[Amps] setting the grid adrift

[Chris Howard]

On Tue, 2006-07-25 at 10:26, Tom W8JI wrote:

> > Here is the conclusion I'm coming too:
> >
> > Setting the grid adrift, if done in a system where there 
> > are other
> > controls on plate overcurrent, doesn't mean certain doom.
> 
> Correct

Yea! good for me.

> 
> > And there are some conditions where it might make sense, 
> > it seems to
> > have a higher sensitivity to certain kinds of faults that 
> > otherwise
> > require more sophisticated detection methods 
> > (speculation).
> 
> No, you have been caught up in a big deception. Someone has 
> been trying to hard sell the idea that all protection 
> circuits are bad because they run 50A through transistors. 
> If you search back through the threads it turn out that 
> claim, having been made for many  years, is entirely false. 
> The person making that claim never even correctly read a 
> schematic.

Have no fear!  I didn't read "all protection circuits are bad",
just some curmudgeonly skepticism.

> 
> Fuses and especially resistors are significantly less 
> reliable than electronic overload circuits for soft faults, 
> and hard faults should be handeled in the anode.

Well... the other issue of course is _COST_.  If a reliable
system is obtainable without investing in a sophisticated circuit
that's a good reason to stick with the simpler method.

As for  "hard faults should be handled in the anode",
I parse that statement as being a design philosophy. 

> 
> > And to me there is some engineering beauty to the idea of 
> > cutting off
> > the element that is suffering... everything everyone has 
> > quoted so far
> > seems to agree that loosing the grid will stop grid 
> > overcurrent.
> > But you then no longer have a working system so you might 
> > as well
> > shutdown HV and do a complete reset.
> 
> No. The point you miss is if there is a hard fault, cutting 
> the grid loose almost ceratinly does nothing. It simply 
> transfers the problem to the cathode system. The only way to 
> stop anode flashover is to remove anode voltage. It is 
> dangerous to the tube and other components when a grid under 
> hard fault is floated, and it is also very likely the grid 
> is well enough insulated to handle anode voltage without 
> breaking down.


(by Hard Fault do you mean arc?  I think so.)
Well here we get to a minor point of contention.  

The textbooks we've been examining, with your generous help,
don't cover what happens when a grid under arc is floated.
So... we don't have good data on what happens, except for
the experience of folks who say things work out ok.

If it were dangerous to the tube..  I would think the experience
stories would be different than what has been reported here.

Or, maybe the position of the pro-fused-grid bunch would be
that arcing is less likely with a fused grid altogether...
I'm a little shakey on this point.