[Amps] Correct Method Of Hi Potting An Electron Tube (3CX-6000A7 / YU-148)

[John Lyles]

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 at telus.net" <jim.thom at telus.net>
> To: amps at 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).
>