Peter,
Yes there will be a difference like that from grid to cathode from being
driven. However, in the case of an arc, we're talking about the power supply
only involved in the arc (think DC only, you have to). In this case, if we had
a grid to anode arc, current would flow from B- through ground (chassis), into
the grid, to the anode, and back to B+. With the respect to B+ and chassis
ground, ground should be 0 Vdc. In other words, the grid would never go over 0
Vdc when an arc occurs. That's what does the damage here. One has to remember
that there's more things going on in the tube when we talk about an arc like
this. This would hold the same for a cathode to anode arc since the cathode is
tied to B- or chassis ground. If a grid is unconnected from ground to where it
floats, it can't become positve either since there's no connection to the
cathode, nor to the negative rail (chassis). The only thing that can happen
here, according to Terman, is that the space charge could cause t
he grid to become slightly negative or to where equilibrium is achieved
between it and the cathode.
Now when it mentions a gassy tube and the ions flying around, the potential of
the grid to the cathode can be changed according to Terman. With respect to
chassis ground though, it can't go above 0 Vdc. What this does is an increase
in the grid current, and it can cause a multiplying effect to where it keeps
building up (see my post quoting Terman about gassy tubes). As this happens of
course, more grid current is produced. Then, if this grid current was to heat
the grid hot enough, we're talking about yellow to white hot in reality, it
could emit some electrons. However, in this case, the electrons that's emitted
are replaced by the electrons bombarding the grid until an equalibrium is
achieved.
The thing is, none of this has anything to do with an arc really. An arc is
formed when the tube is gassy enough that a current forms (arc) from the
ionization of gas say between the grid and the anode. This is in reality a
short in the B+ supply where B- is flowing through the grid back to B+ through
the anode. Because the grid, from the example above, could get hot enough from
some sort of high current flowing through it to where it starts to emitt some
electrons, that's not going to start an arc I don't believe. The metal the grid
is made from, and its plating are very poor emitters of electrons. They were
selected because of this very purpose. The time it would also take for all of
this to happen is not instantaneous, it takes some time to happen. An amp would
most likely be shut off before anything very bad would happen to it. I really
think this may be the case with the guys who have tried the fuses and say they
have worked for them, I'm not sure. I do think a fuse woul
d help the B+ supplies components during and after an arc. The tube from a
major arc, in a really gassy tube, where a seal was breached, and has lost its
evacuation, is bad anyhow and needs to be pitched. The small flash-overs in
mention are small compared to a major arc in a real gassy tube. They're from a
very small amount of gas liberated from the anode or other elemets when heated.
According again to Terman, the gas is generally dealt quickly with, in the
small arc, and it will work fine afterwards. Were talking about little arcs
similar to (maybe a little larger) what a megger or Hipot may cause if ran way
above the specified anode voltage. These hardly make a sound. As Rich says,
they make a "Tink" if I remember. That's what I've heard also as there's not
enough gas in the tube to carry on like one that's lost its evacuation. If it's
a big loud crack, something else is up. That's my opinion on the subject, and
what I've been trying to get across.
Best,
Will
*********** REPLY SEPARATOR ***********
On 7/26/06 at 1:06 AM Peter Voelpel wrote:
>Will,
>
>Again, electrode voltages of tubes are ALWAYS referenced to the cathode of
>a
>tube, not to ground.
>
>I hope you agree that a 3-500Z draws about 100mA grid-current under normal
>conditions.
>If you look at the constant current curves of that tube you will see that
>to
>reach 100mA grid current at an anode potential of 3KV the grid must be at
>about +40V which are between grid and cathode.
>Since the grid is tied to the chassis, you will measure from the chassis to
>the cathode -40V.
>So even if tied to the chassis and the chassis connected to your station
>ground, the grid goes positive to +40V at 100mA grid current
>
>On the Eimac data sheet you will see that as "filament to grid voltage
>-(V)"
>
>73
>Peter
>
>
>-----Original Message-----
>From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
>Behalf Of Will Matney
>
>However, I do not agree that the grid can ever be made positive with a
>potential above 0 Vdc. Also, who's everyone? I've only seen two people
>saying a grid could be positive.
>
>
>
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