Re: Question about grid behavior with triodes
- From: Eiron <E1ron@xxxxxxxxxxx>
- Date: Sat, 29 Sep 2007 09:36:42 +0100
Patrick Turner wrote:
Wessel Dirksen wrote:Hello RAT friends,
I've looked in the books I have but there is not really a useable
explanation over the practical aspects over grid behavior,
particularly with the onset of positive grid current. A few questions
for the knowledgable.
1 Is there a formula over what the load impedance is of a triode
control grid is "normally" and when driven positive?
2 Does grid current only occur when the input signal peak voltage
reaches bias voltage or are there other scenario's?
Wessel
Grid current begins when the grid approaches the same potential as the
cathode
in negativelty biased tubes.
When Eg is raised above Ek, grid I rapidly increases, and there is much
data about how much Ig occurs where it is needed in cases of transmitting
tubes,
although Ig isn't much spelled out for small triodes.
This is because we don't much use small triodes in class A2; A1 is fine
for 99.99% of apps.
If you want to know more about it, make the experiments and measurements
yourself.
Grid current at audio F means that the input resistance to a triode
changes
from near infinite when Eg is -1V or more negative to perhaps 2k only
with Ig
with a 6L6, when grid is driven positive. The Rgin isn't linear.
If you wish to drive a grid positive, then simple experiments
will reveal what kind of low resistance signal source must be used.
So putting 2V rms on the grid of a triode with a fixed bias of 2.4v
may not be such a good idea. Here's a circuit to experiment with -
http://members.lycos.co.uk/fiultra/BatteryT68bis.jpg
--
Eiron.
.
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