John, the basic issue is that is unhealthy for the tube cathode to be
subjected to current flow before it has reached the proper operating
temperature.
If the tube has cutoff bias voltage applied then there will be no cathode
current flow and all is well. The plate and screen voltages can be on or off
during warm up; your call.
In general, it is not smart to have screen voltage on when plate voltage is
off. When the cathode emits electrons you want most of them flowing toward
the plate, not the screen which is much more fragile.
Turning screen voltage off while leaving plate voltage on is not an issue,
in fact it is a good method of switching a tube into standby. The only
precaution is the screen should never be allowed to "float" as the voltage will
tend to rise near the plate potential and that is a good way to destroy a
tetrode tube very quickly. A resistance of a few 100K ohms between the
screen and cathode is enough to prevent the "run away" condition.
Hope this is what you were looking for.
73,
Gerald K5GW
In a message dated 4/11/2016 12:56:25 P.M. Central Daylight Time,
k4dlx@bellsouth.net writes:
New to this reflector.
Hope someone can help me understand tube warm up requirements.
Specifically, What are the necessary conditions for voltages at plate ,
screen and control grid during Warm up for indirectly heated tetrode tubes.
I have seen directions that say no plate voltage during warm up, but some
amps (MLA2500) apply plate voltage and limit cathode current to a few mA by
self-bias during warm up.
What I have in mind for my amp with gu74b (4cx800) is to apply fil voltage
for 3 minutes while screen is at 0v, plate is full on and
G1 is biased to cutoff (approx. -75v).
Is my approach workable?
And
What are the necessary conditions condition for the tub during warm up?..
Thanks in advance for your help
John K4dlx
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