Audio and Video Talk > Valves / Vacuum Tubes

The function of the electron tube (DISCUSSION)

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Ampdog:

--- Quote from: ghostinthemachine on May 28, 2009, 07:55:17 PM --- Each one is probably entitled to their quota of mistakes???

--- End quote ---

Now where have I heard that before ..... Oh yea! The government!

 ;D ;D

Steerpike:

--- Quote from: ludo on May 27, 2009, 07:34:30 PM ---
There are tubes with which one can make the grid more positive than the cathode, but I believe those are a bit rare in audio.

--- End quote ---

Like the Space Charge pentodes of the 1960s, used in car radios, on an anode voltage (B+) of only 6 or 12 volts.
From the outside they look like an ordinary pentode, but you wire the first grid (that you might think was the control grid) directly to B+.

handsome:
speaking of space charge.....the cathode boils of electrons that form a "cloud" around the cathode, known as the space charge. the electrons are negatively charged and initially are all repelled from one another, thus once free of the cathode they sort of bounce away from one another until they stabilize in the aforementioned cloud. cathode (or anode) current is actually the result of electrons being drawn off this cloud and the cloud is then replenished by the cathode. extreme depletion of the cloud will damage the cathode (cathodes have very delicate chemical balances). Applying HT voltage before the cathodes are properly heated i.e. no space charge leads to cathode stripping whereby electrons are forcibly pulled from the cathode. Cathode materials are measured by their 'work function' which is the amount of energy (heat) required to 'loosen' the electrons in the material.

Running the grid above or close to 0 volts (relative to then cathode) makes the grid begin to absorb electrons like the anode. This causes the grid to heat up. As the grid is usually a coil of fine wire the heat can cause it to sag and touch the cathode resulting a dead valve. Too much heat and the grid will melt. Either way distortion of the grid structure will change the characteristics of the valve......

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