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Author Topic: 572B Flashover Mechanism  (Read 4915 times)
KG6Q
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Posts: 71




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« on: March 30, 2011, 04:29:23 PM »

Soon after installing a pair of new 572Bs in a SB-200 amplifier, a loud arc occurred. The only damage evident is a destroyed 200pf grid bypass capacitor (C14 or C15).

I think this was caused by a 572B internal "flashover". Could someone please explain the actual meaning and mechanism of a tube "flashover". Could such a "flashover in a 572B result in HV anode voltage being applied to the 572's grid?
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K7KBN
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Posts: 3490




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« Reply #1 on: March 30, 2011, 06:44:20 PM »

A "flashover" will occur when the potential difference (voltage) between any two points exceeds a critical value.  Lightning is a somewhat extreme example, but applying plate voltage to the grid of a tube, with all the other voltages being somewhere near normal, could definitely produce a small lightning bolt.
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73
Pat K7KBN
CWO4 USNR Ret.
W8JX
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Posts: 12212




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« Reply #2 on: March 30, 2011, 06:47:43 PM »

Tube was likely gassy and when B plus was applied it arced over to nearest ground, the grid, and blew cap. If that is only damage (other than tube) you are lucky.
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--------------------------------------
Ham since 1969....  Old School 20wpm REAL Extra Class..
W9PMZ
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Posts: 629


WWW

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« Reply #3 on: March 31, 2011, 02:21:48 AM »

W8JI has a modification for this problem on his web site.  Although this is for an Ameritorn AL-572, the same principal applies.

The flash over can occur during tuning when the peak voltage could arc and does not necissarly imply a gassy tube.  There is a lot of information on this subject at W8JIs web site.

73,

Carl - W9PMZ
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KH6AQ
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Posts: 7718




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« Reply #4 on: April 01, 2011, 08:22:40 AM »

Yes the tube had a plate-to-grid arc. It will most likely do it again. This is a characteristic of the current crop of Chinese 572B tubes.
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K6AER
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Posts: 4753




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« Reply #5 on: April 01, 2011, 03:17:14 PM »

“Yes the tube had a plate-to-grid arc. It will most likely do it again. This is a characteristic of the current crop of Chinese 572B tubes.”

Not necessarily, once the oxygen molecules have been vaporized you might have a very good vacuum. Many new tubes will have an initial arc and then live 30 years with no problem.
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KM3F
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Posts: 795




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« Reply #6 on: April 01, 2011, 10:53:03 PM »

Oxygen molecules!  Better not be any ox in a vacuum. Only if the glass has cracked will Ox migrate into the vacuum.
The tube is heated to drive out nearly all gasses during the exhausting process.
A getter material is included in the structure to absorb any remaining gas left after the process.
If there is a close spacing (poor design or assembly) in the structure and the plate voltage is high enough it can jump the gap to the nearest lower potential element.
If there is enough gas left, it can ionize to provide a low impedence path to all elements in the structure looking for a compete path back to the other side of the power supply.
If a cap is in  the path and has not enough voltage rating, it to will distruct providing a path.
Granted if the mfger process is defective any of these things can happen.
Been in the biz.
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K6AER
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Posts: 4753




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« Reply #7 on: April 02, 2011, 04:17:12 PM »

“Only if the glass has cracked will Ox migrate into the vacuum.”

Even with a perfect glass tube that has been manufactured  and gettered properly over time, with no use the tube can, with storage time, get oxygen molecules past the pin seals and the tube will collect oxygen molecules inside the envelope. Sometimes it can take as little as a couple of years and sometimes it may take ten years. But all large glass tubes will leak over time.

All of you who have spare glass transmitting tubes should rotate them every year.
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