John,
I think what he was getting at is the speacing between the two elements. That
determines the breakdown potential where in this case, a tube would need to
have more gas in it to arc at a place where the spacing was greater. I read
through sevral text books (RCA power tube book, Terman, and Bill Orrs handbook)
on the subject yesterday evening, and one (I think Bill Orr) was talking about
the difference in the two arcs I spoke of. In a tube that's slightly gassy from
the elements liberating gas when heated, a tube will not arc when the plate
voltage is first applied. It waits until some time thereafter. A gassy tube
from a seal leaking, that has lost its evacuation will cause an arc as soon as
the plate voltage is applied. I've seen this, especially in receiving tubes.
They'll light up like a Christmas tree where pockets of gas will suddenly
ionize all over the place. However, arcs in a tube which has a small amount of
gas will not do this, nor can you hear the arc because of th
e vacuum. There's not enough gass to carry the sound waves. Rich mentioned
testing vacuum caps one day with a hipot. He said the arc sounded like a
"tink". I'd venture to say that this tink was a low volume noise too. From this
though, different volumes of gas will cause arcing at different gaps. The lower
the amount of gas, the closer the gap would have to be to arc. However, sharp
corners do increase the chances of an arc. The cone at the bottom of a 3-500Z
though is a good bit distant away as compared to the gap at the top between the
anode and the grid cage. The grid itself is closer to the anode internally if I
recall. Also, the design of the cone is made, I think to where arcing would be
a minimum. The lip that's bent down or the skirt has a radius which helps
prevent arcing. If the arc hit anywhere, I'd think it would be where the hole
is that the leads come through to the elements.
Best,
Will
*********** REPLY SEPARATOR ***********
On 7/27/06 at 11:16 AM John T. M. Lyles wrote:
>The anode at the top may be a flat surface, at the 'dead head'
>region, like the inside of a metal can. At the bottom of the tube,
>there are edges of the anode and the fil/grid connections may have
>other attachments and sharp edges (like a getter assembly on
>filament). I see no reason to question why it wouldn't arc here just
>as easily as up at the top where the anode/grid may have smoother
>contours.
>
>73
>john K5PRO
>
>
>>Message: 8
>>Date: Wed, 26 Jul 2006 14:08:44 -0700
>>From: R L Measures <r@somis.org>
>>Subject: Re: [Amps] flash arcs
>>To: "Tom W8JI" <w8ji@w8ji.com>
>>Cc: amps@contesting.com
>>Message-ID: <499C6EB0-B570-43FE-88F2-745E616FBCDA@somis.org>
>>Content-Type: text/plain; charset=US-ASCII; delsp=yes; format=flowed
>>
>>
>>On Jul 26, 2006, at 4:12 AM, Tom W8JI wrote:
>>
>>>
>>>> According to Gossling's paper in the Journal of the IEE
>>>> (which means it was certainly peer reviewed!), flash arcs
>>>> is slightly affected by residual gas: the 'clean up'
>>>> effect of the arc can lower the flashing voltage. He
>>>> recommends series resistance in the anode supply (now
>>>> where have we seen that recommended!) but says that it
>>>> isn't generally pracitcable with valves over 10kW. He
>>>> comments that the 'most general visible after effect is
>>>> widespread tree like markings on the grid and filament
>>>> supports. The fine wires in the active part of the grid
>>>> are, however, never damaged.'
>>>
>>> That's exactly the type of damage I have seen. As I posted
>>> earlier, 3-500z's generally mark the grid support cone at
>>> the bottom with very fine lines.
>>
>>Since the top of the grid cone is closer to the anode than the bottom
>>is, why wouldn't the arc take place at the shorter distance?
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