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| Why weren't Vacuum Tubes designed for higher currents |
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| ZeroResistance:
I see that Vacuum tubes are typically rated for medium to high voltage (100's of Volts to few kV) but currents are typically in mA upto a few Amps. What factors limit their use only upto a few Amps? TIA |
| MagicSmoker:
Thyratrons, mercury arc rectifiers and quite a few other exotic tubes were (are) capable of 100s to 1000s of amps. Hot cathode tubes - triodes, pentodes and the like - are limited in current mostly by the rate at which electrons can be emitted from the cathode (space charge limits, basically). |
| Tom45:
I don't know the answer, but that isn't going to stop me from speculating. In the linear region of operation a device dissipates E*I. For transistors, E is typically under 100 volts. For vacuum tubes E is more commonly in the 100s of volts. For a given power dissipation, a tube operating at a voltage 5 times that of a transistor, will max out at a current 1/5 of the transistor having a same power rating. And the tube's vacuum makes it harder to get rid of the power it is dissipating. There are water cooled tubes for high power operation. But these weren't seen in consumer electronics. For example: http://www.tubecollectors.org/eimac/archives/8974.pdf |
| ZeroResistance:
--- Quote from: MagicSmoker on August 13, 2019, 03:45:08 pm ---Thyratrons, mercury arc rectifiers and quite a few other exotic tubes were (are) capable of 100s to 1000s of amps. Hot cathode tubes - triodes, pentodes and the like - are limited in current mostly by the rate at which electrons can be emitted from the cathode (space charge limits, basically). --- End quote --- Agreed! however I wanted to know regarding vacuum tubes. I guess that Thyratrons and the other devices that you stated are "Gas Filled Tubes". |
| ZeroResistance:
--- Quote from: MagicSmoker on August 13, 2019, 03:45:08 pm ---Hot cathode tubes - triodes, pentodes and the like - are limited in current mostly by the rate at which electrons can be emitted from the cathode (space charge limits, basically). --- End quote --- What are these space charge limits? I mean is there a known current (amps) limit beyond which vacuum tubes cannot be used? |
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