Fudgy McFarlen

Edison Effect and Electronic Tubes diodes triodes tetrodes pentodes

Edison knew there was something to this effect and should have put a good man on the job to chase it down. As it is it took another 20 years before Deforest invented the triode.

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

  1. 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. 
  2. The current flow in the diode would have yielded the observation that current is a function of voltage across the filiment to plate.
  3. 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.
By Fudgy McFarlen, ago