>From RLM's web page:
The basic technique behind non-destructive voltage testing is: slowly increase
the applied voltage across the DUT until a small current flows through the
device. Record the V and stop the test. If the voltage were allowed to increase
beyond this critical point, the component could be damaged. The applied voltage
usually does not need to remain on the DUT for any longer than it takes to read
the voltmeter, M1, and the current-meter, M2. For testing devices with lower
breakdown voltages, only half of the doubler should be used. One end of the DUT
is connected to Common instead of to one of the HV OUT terminals. Thus, the
voltage readings on M2 must be divided in half. If the breakdown voltage of the
DUT is expected to be less than 1000v, you may want to connect a DMM from the
output side of R8 to Common so that the voltage can be read more accurately. Be
careful. Most DMMs can not safely tolerate more than 1000v without using a
voltage multiplier probe.
Vacuum-capacitors and vacuum-relays should be tested for gas before
installation. I have seen new, unused vacuum-devices that were defective due to
very slow air leaks. Such leaks typically show up several years after
manufacture. In use, a gassy glass-envelope vacuum-relay can often be
identified because the ionized air inside emits a blue light around the open
contacts. However, the ionization in a defective glass-envelope
vacuum-capacitor is usually deep inside the concentric meshed plates and can
not be seen. Variable vacuum-capacitors should be tested with the plates fully
meshed. When a vacuum-capacitor goes bad in an amplifier, reduced PEP in one
result. However, other things can cause the same problem. When troubleshooting
an amplifier, it is a good idea to routinely test all of the vacuum capacitors
and vacuum relays with a BVT.
Vacuum-capacitors that have been in storage for a long time may develop
"whiskers"--microscopic filaments of copper. Sure, it sounds weird. This
anomaly causes the breakdown-voltage to initially be lower than normal. It is
possible to burn-off these whiskers. During the process, the capacitor may self
discharge--as indicated by a "tink" sound. Repeatedly forcing the capacitor to
self discharge will result in a decrease in breakdown voltage. Five tinks is
usually a good point to stop.
Semiconductor testing is important when building HV power supplies that contain
groups of series-connected rectifier diodes. If one or more of the rectifiers
fails during use, it could start a "domino-effect" and short-out the other
rectifiers in that group. Shorted rectifiers deliver AC to the filter
capacitors. This is not a serious problem for non-polarized capacitors.
However, with polarized electrolytic capacitors, even a small episode of
reverse-current can damage the capacitor--or even cause it to explode. Thus,
one defective 10¢ diode can trigger the destruction of many dollars worth of
good parts. It is better to cull-out bad parts prior to construction.
Silicon-rectifiers: Each diode should be tested individually where possible.
Increasing reverse voltage is applied to the DUT until a leakage-current of
approximately 2uA is detected. [high-current diodes can withstand more reverse
current] At this point it is important to observe the current meter. If the
leakage-current randomly fluctuates without adjusting the voltage, the diode
has a manufacturing defect and it should be discarded. Since it is difficult to
mark each diode, I usually sort diodes into labeled drawers according to PIV.
Individual diodes which test greater than about 1.3kV should be viewed with
suspicion because this usually indicates a doping problem. A forward
voltage-drop test at the rated current can be used to discover whether such a
diode has a problem. At 1A, the forward voltage drop in a silicon PN rectifier
junction should be less than 0.9V.
Transistors are now made with voltage capabilities that are similar to
silicon-rectifiers. Some transistors are rated at 1500V. Testing these devices
is similar to testing silicon-rectifiers--except that a resistor of roughly 100
Ohm should be connected from the base to the emitter, or from the gate to the
Air-variable capacitors: Identifying too-closely spaced points that need
realignment is easy with a voltage breakdown-tester.
Gridded power tubes need a good vacuum in order to function properly. A vacuum
test is made with no filament voltage applied. HV is applied between the anode
and a grid. A healthy 3 - 500Z will typically exhibit less than 10uA of
current-leakage at double the rated anode-voltage.
The BVT can also be used to check the alignment of the filament in a 3-500Z.
When its filament is cold, a healthy 3-500Z can withstand 7 - 8 kV between its
grid and filament. If the filament is not concentric with the grid, the
breakdown voltage will be lower. This problem is usually brought about by
intermittent VHF parasitic oscillations--a condition that generates a large
pulse of cathode and grid currents. The principle is simple: a flow of
electrons is always accompanied by a magnetic force. The larger the current,
the stronger the force. During an intermittent VHF parasitic oscillation, the
magnetic force is sometimes strong enough to bow the hot, tungsten filament
wire helices toward the grid. If the [cold] filament to grid withstanding
voltage of a 3 - 500Z is less than 6kV, a grid to filament short may occur when
the tube is hot.
Testing for parasitic damage in 8874s, 8877s, 3CX800A7s and other oxide-cathode
Such tubes have the following things in common: indirectly heated
strontium-oxide/barium-oxide cathode, high gain, ultra high frequency
capability, and gold-plated grid. The oxide-coating is an efficient
electron-emitter. The gold plating helps to reduce primary electron-emission
from the grid. This improves performance. There is a tradeoff. If the gold
evaporates [sputters], the loose gold particles can cause serious problems.
In a vacuum, gold does not begin to evaporate unless it is heated to more than
1000ºC [1832ºF]. Heating the entire mass of the grid to >1000ºC requires more
energy than is available. However, if there were a way to heat the gold
plating, without heating the entire grid, gold evaporation would be possible.
VHF/UHF energy has a substantial "leg up" when it comes to heating metal
plating. VHF/UHF current travels exclusively on the surface. During an
intermittent VHF parasitic-oscillation, the VHF grid-current can become so
large that the surface of the gold plating briefly becomes hot enough to
evaporate gold. The resulting gold vapour cloud can then move about freely
inside the envelope. As the gold cools, it solidifies into tiny balls. In a low
power microscope, they look like dew drops of water on the petal of a flower.
Some of the evaporated gold lands on the emissive coating. Gold poisons the
cathode's electron-emitting ability--causing a reduction in anode-current and
power output. [A copy of a letter from Eimac describing this phenomenon is
available on request to this author.] Evaporated gold can also land on the
inside of the ceramic anode-insulator. This can cause arcs between the anode
and the adjacent (grounded) grid-ring. An arc is most likely during the crest
in the anode-voltage swing when the tube is not conducting--as the HF
tank-circuit/flywheel swings to its positive voltage peak. At this instant, the
peak anode-voltage is normally about double the positive supply voltage. If the
insulating ability of the ceramic has been compromised by the presence of gold,
trouble is probable. When the anode arcs to the grounded grid, the HV-positive
circuit is virtually grounded. Thus, the HV-negative circuit tries to rise
above ground to the voltage in the HV filter capacitors. This typically causes
damage to components in the HV negative circuit. A common problem is an arc
between the cathode and the heater or an arc between the cathode and the grid.
Test for compromised voltage withstanding ability between the heater and the
cathode--and a burned out heater.
The Loose Gold Test (W6IHA): There is a simple test that can confirm the
existence of loose gold particles without sawing open the suspect
amplifier-tube. The only piece of equipment needed is a BVT. The principle
behind the test: Like charges repel and unlike charges attract.
Procedure: Remove the amplifier-tube from the amplifier. The positive and
negative DC-voltages that are applied between the anode and the grid should be
two to three times the operational anode-voltage.
Loose gold particles can be moved around by changing the polarity of the
anode-voltage. If the anode is positive, the gold particles are attracted
toward the anode-insulator. This causes the indicated leakage current to
increase. If the anode is made negative, the gold particles are repelled and
the leakage current will decrease. If the leakage current is equal with either
polarity, the presence of gas is indicated.
Another method of confirming the presence of loose gold particles is: apply
positive anode-voltage and record the leakage current. Shut down the BVT and,
with the tube vertical and the anode-cooler up, repeatedly bang the
anode-cooler, both vertically and horizontally with a approximately 2oz. soft
face hammer. This will cause some of the loose gold particles to fall to the
bottom where they will be less attracted by the positive voltage at the anode.
Keep the tube vertical. Re-apply positive anode-voltage. If the leakage current
decreases, you have loose gold and you are making progress. If the leakage
current does not decrease, either you need to bang harder or no more
improvement is possible. This procedure has been used by some amplifier owners
to get more operating hours out of gold sputtered tubes. Banging also causes
the gold to fall off the cathode coating. This increases electron-emission.
However, if the tube is turned upside down, the loose gold becomes
redistributed and the banging process must be repeated to move the errant gold
to a safer place.
----- Original Message -----
From: Gary K9GS<mailto:firstname.lastname@example.org>
To: AMPS Mailing List<mailto:email@example.com>
Sent: Sunday, February 12, 2012 6:09 PM
Subject: [Amps] Hi Pot Test Procedure?
What is the proper procedure for Hi-Pot testing a tetrode, in this case
Apply potential between anode and any other element? I did see one
mention of applying potential between the anode and the nearest grid but
is this correct?
Check out K9NS on the web: http://www.k9ns.com<http://www.k9ns.com/>
Greater Milwaukee DX Association: http://www.gmdxa.org<http://www.gmdxa.org/>
Society of Midwest Contesters: http://www.w9smc.com<http://www.w9smc.com/>
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