[AMPS] Re: response on tube

[2]

>I was referring to approx. 20 kV plate voltage. 

//  Thanks.  We are talking x-rays.  

> With 1/2CV^2, the 
>stored energy is large here. Hence the need for good crowbars. They 
>protect 30 gauge wire fine when the are working.
>
>As for arcing inside tubes however, there is no fundamental science 
>that says smaller tubes with closer spacing and lower voltages won't 
>arc and do damage just as easily. Gold balls, parasitics, gas, etc. 
>are all possibilities, 

//  Gold sputtering is not a possibility.  It is a reality.  When an 
R-supp. self-destructs during 75m operation, my guess is that VHF energy 
was the cause.

>but I won't toss out everything to speculate 
>its only one thing for all small tubes. 

//  Who said that failures were due to one thing for all small tubes?

>They can outgas if gettering 
>is not fully activated, such as when turning on an old tube without 
>adequate warmup time. 15-30 minutes is typical for broadcast triodes 
>and tetrodes when a new one or old spare is reenergized first time. 
>Pushing a tube in plate dissipation may also cause arcing, as 
>localized spots are getting hotter than before and outgassing the 
>materials. We had a tremendous problem with a particular warped grid 
>4CW250,000B 7 years ago, as electron beam was focusing on the anode 
>and dislodging and eating into the copper, where the free ions then 
>hit the filament. Once something like this occurs, it is a runaway 
>condition, the tube never gets better, but only worsens in 
>temperment, and number of faults/arcs.

//  I have never seen a case of disappearing gas in a ham type.  How do 
we know that 3­500Zs have a getter?

>Higher gain bandwidth tubes do have an advantage in that they have 
>much lower L in the leadout to the base or cap, and also many modern 
>tubes are well aligned (attention to electron optics), not like the 
>hardware cloth grids of the 1920 and 1930 bottles. 

//  The 8877 has electron optics and low L grid lead-out - yet, about 
half of the corpses that I've autopsied have a plethora of gold particles 
inside.  

>The screen grid 
>for tetrodes today may actually shield the control grid from the 
>anode. I have a 1kW GL851 at home, which is very long, and has 
>absolutely no provision for reduced L or lower Cpg. It was rated by 
>RCA for a max of a few MHz.
>
//  I have never observed VHF regeneration with an 813.  

>Improvements have made tubes easier to use, and pushed the Fmax and 
>the self neutralization freq up. But the final responsibility for a 
>stable circuit rests on the designer of the amplifier. I may be 
>preaching to the choir here.

//  There is still an 8877 amplifier manufacturer who uses no parasitic 
suppressor.
>
cheers, John

-  R. L. Measures, 805.386.3734,AG6K, www.vcnet.com/measures.  
end


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