of the cathode

Gain and transconductance (II)

At first sight, the amplification factor defines alone the tube performance. This is not true because the load resistance influences much the amplification factor of the circuit. The more high is the resistance, the more gain offers the circuit, what reduces the value of that resistance, and the gain drops. 

The gain as it was defined earlier must be considered as a circuit characteristic rather that the one of the tube, and this is for that reason that electronicians introduced the transconductance (existing in FET transistors too). 

The transconductance is defined as the ratio between the variations of anodic current and the variations of the grid voltage. It is expressed in mho or Siemens.

Polarization of the cathode

Instead of using a battery to polarize the grid, power amplifiers working with triodes usually take advantage of another system : the cathode polarization, a solution much more flexible and simpler to set for manufacturers.

In the first circuit involving a triode, we used a battery that we connected to the grid to give it a polarization voltage and we placed the cathode to the ground because of the negative value of its voltage. We were forced to make this circuit because the cathode voltage (potential) was higher to the one of the grid.

Manufacturers proceed in another way, much less intuitive but more efficient. They place the grid to the ground in inserting a small resistance in the cathode. When there is current in the tube, a voltage drop will occurs in the cathode resistance. Its potential will be higher than the one at ground. In other words the cathode is positive in relation to the grid and thus the grid voltage is well negative in relation to the cathode.

Like in most electronic circuits, the condensor is mandatory because we are working with alternative current. Indeed, current variations could produce voltage variations and thus a variable polarization. This condenser suppresses all voltage variations in derivating its alternative component to the ground.

Vacuum tube manufacturers are mainly represented in the U.S. and are seen today as a reminiscence of the past, with the exception of the special purpose types used in broadcast and image sensing and displays. Today Eimac, GE, RCA, and the like are continuing to provide several series of tubes, but in a rate much slower than in the past, their main activity being concentrated to the production of audio or transmitting amplifier tubes and beam power tetrodes. Their competitors are the Russian manufacturers but which tubes quality is not always on top. When the product is of quality (i.e. 3-500Z triode sold by RFParts) the amateur can do a substantial saving vs their US couterpart, which can be up to 10 times more expensive !

That the light be : use your tube as a bulb !

At right, two 3-500ZG in full activity in their socket in a QRO HF-2000 amplifier showing a light glare in the dark. Below, a 3-500Z triode outside its socket, usually installed in a Kenwood TL-922. To light it its filament has been powered under 5V at 14.7A. No need of high voltage. How to get this energy ? If you own a 230V/6V at 75VA transformer, add a 100W resistance in serie with the 230V primary to get 5V on the filament. The resistance will dissipate 15W.

You can also use a 12V lead or gel battery with a coil resistance of 0.47W able to dissipate 100W. At last the best solution is using a stabilized power supply providing 5V at 15A. 

If you want to see a huge pentode tube GU-81 light, you must connect the filament pins 1 and 2 to a 12.6V at 11A power supply. Remember that when you connect the voltage to your tube, the filament current will be above 10A. Be careful and don't exceed the nominal filament voltage !

After have reviewed vacuum tubes and how they work, it is time to explore their main function in HF amplifiers, I mean the signal or power amplification. 

Characteristic curves of a transistor

Although vacuum tubes were promised to a bright future, in 1947 at Bell Laboratories, Bardeen, Brattain and Shockley built the first transistor which, like a vacuum tube, was able to amplify signals.

The transistor is an active electronic component constituted of two PN junctions (in fact there are diodes made of a sandwitch of P material and N material constituing respectively the anode and the cathode) placed side by side to form a NPN component.

 In the early 1950's, the tiny but very efficient transistors began to replace the big and less reliable vacuum tubes in radios and other electronic equipment. For decades transistors have been used to swith currents or voltages, to amplify, convey frequencies, mix them, among thousands other things. 

From 1960 it replaced amplifier vacuum tubes when IBM introduced its first IBM 1401 computer. To understand how work a 1 kW amplifier, there is nothing simpler than a transistor to understand is functioning.

A transistor works like a switch : saturated it let the current through and closed it blocks it. We can also use it in an intermediate state where it works like an amplifier. Let's explain this principle.

If we trace the characteristic curves of a transistor on an oscilloscope we get the graph displayed at left. On the load line traced in blue three points are highlighted: the point of saturation, the operation point or quiescent point and the point were the load is blocked.

The operation point is variable. If we modify the transitor polarity (in other words if we change the contineous voltage applied on it) it will move along the load line between Vc=0 and Vce=Vcc. Between these extrema, the collector current varies contineously, producing a drop of voltage in the collector resistance. 

This drop of voltage follows the current variation according U=RI. The voltage U=0 when the transistor is blocking (below at right), then increases along the load line to reach U when the transistor is quasi saturated (above at left). 

Our objective is to use this transistor between both extrema, in a way to avoid blocking and saturation, thus in a narrower band along the load line.

The pattern of this voltage fit exactly the current one that we want to use in our amplifier, excepting that in practice our external polarisation elements are fixed and we cannot move the operation point. So how can we move it along the load line ?

The only variable in your system is the input voltage that we need to amplify and that we can apply on the base of our transistor. 

The signal being alternative, positives alternations will be added to the actual polarisation voltage what will increase the polarisation of the Base-Emitter junction. This effect wil increase the current in the Base (Ib), thus in the collector (Ic). In a similar but inverted way, negatives alternations will be substracted from the actual polarisation, reducing the current in the Base, thus in the Collector too.

As we can see on the drawing at left, in the upper left part of the graph a small alternative voltage is applied at the entry. This variation is find amplified below at right in the form of voltage between the transistor Collector and Emitter.

But we need to route amplified alternative signals to the Base and recover them on the Collector. How to proceed ?

Only one component is able to let through alternative signals and to stop the continous one  : the condenser. Its second characteristic is to display a reactance (resistivity) depending of its capacity and working frequency. So we will need a specific condenser showing the smallest reactance, acting like a circuit-breaker for alternative signals. These condensors are named coupling condensers.

When we create an amplifier, we send signals to amplify to the Base of our transistor and we "load" the emittor diode. We can then calculate the direct resistivity of this diode. Long computations allow us to define that the resistivity (Re) of a emittor diode in AC is equal to 25mV/ Ie. This approximation will allow us to estimate easily the amplification gain.

Next chapter 

Amplification classes

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