It is best to contact the tube manufacturer for their recommended hi pot
levels as well. For instance, on very large tubes, the hipotting of G1
to K may have a time limit, like a few minutes. For the anode to G1 (or
G2 for a tetrode) it is best to not run more than 5 minutes. There is
always some risk, and if the high potter had any ripple it can cause
vibration of the elements. Flashover on the ceramic is not good, so
always clean off the tube ceramic first, using alcohol wipe. Keep sharp
objects like clip leads away from the ceramic so as not to cause
ionization and flash over. And most importantly, be aware of the hidden
Xray hazard. Whenever testing over around 15 kV and higher, there is a
good chance you are producing xrays, even without filament being lit.
The leakage current that is being drawn x voltage applied gives you an
estimate of how much cold electron emission power there is. Reversing
the polarity may show a completely different current as the electrons
are now emitted from another (sharp) edge. It is smart to give extra
distance between you and the tube when you go above 20 kV, as distance
is one of the ways to reduce exposure.
I hi pot ever new tube we buy, but these are tubes over a hundred
thousand dollars a piece so it is important before socketing them.
73
John
K5PRO
Message: 3
Date: Tue, 11 Jul 2023 08:24:48 -0700
From: "jim.thom jim.thom@telus.net" <jim.thom@telus.net>
To: amps@contesting.com
Subject: [Amps] Correct Method Of Hi Potting An Electron Tube
(3CX-6000A7 / YU-148)
On the 3CX-6000A7 triode, the tube is 1st hi-pot tested between the
anode and grid (and a temp short between grid and cathode). Then the
polarity is reversed. Leakage should be < 5 ua in both cases. On a
3CX-6000A7, a good tube will hi pot test to 25 kv. (On a 3CX-3000A7, it
should hi-pot test to 20 kv. A good 3-500Z should hi-pot test to > 12
kv)
Then the tube is hi-pot tested between the grid and cathode (and a temp
short between the grid and anode). Then the polarity is reversed. A
good 3CX-6000A7 should hi-pot test to 5 kv (same 5 kv with the smaller
3CX-3000A7).The hi-pot tester has to be a DC type...and also incorporate
current limiting. It's an invaluable tool. It's also used to test vac
caps, (fixed and variable, glass and ceramic), vac relays, connectors, disc
caps, doorknob caps, and coax connectors, and also cables...and anything
else you can think of. It just saves a lot of grief..... knowing that the
parts you installed meet spec.
(I use the small, compact hi-pot tester sold by Tom Rauch, W8JI, through
his business, CTR engineering. It's a 0-15 kv DC unit, and incorporates
dual current limiting. It operates on anything from 12-18 vdc. That
comes in handy for a hamfest, etc. The unit also has a red led on it to
depict leakage current. I use a Fluke 87 DVM, in series with the return
leg, for a more accurate reading of leakage current. Both methods work).
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