[Amps] Arc distance and plasma

[Will Matney]

Steve,

Keep in mind that there's a difference in the gas ionized you see as a purple glow and that of an arc. However, these ions go in the opposite direction of electrons in a tube, and strike the cathode which can damage it. They also start to neutralize the space charge which causes anode current to rise until you have a snowball effect, and the tube can eventually burn up from the high current. This can go on until the cathode can't emit any more electrons and the tube is said to saturate. This is where you'd see a white hot anode and the glass actually getting hot enough to melt.

It's according to how much a tube has went soft (gassy) as to where any getter can clean it up. The gas has to be absorbed by the getter material, and the getter has to be so that it will absorb all gasses in the tube. If the glow is still in the tube after it has set and baked for a while, or baked several times, I'd venture to say it never will. What will happen is that the cathode emmision will start to eventually drop off from being damaged from the ions striking it. This can also cause material from the cathode to be deposited on the control grid thereby damaging it that much more.

Best,

Will

*********** REPLY SEPARATOR  ***********

On 7/28/06 at 4:34 PM Steven Cook wrote:

>Tom, et al.,
>
>The 3-500Z's in my TL-922 both exhibit a purple "plasma" above the top of 
>the plate inside the glass envelope.  The intensity seems to change with 
>plate voltage (CW vs. SSB) setting.  Also with frequency.  Does not seem
>to 
>affect tube function -- output power about 1300W with 100W input.
>
>Is this due to gas inside the tube.  Will it eventually getter out?  Does 
>the presence of this gas have a deleterious effect on the tube life 
>expectancy?
>
>Thanks in advance...
>
>Steve (((WG7K)))
>
>----- Original Message ----- 
>From: "Tom W8JI" <w8ji at w8ji.com>
>To: <amps at contesting.com>
>Sent: Friday, July 28, 2006 7:51 AM
>Subject: [Amps] Arc distance and plasma
>
>
>> Rather than claim I can look at a tube and magically tell
>> what happened years ago with my magnifying glass,  I thought
>> I would add a picture of a 3-500Z under RF plasma
>> conditions.
>>
>> The photos clearly show then concentration of plasma during
>> a hard fault.
>>
>> http://www.w8ji.com/fault_protection.htm
>>
>> Enjoy.
>>
>> 73 Tom
>>
>>
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>
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