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On 7/23/06 at 1:04 PM Peter Chadwick wrote:
>Tom said:
>>Not quite a peer reviewed engineering text is it Will?<
>He does tell you not to leave grids open, though.
Correct Peter. Leaving the grid open would cause the tube to act as a diode. It
would be un-controlled and most likely heat up to the point of ruin.
>When you have grid current, you have grid dissipation. Which is why tubes
>designed for Class C had carbon coatings or whatever to reduce emission.
>See the RCA TT4 Tube Manual. It's fair to say that flash over dumping a
>lot of current into the grid could well heat it enough to get emission.
>Whether 'arc' is the right term is arguable, in that 'arc' implies a
>plasma effect, which you don't get in a vacuum. However, 'flashovers' in
>high power tubes with a good vacuum have been written up many times over
>the years, with a number of explanations. In tube manufacturing circles,
>they were said to be 'barnacles', and could be removed by controlled
>energy flashovers - there was an article in about 1934 in the IEE journal.
>As I recall, flashovers are more prevalent with new tubes.
The thing is, flash-overs won't cause as much heat as a continuous arc would.
Here's something to think about on a quick arc. If you've ever discharged
capacitors to ground with a screwdriver, you'll note the terminal or lead was
still cold after the arc. Doing the service I've done at let's say some lower
voltages (around 900-1200 Vdc), it was common prectice to discharge a bank of
capacitors this way. I have seen where this had caused an arc to the capacitor
lead where it actually caused a small burn on the wire, but no heat. Inside the
tube, the grid would be hotter, at the operating temperature of the tube, but a
flashover in my opinion wouldn't cause the grid to get any hotter than it
already is. The flash-over just happens to quick for any amount of heat to
cause a problem. Even in welding (which I've done a lot of) we use what's known
as tack welds to hold the parts in place for welding. A tack is a quick arc
between the parts which allows a small amount of filler meta
l to bridge the joint. In most cases, after a tack has been placed, one can
handle the part by your bare hands (according to the size of the tack). That
would be about equal to if not more than a tube arc with plasma. The heat
quickly dissapates into the rest of the part from the point where it
originated, thus staying cool.
>Sorry, I just don't buy grid fuses ( and even more so, resistors as fuses)
>as a good idea. I believe I'm not the only one, though.
Peter, I'm totally ok with that. To each his own I say. I've never used a grid
fuse either! My problem is with some on the theory which I know about, have
read about, and have been taught about to be different.
>73
>Peter G3RZP
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Best,
Will
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