Edison discovered the Edison effect about 1880. Had he not been so otherwise occupied with getting a working light bulb he might have invented the diode and triode in rapid succession. In the following video Handy Jack makes an Edison Effect light bulb and shows the effect starting at T=32m30s. Skip ahead to that point if you do not want to watch the build sequence. The effect manifests as a diode effect. Something that the radio guys must have overlooked because they were using metal filings in a tube with a tippy tapper to do their detection.
The possibilities
Edison would have been 20 years ahead of Marconi had he handed this off to one of his lab guys.
Edison Effect PDF – 88 pages of detail. He tried to use it to regulate a generators output. So he more or less knew there was something there.
Possible lines of reasoning
- Observation of diode effect was direct. – a research effort to optimize that alone was warranted at least in retrospect. This seems so obvious that Edison et al must have not recognized the full value.
- The current flow in the diode would have yielded the observation that current is a function of voltage across the filiment to plate.
- The observation of current varying due to voltage leads to the idea of a phantom plate – a whispy mesh in between filiment and plate that effectively modulates the voltage to the plate and thereby modulates the current.
Next came the tetrode and the pentode – these were refinements. Most of the discovery was in the diode and triode.
- Screen grid was added next – This resulted in a Tetrode The screen grid tube provides much smaller control grid to anode capacitance and much greater amplification factor than a triode. Radio frequency amplifier circuits using triodes were prone to oscillation due to the grid to anode capacitance of the triode. In the screen grid tube, a grid referred to as the screen grid, shield grid or sometimes accelerating grid is inserted between the control grid and the anode. The screen grid provides an electrostatic shield between the control grid and the anode, reducing the capacitance between them to a very small amount. The anode characteristic of a screen-grid valve is thus quite unlike that of a triode. Where the anode voltage is less than that of the screen grid, there is a distinctive negative resistance characteristic, called the dynatron region or tetrode kink. Dynatron oscillator
- Supressor grid was added to solve the negative resistance problem introduced by the screen grid.
Tetrode Kink – from wikipedia tetrode
Typical Bias voltages of Pentode
Per: Pentode Valve / Vacuum Tube
- Cathode: As with other forms of vacuum tube or valve, in the pentode the cathode is heater and emits electrons into the evacuated envelope.
- Control grid: As with other forms of vacuum tube, in the pentode the control grid is commonly maintained negative with respect to the cathode and it is used to control the flow of electrons between the cathode and anode.
- Screen grid: The screen grid is operated at a fixed positive potential, but below that of the anode and it provides screening between the anode and control grid, which considerably improved the performance. It is decoupled to ground using a capacitor to ensure that the screening is effective.
- Suppressor grid: In the pentode valve / vacuum tube, the suppressor grid is generally maintained at a low voltage, often connected directly to the cathode. Its function is to create a lower voltage region between the screen grid and the anode. It suppresses the secondary emission where high energy electrons hitting the anode at high speed have a tendency to bounce off. This effect causes a kink in the response curve of tetrode valves. In this way it enables the pentode to provide a high amplification factor along with the ability to operate at high frequencies.
- Anode: The anode or plate is operated at a high potential and it is the electrode to which electrons are ultimately drawn in the pentode or any other valve.
The history of the various tube designs are helpful in understanding tube design and introduction of tetrode and pentode
Quick Overview of Circuit Topologies
Chinese Tube Factory Making EL-34-B
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