[AMPS] HV fuses

[Rich Measures]

>Rich Measures said:
>
>>A correctly sized "glitch" resistor does not need to be replaced after
>an 
>>"event".  A HV fuse may not prevent gold-sputtering damage to the grid
>of 
>>the tube from VHF parasites.  {see photos in September 1990, QST}
>
>>Limiting peak current discharge current is quite probably more
>important, 
>>and a fuse  isn't fast enuff. 
>
>Yes I think the fuse cannot be fast enough.  However, I am begining to
>believe that the amount of damage sustained by an internal flashover
>(i.e. gold sputtering) can be lessened by the addition of a HV fuse.

If the fuse were helpful there would be no gold sputtering.  

>Just for clarification, my reference to the word "flashover" is an
>electric arc
>thru free space from the high voltage DC line to a lower potential bus.

(to the grid collet, which is grounded)

>
>Internal, external, or whatever,  due to gas, 

If the tube is gassy -  trash-time.  

> parasitics, foreign
>particles or any other reason.  I think repeated flashovers cause an
>accumulated damage that can eventually kill the tube.  

A cathode-grid short caused by a cathode flake is fatal.  However, a 
moderate amount of loose gold melt-balls can be safely settled into the 
base of the tube by tapping in the vertical plane. 

>The lesser the amount of energy passed by each event, the better the going 
gets in the
>long term.  
>
True, Alex.  Reducing VHF amplification is also useful.  


Rich...

R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures  


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