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February 6th 04, 12:22 AM

In article , (Onno de
Vries.) wrote:

My name is Onno de Vries and i'm living in the Netherland and building
big amps is my hobby.
I just bought a 4CX 5000A on a radio market here for a good price (180
Euro) including socket.
Becourse the price was so good i took the challenge to build a 88-108
Mhz final from it.
I allready build many with all diferent kind of tube's but after some
experimenting with this tube i found out that the anode resonator a a
bit of a problem course of the high cappacity of the tube.
I try to do it with a double output coil (together Almost a half wave)
wich you see in manny designs for 144 Mhz amateur amps using for
example the russian tube GU84.
This works but it has a terrible efficiency,about 30%.
I hope that someone have experiance with big transmitters with
4CX5000A or simular in the final wich has about the same anode
cappacity and inpedance.
I hope that somebody can give me some Info about making a good and
high efficiend tank with these kind of tubes.

Hi Onno,

RCA used the 4CX5000A as the final in their model BTF-10D 10kW FM
transmitter. The manual doesn't include any photos that show the complete
anode cavity, or even photos that show all the details. It's been nearly
forty years since I have had hands on one of these, but basically there
are two adjustable lines used as inductors in a pi network connected
between the tube anode and the transmission line to the antenna. The
first inductor is in parallel with the anode capacity. I will review the
manual and see if I can figure out any of the details.

Regards,

John Byrns

Surf my web pages at, http://users.rcn.com/jbyrns/