It pays to read the manual!!

nsaspook

Joined Aug 27, 2009
13,086
IME: cathode poisoning is most often caused by undervolted heaters or standing around for long periods with too little anode current - apparently the normal electron flow sweeps up positive ions that bombard and contaminate the cathode coating.
We use this effect (halogen cycle with fluorine) to extend cathode life in high power ion sources. With a high AMU process gas the net movement of cathode metal ions is to the chamber liner walls (made of the same material as the cathode. If we switch to a fluorine containing process gas at the correct operating conditions the direction reverses from the walls to the cathode rebuilding the ionization surface without downtime.

http://en.wikipedia.org/wiki/Halogen_lamp


http://www.google.com/patents/US5943594
This is for an obsolete directly heated source cathode but the same effect can used on IHC.
 
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ian field

Joined Oct 27, 2012
6,536
Where you have the powersupply using tube rectifiers it probably doesn't matter. But if the power supply uses solid state devices then there is HV before the cathode is fully heated. In marine radars they have a built in timer so the Magnetron is fully heated before the HV is applied. And the radar power switch has a standby position that keeps the magnetron heated if you dont want to wait for the warmup period.
There was pretty much a long running controversy when people started replacing valve rectifiers with solid state, valve rectifiers took just as long as the power valves to warm up, with silicon - bang; the HT was right there, some people had the misfortune of stripped cathode coating because of full anode current before the space charge got going.
 

nsaspook

Joined Aug 27, 2009
13,086
Operating a tube in "space-charge" mode at low voltage is (was) a cool thing to play with before transistors. There were special tubes designed to work at 12vdc for cars that used grids for electron acceleration but lots of 'normal' tubes could be adapted to work by running the cathode very hot.
http://oldradiobuilder.com/12vsuperhet.html

Maybe I'm just a old fart but we should still teaching tube electronics to new students for something longer than just an historical footnote. Not for the ability to work on tube equipment but for the ability to see the applied physics of charge in motion at a visible scale early.
 

ian field

Joined Oct 27, 2012
6,536
Maybe I'm just a old fart but we should still teaching tube electronics to new students for something longer than just an historical footnote. Not for the ability to work on tube equipment but for the ability to see the applied physics of charge in motion at a visible scale early.
Its how us "old farts" get one over on the college boys who think they know everything.
 

nsaspook

Joined Aug 27, 2009
13,086
Its how us "old farts" get one over on the college boys who think they know everything.
They know it at the scientific theory level but not at the practicing engineering level of what works and how it works in real devices. Unless you work in a specialized field that still deals with tubes or tube like devices the information seems esoteric today but having that background makes it easier to understand complex solid-state devices where charge/hole movements are invisible, difficult to mentally picture in solids and are moved by electric fields just like in vacuum tubes.
 
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ian field

Joined Oct 27, 2012
6,536
They know it at the scientific theory level but not at the practicing engineering level of what works and how it works in real devices. Unless you work in a specialized field that still deals with tubes or tube like devices the information seems esoteric today but having that background makes it easier to understand complex solid-state devices where charge/hole movements are invisible, difficult to mentally picture in solids and are moved by fields just like in vacuum tubes.
My most recent encounters of the thermionic kind were microwave oven magnetrons.

For a while I worked for a back street bodger with a strong emphasis on getting stuff out the door.

We were getting a hell of a lot come in with low emission, our best guess was that excessive use of defrost mode was the likely cause, I found that if you jam the cooling fan and run continuous full power for about 15 minutes, the emission often recovered good as new - we almost never got any returns on microwaves for any reason, so it must've been working pretty well.
 

ian field

Joined Oct 27, 2012
6,536
Had a bad tube in the shunt regulator of a 10kV power supply this week. Thank goodness for Russia when you need modern components.
This is the biggest shunt triode I've encountered:-

http://www.r-type.org/pdfs/pd500.pdf

They were im most of the very early Btitish CTVs, all 3 power valves of the horizontal output stage were enclosed in a ventilated steel box to contain any X-rays, any covers that could be removed from the box were protected by safety interlock switches.

All that was expensive to manufacture, so it didn't take manufacturers all that long to figure out more efficient ways of controlling X-rays than a shunt regulator - which itself was as guilty as the other 2 valves put together!
 

snav

Joined Aug 1, 2011
115
Wouldn't it be better if you just designed and built a sequence-timer circuit and use a single switch instead of performing that operation manually every time your son intended to use it?
One of the reasons the 5AR4 is so popular, Delayed B+ with indirect heater over 5U4. Still, standby also extends system life while not in use also.
 
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