>Looking for tech.specifcations on the 290A Eimac tube
>I would expect that data can be similar as for the nCX5000 types
>It sits in a SK-291A socket.
>
// The 290 is a Class AB1 pentode with an oxide-cathode and a delicate
2w grid. The heater needs 17A at 5.7 - 6.3 volts. Anode ratings for AØ
are 6000V, 2A. For SSB, 7000V should be ok. To get 2a, the
screen-potential needs to be c. 900v. Since the 290's screen is subject
to reverse-current, a zener string shunt regulator would be good. Zener
current needs to be c. 65mA. To protect the delicate screen from
over-current, the zener string should fed from a power resistor connected
to a 3000v or more source. I would use a string of 24v - 30v, 5w zeners
mounted on perforated board, near the cooling airflow. [note - "5w-rated"
zeners should not be operated above 2w without liquid cooling]. With a
150w transceiver, the needed 120V-peak RF grid voltage can be achieved
with a 50-ohm grid termination. My guess is that 9k-out PEP should be
possible on SSB. Back off the mic. gain until the anode-I doesn't
modulate (a.k.a. Class A) and you should have around 1k out. . .
"Speak softly and carry a big stick."
cheers, Chris
- R. L. Measures, 805.386.3734, AG6K, www.vcnet.com/measures.
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