I contacted a fellow who has first-hand experience in
high power hi-potting but who is not on this list. I sent
him the letter inquiring about - basically - "how to."
He is an engineer at Continental Electronics, a high-power
transmitter manufacturer, and does installations, among
other tasks.
Here is his reply to me:
===========================================
The small tubes would have to be treated carefully, since the grid wires
are quite small. In general, I would recommend the following:
Start with a simple continuity test through the filament to check for an
open filament.
Next, check for shorts between the filament and the control grid, and
from the control grid to the screen.
Now, perform the hi-potting with the following general recommendations:
1. First, add a 1 megohm resistor in series with the Hi-pot output to
limit the fault current.
2. Connect all leads together except for the lead that you are
hi-potting.
3. For the anode, hi-pot to no more than twice the recommended anode
voltage. This takes care of voltage swing on the anode during
modulation.
4. For the grids, hi-pot to no more than the maximum recommended
voltage according to the tube data sheet.
5. If you have a DC hi-pot, apply the voltag polarity so that the
internal circuit is "reverse biased". This means that for the anode,
apply negative voltage to the anode. For the control grid, apply
negative voltage to the grid and positive to the cathode and to the
screen grid.
6. Leakage should be down in the very low micro-amperes if at all, ie
zero to perhaps 20uA as a guess. Anything in the 100's of uA would
probably be way out of line.
In the larger tubes and vacuum capacitors, a tube or capacitor which is
showing high leakage can sometimes be cleaned up by bringing the hi-pot
up to a voltage where the leakage starts to increase, and holding it at
that voltage. This can have two effects. The first is, via corona, to
polish the internal electrodes and remove any sharp points from which
corona leakage was occurring. There may also be some improvement in the
internal vacuum due to gas molecules being accelerated into and embeded
in the metal by the high voltage.
With some tubes and capacitors, I've seen dramatic improvement this way.
I've taken a vacuum capacitor which was rated for 40kV, but showed high
leakage and some snaps at about 10kV. By appling the hi-pot and
gradually bringing the voltage up over an hour or so, I was able to
bring the capacitor all the way up to the rated 40kV rating. Anytime a
device has been out of service for an extended period of time, this is
recommended. If you just apply the high voltage to the device, it may
be damaged by the high current avalilable from the transmitter supply.
Hope this helps,
Best regards,
Paul Lynas
=============================================
I don't know what Paul meant in the opening statement about
"small tubes" - might mean those with plate dissipation ratings
of less than 100 kw :-)
- JimT
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