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| Why weren't Vacuum Tubes designed for higher currents |
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| ZeroResistance:
--- Quote from: Gyro on August 13, 2019, 04:21:36 pm --- Once you bias the anode positive and the tube starts conducting, these electrons flow towards the anode. If you pull too much current, the space charge is exhausted and you start physically 'ripping' electrons from the Cathode surface, irreversibly damaging it (particularly if it is an oxide coated one for higher emission). The space charge also helps protect the cathode surface from positive ion bombardment. --- End quote --- Wouldn't the electrons be returned back to the Cathode after hitting the Anode and therefore replenishing it? In that case why would the electrons in the Cathode get exhausted? I didn't get the "positive ion bombardment" part too... |
| hfleming:
Here are 2 datasheets, one is for a 5MW magnetron, the other is for a 66kW broadcast tube |
| Gyro:
--- Quote from: ZeroResistance on August 13, 2019, 07:45:41 pm --- --- Quote from: Gyro on August 13, 2019, 04:21:36 pm --- Once you bias the anode positive and the tube starts conducting, these electrons flow towards the anode. If you pull too much current, the space charge is exhausted and you start physically 'ripping' electrons from the Cathode surface, irreversibly damaging it (particularly if it is an oxide coated one for higher emission). The space charge also helps protect the cathode surface from positive ion bombardment. --- End quote --- Wouldn't the electrons be returned back to the Cathode after hitting the Anode and therefore replenishing it? In that case why would the electrons in the Cathode get exhausted? I didn't get the "positive ion bombardment" part too... --- End quote --- Don't forget there's an external circuit - electrons reaching the anode will pass into the external bias 'supply' ie. circuit. In fact some electrons can be knocked off the anode (secondary emission) which is responsible for the characteristic kink in a Tetrode's characteristics. This is resolved in the Pentode by adding a coarse negatively charged Suppressor grid between the anode and other electrodes to repel secondary emission electrons back to the anode. The secondary emission kink is also suppressed in the Beam Tetrode (aka Kinkless Tetrode) which creates a 'virtual' screen grid in the voltage gradient to the anode (it's complicated). It's worth googling Tetrodes and Pentodes at this stage [Edit: and taking a browse through some of the books in Peter Millett's archive]. The Positive Ions are formed by collisions with trace gas atoms remaining in the tube. Being positively charged, they are strongly attracted to the cathode, and being relatively massive, they can severely damage the emissive coating on the cathode. This is why Tungsten is used for very high voltage tubes, even though the work function is lower, the surface is immune from high energy ion bombardment. |
| chris_leyson:
Beam tetrodes were designed to get around the Mullard pentode patent and eliminated the tetrode kink hence valves like the KT66 or KT88, KT standing for kinkless tetrode, allegly. One thing you don't see with vacuum tubes is the non-linear voltage controlled junction capacitance that you get with semiconductor devices. Less harmonic distortion to some extent. |
| ArthurDent:
Here is a photo and close-up of a Tung-Sol CTL-6336A dual triode with an octal base. It was typically used as the series pass tube in higher current power supplies than the 6AS7 dual triode mentioned earlier and could handle about 4 times the current or about 450 ma per triode section.. The problem with vacuum is that it sucks as a conductor, both for electricity and for temperature transfer. Conduct may not be the technically correct explanation but that is the effect and easier to visualize. The 6336A didn’t use steel plates like the 6AS7 and other lower current (and less expensive) tubes, but had machined graphite plates that wouldn’t warp if they got hot enough to glow red. The heater was 6.3 volt @ 5 amps or over 30 watt and the plate dissipation was 30 watts for each of the triode sections. The electrons had to be boiled out of the cathode and were literally fired at the plate and their flow was controlled by the potential on the control grid. The spacing between elements of this triode was quite small to increase the efficiency by decreasing the distance the electrons had to travel through the vacuum. To get higher power, some transmitting tubes didn’t have their plates inside a glass envelope but made the plate the outside of the tube so the heat could be more easily dissipated. This also meant the outside of the tube would have a high lethal voltage on it. The plates could be water cooled and here is the datasheet on a high power water cooled tube. The filament current of this tube is 640 amps and the anode current is 125 amp max. If you’re wasting tens of kilowatts just to boil electrons out of the filament, you can see why much higher efficiency solid state devices are so popular for transmitters today. http://www.tubecollectors.org/eimac/archives/8974.pdf |
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