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

[R L Measures]

On Jul 21, 2006, at 3:20 AM, Peter Chadwick wrote:

>>> If the tube is not gassy, and the grid is floating, the
>>> electron
>>> stream from the cathode probably raises the grid-cathode
>>> (fil-CT)
>>> potential to a few volts negative
>
> So if it doesn't matter, why do tube manufacturers specify a  
> maximum grid - cathode resistance for a lot of tubes?

To keep the grid potential from wandering and degrading performance,  
not to prevent a catastrophe.

- note - If there's someone here who is presently working on a 3-500Z  
amplifier, please float the grid and report back what happens.

> Even low power audio tubes used in Class A?
> My experience is that exceeding the the maximum grid-cathode  
> resistance can (but not always does) lead to a plate current  
> runaway, even in tubes with no measurable gas (as measured by grid  
> current)

Why doesn't the effect of arriving electrons take the grid in the  
negative direction?

> 73
> Peter G3RZP
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R L MEASURES, AG6K. 805-386-3734
r at somis.org