[AMPS] hi pot warning

[John Lyles]

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. 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 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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