[AMPS] hi pot warning

[km1h@juno.com]

On Fri, 8 May 1998 13:13:14 -0600 John Lyles <jtml@lanl.gov> writes:
>Hi,
>
>Not to enter the parasitic discussion, I just wanted to pass on the 
>advice
>I got from both Econco and CPI/Eimac over the years. When high potting 
>the
>grid/filament of big tubes, especially those in hundred KW ratings, be 
>sure
>to use pure DC or keep the time on the high pot to a minimum, few 
>seconds
>for example. A lot of commercial Hi-potential machines have AC ripple 
>at
>the mains frequency, at least the older big units did. Modern high 
>freq and
>switching power supplies are exempt from this warning. 

John..what would you think about the typical "ham" level hi-pot setup? 
It would be a bug zap xfmr with typically a 0.2 MF/10KV filter cap. I
have built one here that will Variac up to about 12.5KVDC but I am
getting the impression that it will not do anything useful for tube gas
testing. It certainly works fine for testing air and vacuum variable
caps.


Low frequency
>resonances can be excited in the structures, causing them to vibrate 
>and
>possibly cause a short. I don't know where the bottom end of this 
>warning
>falls, such as 10kW, 20, etc. A mechanical engineer with knowledge of 
>the
>tube structure would know. I know that the tubes for modulators in big
>transmitters had certain frequencies that they could sing at, if 
>excited
>mechanically.
>
>In the Philips paper from 60 years ago, that I posted having found a 
>month
>ago, there was reference to the proper amount of energy limiting R in 
>the
>plate circuit, to prevent filament bending due to the forces from the
>occassional arc from "Rocky Point" effect in big tubes. While I have 
>heard
>this debated constantly for weeks here, the analysis was published a 
>long
>time ago by experts in one of the major tube companies.

I remember that posting John...and it should have stopped any further
speculation. Perhaps you would repost that for all to see...again.

73  Carl  KM1H



 I tried to do 
>an
>OCR (optical character recognition) on the TIFF images that I scanned, 
>but
>the program (Omnipage) said no, I had not saved it with 300 dpi 
>resolution.
>So, I am unable to post the text here, but read it several times now.
>
>John
>K5PRO
>
>
>
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