Types of precipitators[ edit ] There are two main types of precipitators:
Plotting plate current as a function of applied grid voltage, it was seen that there was a range of grid voltages for which the transfer characteristics were approximately linear. To use this range, a negative bias voltage had to be applied to the grid to position the DC operating point in the linear region.
This was called the idle condition, and the plate current at this point the "idle current". The controlling voltage was superimposed onto the bias voltage, resulting in a linear variation of plate current in response to both positive and negative variation of the input voltage around that point.
This concept is called grid bias. Many early radio sets had a third battery called the "C battery" unrelated to the present-day C cellfor which the letter denotes its size and shape. Later circuits, after tubes were made with heaters isolated from their cathodes, used cathode biasingavoiding the need for a separate negative power supply.
For cathode biasing, a relatively low-value resistor is connected between the cathode and ground. This makes the cathode positive with respect to the grid, which is at ground potential for DC. However C batteries continued to be included in some equipment even when the "A" and "B" batteries had been replaced by power from the AC mains.
That was possible because there was essentially no current draw The principle of pulse industrial vacuum these batteries; they could thus last for many years often longer than all the tubes without requiring replacement. When triodes were first used in radio transmitters and receivers, it was found that tuned amplification stages had a tendency to oscillate unless their gain was very limited.
Eventually the technique of neutralization was developed whereby the RF transformer connected to the plate anode would include an additional winding in the opposite phase.
This winding would be connected back to the grid through a small capacitor, and when properly adjusted would cancel the Miller capacitance. This technique was employed and led to the success of the Neutrodyne radio during the s. However, neutralization required careful adjustment and proved unsatisfactory when used over a wide range of frequencies.
Tetrodes and pentodes[ edit ] Main articles: Tetrode and Pentode Tetrode symbol. From top to bottom: Schottky invented the tetrode tube in A positive voltage slightly lower than the plate anode voltage was applied to it, and was bypassed for high frequencies to ground with a capacitor. This arrangement decoupled the anode and the control gridessentially eliminating the Miller capacitance and its associated problems.
Consequently, higher voltage gains from a single tube became possible, reducing the number of tubes required in many circuits. This two-grid tube is called a tetrodemeaning four active electrodes, and was common by At certain values of plate voltage and current, the tetrode characteristic curves are kinked due to secondary emission.
However, the tetrode had one new problem. In any tube, electrons strike the anode with sufficient energy to cause the emission of electrons from its surface. In a triode this so-called secondary emission of electrons is not important since they are simply re-captured by the more positive anode plate.
But in a tetrode they can be captured by the screen grid thus also acting as an anode since it is also at a high voltage, thus robbing them from the plate current and reducing the amplification of the device.
Since secondary electrons can outnumber the primary electrons, in the worst case, particularly as the plate voltage dips below the screen voltage, the plate current can decrease with increasing plate voltage.
This is the so-called "tetrode kink" and is an example of negative resistance which can itself cause instability. The so-called dynatron oscillator thus operated on the same principle of negative resistance as the tunnel diode oscillator many years later. Another undesirable consequence of secondary emission is that in extreme cases enough charge can flow to the screen grid to overheat and destroy it.
Later tetrodes had anodes treated to reduce secondary emission; earlier ones such as the type 77 sharp-cutoff pentode connected as a tetrode made better dynatrons. The solution was to add another grid between the screen grid and the main anode, called the suppressor grid since it suppressed secondary emission current toward the screen grid.
This grid was held at the cathode or "ground" voltage and its negative voltage relative to the anode electrostatically repelled secondary electrons so that they would be collected by the anode after all. This three-grid tube is called a pentodemeaning five electrodes.IndustrIAl hePA vAcuuM cleAner OWNER’S/OPERATOR’S MANUAL WORKING PRINCIPLE ii.
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