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

[Bill Turner]

ORIGINAL MESSAGE:

On Fri, 21 Jul 2006 12:20:37 +0200 (CEST), you wrote:


>So if it doesn't matter, why do tube manufacturers specify a maximum grid - cathode resistance for a lot of tubes? 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)
>73
>Peter G3RZP

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I suspect they specify a maximum grid-cathode resistance to compensate
for variations between tubes, especially after many hours of operation
when one may become slightly gassy (but still useable) and another one
is not. 

In the case of small receiving type tubes, I can attest that one in
good condition will operate quite well with no c-g resistor at all,
depending on the accumulated electrons on the grid for bias. 

I am not recommending such a method of bias, only noting that it can
work, sometimes. It is not good design to omit the c-g resistor but it
can provide some enlightenment as to how tubes really work. :-)

Bill, W6WRT