[Amps] Voodoo Magnetic Fields

[Gary Schafer]

Thanks Ian,

Very informative!

73
Gary  K4FMX


Ian White, G3SEK wrote:
> 2 wrote:
> 
>>
>>
>>> 2 wrote:
>>>
>>>>> High enough to initiate a plate supply short through the tube?
>>>>
>>>>
>>>> If the anode-grid path shorted, there would be an arc-mark on the grid.
>>>> I have not seen one in a grid-fil shorted tube -- nor have I found a
>>>> shorted tube that was gassy.  I doubt that Mr. Rauch's disappearing gas
>>>> theory is possible without direct intervention from the Fairy 
>>>> Godmother.
>>>>
>>> Since you persistently refuse to understand how a getter works, or to
>>> accept that arcs can happen in tubes that appear perfectly good, you're
>>> unlikely to find much evidence to change your mind.
>>>
>> Ian -- Please explain how a gassy, shorted 3-500Z is gettered between its
>> removal  from an amplifier and its being tested for gas with a high-pot a
>> minute or so later?
> 
> 
> We keep going around this argument in cycles of a few months; and every 
> time you act as if nobody had ever explained all this before. I am only 
> explaining it this time for the sake of any new arrivals.
> 
> 
> 
> The materials of which tubes are made - especially the metals - contain 
> trace amounts of trapped gases. When the tube is manufactured, it is 
> induction-heated to a very high temperature (way above the normal 
> operating temperature) to drive out as much as possible of these gases 
> while the tube is still connected to the vacuum pump. When the tube 
> cools down, it is sealed off.
> 
> But more gas continues to slowly evolve into the "vacuum" space. This is 
> a perfectly normal process, even in a tube with perfect vacuum seals 
> (leakage is a totally separate problem). To maintain the quality of the 
> vacuum throughout the life of the tube, the manufacturer creates a 
> specially activated metal surface inside, called a "getter". The getter 
> will react chemically with any gas atoms that strike it, and will keep 
> them trapped on the surface. It's a kind of passive, maintenance-free 
> vacuum pump.
> 
> There are two types of getter. In receiving tubes and small glass 
> transmitting tubes the getter is the silvery film of barium metal that 
> you can see through the glass. However, barium can only operate at low 
> temperatures - at high temperatures, it would evaporate and become part 
> of the gas problem.
> 
> In transmitting tubes, which have top operate at high temperatures, the 
> getter is some other chemically active metal that is less volatile, but 
> need to be at a high temperature in order to operate at its best. In 
> ceramic-metal tubes, the getter is generally mounted at the top of the 
> cathode pillar, which is about the hottest point inside the tube. In 
> glass-metal tubes like the 3-500Z, the getter is the dull grey zirconium 
> metal on the outside of the anode, and it operates best when the tube is 
> running hot.
> 
> When the tube is hot, there are two competing processes going on. On the 
> one hand, very small amounts of gas are still being evolved. On the 
> other hand, the getter is mopping it up... but that can't happen until 
> those gas atoms have bounced around inside the tube until they actually 
> strike the getter. Not every impact on the getter surface will hit a 
> chemically active site that will react with the gas atom and trap it, so 
> the trapping process takes time.
> 
> Also, the evolution of gas out of a piece of metal is not a steady 
> process. The gas tends to come out in pulses of several atoms at a time. 
> Small pulses are common; larger pulses are rarer; and very large pulses 
> are rarer still.
> 
> If one of these very large pulses of gas reaches the surface and enters 
> the space inside the anode, then as I said, it takes a little time to 
> diffuse around to where the getter can mop it up. In the meantime, there 
> is a temporary higher pressure inside the tube - and it only takes 
> microseconds (or less) for the tube to arc.
> 
> Arcs in high-voltage transmitting tubes are a very well-known 
> phenomenon, almost as old as radio itself. In Eimac's words [1], "An arc 
> is a self-sustained discharge of electricity, between electrodes in a 
> vacuum environment... The arc supports large currents by providing its 
> own mechanism of electron emission..."
> 
> Arcs can happen at any time in the life of the tube, but notably in its 
> early life while gas is till being evolved, and after the tube has been 
> stored for a long time (cold, and therefore with very little getter 
> function). You may never encounter one; but neither should you be 
> surprised if you do.
> 
> When an arc happens, the current through the tube is limited mostly by 
> the external power supply... until some other circuit component stops 
> it. Hopefully this will be a fuse or some other protective device, but 
> unless the current is limited by a "glitch resistor" the surge can do a 
> lot of damage.
> 
> Therefore Eimac recommends that precautions are taken to limit the 
> amount of energy dumped into the tube, and to limit the current to maybe 
> 40 amps [1]. If these precautions are taken, the tube itself may not be 
> damaged.
> 
> If the arc is extinguished quickly and not too much energy is dumped 
> into the tube, the tube can recover completely. There may not be much 
> visible evidence that the arc ever occurred (depending on the tube 
> construction). If the tube was hot, the getter can collect the gas 
> within a few seconds.
> 
> So it's all a matter of time-scales. An arc can happen faster than the 
> getter can handle the gas release - but the getter can do its job faster 
> than anyone can possibly pull the tube out of the amplifier to test it.
> 
> 
> [1] FAULT PROTECTION. Varian, EIMAC Division, Application Bulletin #17,
> January 1987  (see www.ifwtech.co.uk/g3sek/misc/bull17.pdf)
> 
> 
> 
> Having written far more than I'd expected, in order to tell the whole 
> story, I am never going to write all this again. Let's take comments and 
> corrections this time around, and I'll park it on my web site ready for 
> the next time.
> 
>