4CX250B

FM does it with frequency!
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Lee_Wheeler
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4CX250B

Post by Lee_Wheeler » Mon Nov 28, 2016 8:16 pm

What is the consensus on 4CX250B tubes these days? I have not had any need to replace any in at least 20 years and I read some comments here and there that Eimac changed the design at some point to more favor the 450 MHz rigs and they don't do well in FMs any more. I also got a quote on them today for over $1100 for a pair of Eimacs.

Does anyone have any experience with the Chinese National brand tubes?

For what it is worth the transmitter is an old Harris FM 20K - BANG. :?

...Lee

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BroadcastDoc
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Re: 4CX250B

Post by BroadcastDoc » Tue Nov 29, 2016 6:57 am

Future generations of my family will have ringing ears from from me tuning those up 15 years ago. :lol:
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TPT
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Re: 4CX250B

Post by TPT » Tue Nov 29, 2016 7:06 am

I think --it's early and the caffeine hasn't kicked in--that I have a couple--New/Old Stock Cheap. Send me a PM

There are two kinds of these small tubes. Some are designed for "ICAS" e.g., intermittent commercial amateur service, where the tube is not on for that long a time --and broadcast, 24/7 operation. The ones I have are the former, in my old 10K I got about 6~9 months out of them.

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RGORJANCE
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Re: ringing ears from from me tuning those up 15 years ago.

Post by RGORJANCE » Tue Nov 29, 2016 8:15 am

Ok, here's the "rest of the story".

The FM-20G and H rigs were reported to be louder that Niagara Falls (later verified). As I recall, the blower change to a "more pleasing sound" began in the H2 series. "Oh what a relief it was"!

The BANG usually was the result if a dust explosion, and were truly spectacular! However, I did manage to see a gigantic super nova on a 20 in the test lab during a trip to Quinky back in the day. Beautiful flash of light, an enormous bang and a gracefully ascending mushroom shaped cloud rising into the steel reinforced ceiling. All followed by a strange silence interrupted by lots of loud laughter!

4CX250 life was about 6-9 months. Several folks greatly extended that with fil. voltage control on those jugs

Someone mentioned a 10K....if it was a Harris, the driver tube in that rig was a single 4CX300, not a 4CX250
(a pair was used in the FM-20 series).

Fossil

TPT
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Re: 4CX250B

Post by TPT » Tue Nov 29, 2016 7:51 pm

Thought that would be too easy--but try RF parts--that's where I found whatever it was the 10K needed. At that time, that driver tube had been discontinued, but RF parts had the NOS ICAS bottles. Made a new Nautel an easy choice when the 10K later died.

Joe Friday
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Re: 4CX250B

Post by Joe Friday » Wed Nov 30, 2016 12:25 am

I could get about 11-12 months out of a set of Eimac 4CX250BC's in the 20K that I looked after several years ago. We switched to the Svetlana brand and the life went to about 15 months. It looks like Svetlana 4CX250B's are no longer imported to the US.

In terms of the National brand, I don't have any experience with that tube, but have had good luck with one of their 3CX3000A's in an old CCA rig. Running well after about two years.

Best of luck!

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Re: 4CX250B

Post by ChuckG » Wed Nov 30, 2016 6:13 pm

The last time I had to buy that tube, I got a "pro-tek" from D&C Electronics. Chinese-made I'm sure. It was still making full power when the transmitter was retired some 15-16 months later.
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Lee_Wheeler
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Re: 4CX250B

Post by Lee_Wheeler » Thu Dec 01, 2016 4:51 pm

I think I have a cheap solution and it was staring at me all along. At the doomsday Aux site there is an old RCA BTF-20E back in the storage room, from the logs it was making full power when retired 15 years or so ago. I popped open the door and there was a shiny pair of Eimac 250Bs. I pulled them this afternoon and I will give them a shot. Since they are going into another aux transmitter that I only use when I change tubes on the FMD-50K it sees well under 10 hours a year.

The signal from the aux transmitter / antenna at the main site is much better than the signal from the doomsday site so if I need to go to an aux it is my preference to go the the 20K. The main thing with that transmitter is that it needs to come up when called upon to do so and it would be nice if it maintained power for more than an hour.

...Lee

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Re: 4CX250B

Post by jammerdave » Thu Dec 01, 2016 5:29 pm

If those have been just sitting that long, be sure to bake the filaments for a day to de-gas them...

df

Lee_Wheeler wrote:I think I have a cheap solution and it was staring at me all along. At the doomsday Aux site there is an old RCA BTF-20E back in the storage room, from the logs it was making full power when retired 15 years or so ago. I popped open the door and there was a shiny pair of Eimac 250Bs. I pulled them this afternoon and I will give them a shot. Since they are going into another aux transmitter that I only use when I change tubes on the FMD-50K it sees well under 10 hours a year.

The signal from the aux transmitter / antenna at the main site is much better than the signal from the doomsday site so if I need to go to an aux it is my preference to go the the 20K. The main thing with that transmitter is that it needs to come up when called upon to do so and it would be nice if it maintained power for more than an hour.

...Lee

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