>
>----- Original Message -----
>From: "John" <jlockj@earthlink.net>
>To: <K1LE@arrl.net>
>Sent: Saturday, December 21, 2002 9:18 PM
>Subject: 3CX800A's
>
>
>> Hi Jeff,
>>
>> re: snip
>>
>> > Now most ham amplifiers are single phase and the current from the power
>> > supply will have
>> > zero crossings and extingish the arcs.
>>
>> snip
>>
>> Zero crossings...hmm...sounds like not enough filter C ! ;-)
>>
>> Jeff - K1LE - CT ><>
>>
>> snip
>>
>>
>> When you have a short circuit, it's tough to have enough C to keep the
>> voltage steady!
>>
>
>Hi John,
>
>I was being slightly facetious as "zero crossing" is something that is
>relied upon to interrupt arcing across the contacts of an AC circuit
>breaker. This is why the DC interrupting rating of a given breaker or switch
>is much less than the AC rating.
>
>As I see it, the short circuit current should be limited by the glitch R
Amen. Circuit-breakers are not nearly quick enough.
>
>(along with other R's and Z's in the fault circuit) and the fault duration
>would be such that the filter C would not be discharged to the zero line.
One problem is that, during an "oscillation condition" (Eimac/WB Foote)
the top layer of gold can be evaporated off of the gold-plated grid
before the filter cap discharges to 0V. By limiting peak fault current
with a glitch R, damage can be avoided. If this is not done, the energy
available may be sufficient to release a cloud of gold (Au) vapour into
the vacuum which quickly condenses (@1,063.0°C) into Au meltballs. There
are about the diameter of talcum powder. Loose gold meltballs inside a
vacuum is not good news since Au is a good conductor.
- Au meltballs can be seen at:
http://www.vcnet.com/measures/8877.gs2.JPEG
- however, there are two different particles laying next to tube in the
photo. The large particles are Cu shavings from the saw that was used to
cut open the 8877. The barely visible particles are gold meltballs. A
30x - 50x microscope is needed to view Au meltballs.
>
>However, I do understand that a low Z long duration fault could allow the
>zero point to be seen on the B+.
>
>Maybe someone has observed B+ voltage during a tube flash-over, and can
>comment and / or correct my thinking.
Such would likely be too brief to observe.
- After observing the insides of numerous autopsied 3-500Zs, 8874s,
3cx800A7s and 8877s, I have yet to see a (Rocky Point theory) "barnacle"
or an arc-mark -- even in tubes that had leaky seals/bad vacuums.
- My guess is that when a HV+ arc occurs through Au meltballs in a 8874,
3cx800A7 or 8877 , the Au remelts and recondenses into mint-condition
meltballs -- so obvious damage is not to be seen.
>
cheers, Jeff
- Rich..., 805.386.3734, www.vcnet.com/measures.
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