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Author Topic: 4CX350FJ for Hf amplifier?  (Read 4867 times)
5B4AET
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Posts: 41




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« on: August 19, 2011, 08:14:18 AM »

Hello everyone,

Recently I have decided that I would try and buit an HF amplifier.
My plans go for SSB output NO more than 1Kw (say 700-800w) and will use tube/tubes. This will be my first HF amp.
I have kept for many years (for future use...) four 4CX350FJ which I dont know if they are any good (externaly are in excellent condition).
Q1. Is there a way to test these tubes (4CX350FJ) and know if they are any good before installing them on the amp?
Q2. Are they a good choice or should I use different tube (e.g. 3-500Z)? Any suggestion is most welcome.
Q3. Bearing in mind that I dont have much experience in such amps, would it be a good idea to built one    from the scratch or buy an old cheap one rebuilt/restore it?

Thanks for your time

Panicos

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AD4U
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Posts: 2157




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« Reply #1 on: August 19, 2011, 04:17:08 PM »

For your first amp I would suggest that you NOT use a 4CX350 or any of the 4CX tubes.  Collins successfully used the 4CX1000 in their 30S1, but Collins engineers had a lot of experience. The 4CX family of tubes are tetrodes and unless you carefully design and build your amp, you may have instability problems.  

Any of the glass triodes (3-500 for example) or most of the 3CX types would make your project much simpler.  After all the 3-500 family of glass tubes were designed to be used in grounded grid amps.  The 4CX family of tubes were not!

Dick  AD4U
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W8JI
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« Reply #2 on: August 19, 2011, 06:13:06 PM »

Much more expertise is required for a tetrode to be clean and reliable.

For a first amp, I would use a grounded grid triode like a 3-500Z.
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K8AXW
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Posts: 3723




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« Reply #3 on: August 19, 2011, 08:11:22 PM »

Panicos:  I also vote for the 3-500Z but would also add, since you're not an experienced linear amplifier builder, I would recommend you find a good circuit in a reliable book.

No point in trying to reinvent the wheel!
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5B4AET
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Posts: 41




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« Reply #4 on: August 20, 2011, 02:13:26 AM »

To start with many thanks for your help.

To be honest, one of the main reasons I considered the 4CX350 was because I have 4 in the shack.
So I will go probably with the 3-500 or any other 3CX that will give the 800w I want.
Since I will have to buy the tube anyway, pls suggest a tube which is tested and "easy" to use and any reliable source for the linear amp circuit.

Panicos - 5B4AET

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K8AXW
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« Reply #5 on: August 20, 2011, 09:31:35 AM »

Panicos:  If your email address is correct on QRZ.com, perhaps I can send you via email information.

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5B4AET
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Posts: 41




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« Reply #6 on: August 21, 2011, 01:45:59 AM »


Many thanks Allen, it is correct.
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K8AXW
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« Reply #7 on: August 21, 2011, 06:51:17 AM »

OK Panicos, let me get the information together and email you.  Good luck.
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5B4AET
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« Reply #8 on: August 21, 2011, 11:56:18 AM »


Many thanks Allen, looking forward!!!
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5B4AET
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Posts: 41




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« Reply #9 on: August 22, 2011, 12:17:56 PM »


Received the plans just fine.

Many thanks All.

 
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K8AXW
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Posts: 3723




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« Reply #10 on: August 22, 2011, 09:28:57 PM »

You're welcome.  Hope the plans gives you something to go with.  Good luck.
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ZENKI
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Posts: 916




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« Reply #11 on: August 26, 2011, 03:09:47 PM »

Why dont you use a GM3SEK Tetrode board. I have used these on my VHF amplifiers. The regulated protected screen  supply is really worth the money he asks for these boards.

The J series of Eimac tubes produce excellent IMD performance. I have a 6 meter amp that uses the 4cx600J using a GM3SEK regulator board. 2 tone 3rd ORDER IMD is -44db below one of two tones, its incredible.

I agree with Tom, the abundance cheaply available Russian tetrodes are really hard to get going with a clean signal. The problem with ham transceivers ALC oveshoot, bad EBS design, poor power control on radios, and generally poor IMD performance from terodes has taken us backwards  in terms of reducing IMD on the ham bands.

You hear more crap signals from  badly designed tetrode amplifiers than from triode amplifiers. Tetrode amplifiers commercial or homebrew should not be  the first amp of choice for new hams. It takes a lot of effort making certain you have a clean signal when using a tetrode.  Its so bad now that I can spot people who are using tetrodes on  CW and SSB just by listening very carefully using a SDR receiver. The characteristic of RF on the signal and IMD from these tetrode signals makes them  easy to spot. Most of these amps have poor EBS design that causes these issues and gives signals such a unique filthy fingerprint. Whatever design you use make sure you dont use any EBS, all the designs have faults and cause problems.

I wish everyone would go back to the good old humble single 3-500 homebrew amps, even the 813's were better  than most of these crap tetrode amps using russian tetrodes. If they were all as good as the 4cx350fj, they would of made a good choice.



Hello everyone,

Recently I have decided that I would try and buit an HF amplifier.
My plans go for SSB output NO more than 1Kw (say 700-800w) and will use tube/tubes. This will be my first HF amp.
I have kept for many years (for future use...) four 4CX350FJ which I dont know if they are any good (externaly are in excellent condition).
Q1. Is there a way to test these tubes (4CX350FJ) and know if they are any good before installing them on the amp?
Q2. Are they a good choice or should I use different tube (e.g. 3-500Z)? Any suggestion is most welcome.
Q3. Bearing in mind that I dont have much experience in such amps, would it be a good idea to built one    from the scratch or buy an old cheap one rebuilt/restore it?

Thanks for your time

Panicos


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