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Re: [Amps]Grid fuses (was: ÂLife and gain of 3-500Z)

To: Chris Howard <chris@yipyap.com>, amps@contesting.com
Subject: Re: [Amps]Grid fuses (was: ÂLife and gain of 3-500Z)
From: Peter Chadwick <g3rzp@g3rzp.wanadoo.co.uk>
Reply-to: g3rzp@g3rzp.wanadoo.co.uk
Date: Fri, 21 Jul 2006 19:11:18 +0200 (CEST)
List-post: <mailto:amps@contesting.com>
We have a number of possible scenarios:
1. We get, for some reason, an arc from plate to grid. The resistor/fuse/what 
have you opens. Because once the arc is struck, it's a low impedance with 
around 50 volts across it, the grid starts to hare off towards the plate 
potential. The arc doesn't immediately extinguish because it's got the grid 
plate capacity to charge. This gives the chance for either the grid - cathode 
to arc, or the grid to start pulling lots of current from the cathode because 
it's trying to get to the B+ line, or both. The arc may well extinguish once 
the cathode reaches saturation, since that will try to clamp the grid at some 
lower voltage than the B+, and the grid plate spacing may then be enough that 
the arc can't maintain itself. Either way, we may well have a bad tube as well 
as a fuse or resistor to change.
2.We get, for some reason, an arc from plate to grid. The resistor/fuse/what 
have you opens. The arc extinguished, teh grid floats. Internal plate grid 
leakge starts pulling the grid positive. Depending on the extent, and the drive 
conditions prior to the arc, we can have a variation of scenario 3, where the 
grid is pulled positive by leakage as well as the secondary emission.

3. We overdrive the grid, and it gets hot, and the resistor/fuse/what have you 
opens. The grid is now floating, is hot and emitting electrons, so it goes 
positive atracting more electrons from the cathode, which hit it with kinetic 
energy so it gets hotter so it emits more so it gets more positive so it 
attracts more electrons...........and  so on until we have a run away plate 
current situation.
Depending on such factors as the actual voltages, the grid current and 
temperature, it may not happen with some tubes and may with others.
What happens if the cathode is cold?  Two capacitors in series across a direct 
voltage distribute the volts across them in inverse relation to teh capacity - 
the smaller the cap, the higher the volts. So depending on the internal 
leakage, the grid under those conditions will float to some voltage, which 
could lead to an exacerbations of scenarios 2 or 3 above.
With small receiving tubes, you rarely run much grid current or get arcs.

73
Peter G3RZP
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