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Re: [Amps] Hi Pot Test Procedure?

To: <>, "AMPS Mailing List" <>
Subject: Re: [Amps] Hi Pot Test Procedure?
From: "B Osburn" <>
Date: Sun, 12 Feb 2012 19:24:43 -0700
List-post: <">>
>From RLM's web page:
Testing Components
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 
type tubes:
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<> 
  To: AMPS Mailing List<> 
  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 
  a GU74B?

  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?



  Gary K9GS

  Check out K9NS on the web:<>
  Greater Milwaukee DX Association:<>
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