The screen requires a path for current flow to the cathode. If this path has
too much resistance or is an open circuit, the screen grid will tend to rise
to the plate potential. You can look at the tube curves to see what happens
to the other currents when screen voltage is this high.
A typical design might protect the screen from this condition by means of a
50K resistor between the screen and cathode terminals. With this resistor in
place, failure of the screen supply will result in very low or no plate
current.
I have no experience with using solid state switches in this application. I
do know that a relay will do the job.
Lifting the screen voltage above chassis is no problem as long as the above
screen to cathode resistor is in place. In fact, many designs use this method
as a means to switch from standby to operate.
73,
Gerald K5GW
In a message dated 12/19/2008 9:11:20 A.M. Central Standard Time,
vk3pa@vk3pa.com writes:
Tnx all, so far so gud! just 2 more ques,
When using "tetrodes" what happends if u loose Screen grid supply?
(we all know if we loose plate supply, the screen will act like the anode
es draw excessive current)..
I have seen a circuit using a switching transistor on the bottom end of
the Screen (earthy end) es was refered to a EBS circuit..it lifted the
screen supply above chassie!!
es Can i replace a Si NPN transistor with a mosfet in the electronis bias
switching circuit es if so do we use "N channel" type say 160 volt 7 amp
type?? 73 de vk3pa
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