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Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
If you increase the voltage enough, the tube will arc internally. This can be messy, as it often arcs from plate to grid (or screen in a tetrode) blowing holes in the grid and dumping your HV supply into the grid circuit.
The tube can withstand more overvoltage cold than when it's hot.
Generally, tubes withstand overvoltage better than overcurrent. You can crank the plate voltage to 2, 3, 4 times the limit in the tube datasheet, and it'll probably live.
You can think of them as the dual of IGBTs: these can take overcurrent easily, but the maximum voltage rating is non-negotiable.
"With tubes you pay for current, with transistors you pay for voltage".
Registered Member #2463
Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546
Proud Mary has nailed it with the way the tuned load in Class C is infact what is oscillating after beng kicked by the tube switching on. Between grid pulses the anode to cathode voltage can be several thousand volts, at the resonant frequency as well as having a DC component equal to anode supply. That is why many TV schematics have a warning "do not measure" pointing to the anode of the line output tube. 6JE6 anode voltage is 7500 peak pulse and 175 DC with a cathode current of 315 mA.
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
Proud Mary wrote ... As a rule of thumb, the small valves found in consumer electronics such as television receivers are dependent on convection and radiation for cooling, and so are not designed to run with their anodes at red heat. ...
Here is a failure mode caused by, I bet, protracted overheating of the plate in a popular audio tube.
This tube has gone to air in a way that lets you literally touch the plate metal.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
I got mixed answers to tube preheating - should the heater always be allowed to heat up fully before applying anode power, or is "cold start" (when you turn both anode and heater at the same time) ok?
Registered Member #2463
Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546
This depends on the valve. Mercury Vapor rects need preheat. . Some are made wiith controlled warmup characteristics so that a system will come up all together. eg: 12BH7A Warmup time for a valve has a spcific definition.
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
You should always warm up the filaments prior to applying voltage to the tube. This ensures the filament current has stabilized (heating of the filament will vary its resistance) and that the rate of thermionic emission has peaked and stabilized.
The warm-up is typically proportional to the size (mass) of the filament, so small tubes may only take a few seconds while very large tubes could take several minutes.
In many tubes, the filament is connected directly to cathode so you also have this thermal mass which needs to heat up as well, which will lead to longer filament warm-up times.
That said, i always allow my VTTC tubes to warm up for at least a few minutes before applying power.
Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
If the filament is not warm enough, you will risc cathode depletion as the thorium on the tungsten filament will vaporize as its not hot enough to give off ions instead.
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Vacuum tube life in tesla coil use
