[Amps] setting the grid adrift

[Will Matney]

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 the 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 would 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 at contesting.com [mailto:amps-bounces at 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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