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Author Topic: FU728F & 4CX1500B Question  (Read 24499 times)
KO4NX
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Posts: 179




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« on: January 21, 2013, 07:00:55 AM »

Hello All:

With all the discussion regarding amplifiers and other parts being imported from China, I have decided to look up the FU728F.  In my search, I seem to be getting a bit of conflicting information. Is the FU728F really a drop in replacement for the EIMAC 4XC1500B? 

It appears as though some U.S. Manufactures offer this tube as  a low cost alternative to the EIMAC tube.  My question is, what is the cost of a FU728F, and where in the USA would you buy one?

73

Rich, AJ3G
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N3QE
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« Reply #1 on: January 21, 2013, 08:31:02 AM »

Nearly drop-in. Some previous discussion at: http://lists.contesting.com/archives//html/Amps/2012-06/msg00315.html
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KO4NX
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Posts: 179




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« Reply #2 on: January 21, 2013, 08:41:19 AM »

 I had read that, but I have also read some other threads which seem to contradict one and other. What really prompted me to question compatibility was the fact RF Parts seems to sell two tube sockets. One is specified for the FU728F while the other for the 4CX1500B. 

Now the question is; who sells the FU728F in the USA??

73

Rich, AJ3G
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W9GB
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Posts: 2600




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« Reply #3 on: January 21, 2013, 09:07:09 AM »

Emtron Amplifiers have switched to the lower cost Chinese FU728F ceramic tetrode.
http://www.eham.net/reviews/detail/2323

As noted by N8RI in 2010,
I've had the Emtron DX-2sp now for about 4 months. I have the new version with QSK as a standard feature.
Construction is modular with many components common across the line.

Emtron changed to the Chinese FU728F tube, which except for the filament voltage is directly interchangeable with the 4CX1500B. Emtron has included a voltage tap on the transformer for the 4CX1500B filament.
The centering pin in the socket for the FU728F is *slightly* larger than the one for a 4CX1500B, but Rudi is using custom sockets that will fit both tubes and I'd call the tank circuit a "heavy duty, work-of-art".
With this change of tubes, Emtron has answered the tube availability question.
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KO4NX
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Posts: 179




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« Reply #4 on: January 21, 2013, 09:28:18 AM »

I think I am getting the picture now.  I think if EMTRON is using a custom socket to fit the EIMAC and the FU728F then it might not be plug and play from a mechanical standpoint.  Changing the filament voltage seems pretty easy, but installing a new socket might be a bit harder than one might think!

73

Rich, AJ3G
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W8JX
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« Reply #5 on: January 21, 2013, 09:47:02 AM »

I think I am getting the picture now.  I think if EMTRON is using a custom socket to fit the EIMAC and the FU728F then it might not be plug and play from a mechanical standpoint.  Changing the filament voltage seems pretty easy, but installing a new socket might be a bit harder than one might think

It seems the the center guide pin or post is slightly larger and will not fit into a standard 4x1500 socket but it is possible to make a FU72BF socket accommodate a 4cx1500 requiring only a filament voltage change.
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ZENKI
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« Reply #6 on: January 22, 2013, 01:27:49 PM »

One thing I can tell you is that Fu728 is no match for the 4cx1500B's  excellent IMD performance. Its 3rd order IMD figures  is about  15db worst than the 4cx1500B .
The 4cx1500B was designed for linear SSB service by Eimac and is one of the best tubes around for IMD performance. There is a Chinese manufactured 4cx1500B.
I dont have any knowledge of how good it is. I have a good supply of genuine Eimacs which are used in a Collins 30S1.

You can find  tons of FU728's on the Chinese Ebay site called TaoBao. Many of those tubes appear to be rejects and suffer from flash over syndrome so be be warned. I know
several hams  who have tried to use them in their Emtron DX3's  with not very good tube life or immediate flashover even after conditioning the tubes. Tao Bao is like Ebay fully of crooks.

Rudy from Emtron  is apparently dealing with one of better suppliers and tubes sourced through him are reliable so I am told by owners of his amps. There will be skeptics about this claim  since he charges more than the internet price for the tubes. Special hand selection of tubes is a well known price gouging marketing technique used by many of the Russian Tube sellers.

Another thing to watch out for is that the input capacitance of the FU728's are higher so if you using them in amps like the Harris 110A you might have trouble on the higher bands with the amp taking off. Worst case scenario
is that you wont get the same output power as a pair  of genuine 1500B'S You wont find this information on the Emtron page for the DX3SP, it has very poor efficiency on the higher bands because of the input capacitance issue.
Really using 2 of any tubes in any amp design is bad engineering practice. You better off just using a single tube to achieve your power output objectives.





Hello All:

With all the discussion regarding amplifiers and other parts being imported from China, I have decided to look up the FU728F.  In my search, I seem to be getting a bit of conflicting information. Is the FU728F really a drop in replacement for the EIMAC 4XC1500B? 

It appears as though some U.S. Manufactures offer this tube as  a low cost alternative to the EIMAC tube.  My question is, what is the cost of a FU728F, and where in the USA would you buy one?

73

Rich, AJ3G

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KO4NX
Member

Posts: 179




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« Reply #7 on: January 22, 2013, 01:36:58 PM »

Hello Zenki:

In this case I have a more than enough 4CX1500B's to last a life time. The reason I asked the question was to see if I could take an amplfiier which was built around the FU728F and easily drop in a 4CX1500B. I think the answer to the question is NO, as I would have to install a new socket, and change some of the voltages.

The other odd item is, I cannot seem to find an English written specification on the internet for the FU728F.  Found plenty of information on the Sockets, but nothing legitimate on the tube itself.

73

Rich, AJ3G
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ZENKI
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Posts: 906




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« Reply #8 on: January 22, 2013, 01:43:07 PM »

 FU-728F POWER TETRODE
Cathode:  Oxide-coated unipotential
Heater:  8.5-9A @ 9V
Cathode warm up time:  3 min (Min)
Inter-electrode Capacitances:  (grounded cathode connection):
Input:  103 pF
Output:  13.3 pF
Feedback:  0.03 pF
Transconductance:  >= 14.3 mA/V
Maximum operating temperature:
Ceramic/metal seals: 250°C
Anode core: 250°C
Cooling:Forced air
Air-flow requirements:  3000 l/min
Net weight:  950g
Class AB1 radio frequency SSB linear amplifier (below 110MHz):
 Ua = 3000v
Ug2 = 350V
 Ug1 = -79V (zero-signal plate current Ia = 250mA)
Plate dissipation:  1200 W
Pg2 dissipation: 12W,
Pg1 dissipation: 0W
Ia = 0.9A
Output power:
Single-tone: 1800W
Two-tone: 2500W

I have a dimension sheet somewhere. As you can see its not a 1500 watt dissipation tube.

Hello Zenki:

In this case I have a more than enough 4CX1500B's to last a life time. The reason I asked the question was to see if I could take an amplfiier which was built around the FU728F and easily drop in a 4CX1500B. I think the answer to the question is NO, as I would have to install a new socket, and change some of the voltages.

The other odd item is, I cannot seem to find an English written specification on the internet for the FU728F.  Found plenty of information on the Sockets, but nothing legitimate on the tube itself.

73

Rich, AJ3G
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KO4NX
Member

Posts: 179




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« Reply #9 on: January 22, 2013, 03:46:42 PM »

Hi Zenki:

Thanks, that is very helpful information. Did you have this information stashed somewhere, or is there online specification sheet you can point me to?

Rich
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KE3WD
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Posts: 5694




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« Reply #10 on: January 24, 2013, 05:58:39 PM »

I'll stop at the "FU" prefix, thanks. 
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KO4NX
Member

Posts: 179




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« Reply #11 on: January 24, 2013, 06:28:21 PM »

I'll stop at the "FU" prefix, thanks. 

Thank you!
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ZL4IV
Member

Posts: 42




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« Reply #12 on: February 03, 2013, 01:48:24 AM »

My Emtron DX-3sp takes the 4CX1500B in the same socket but with about 1kw drop in power output but I nobody would notice this at the other end. There are 3 main types of this FU728F, only one is quality and Rudi buys from this factory. I would no buy from eBay or any other site unless I was given a close up shot of the markings to confirm the factory.
The spec's given here are not quite correct. They are a beefed up military version of the 4CX1500B.
The DX-3sp will do 5kw pep at about 80-90w drive.
Still like my DX-3 though with the GU78B. I like single tube amps.
ZL4IV
 
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VK2UW
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« Reply #13 on: April 05, 2013, 01:43:13 AM »

About the FU-728F tetrode, the tube is bigger in height than the 4cx1500b and has far better cooling and this also allows for a higher PEP rating than the older Eimac 4cx1500B, and has also been tried and tested in these Emtron Amplifiers as the bases are the "4CX1500B bases", so no need to change the bases if you want to use 4CX1500B tubes but you will notice the PEP rating will drop quite dramatically.

Please note:  Yes these are the "4CX1500B Bases" not the FU-728F type so no need to change if using either.

Power using the Eimac 4cx1500B will be very noticeable as it is not in 10's of watts that it drops, it is in 100's of watts using the older Eimac 4CX1500B.

The FU-728F is far superior and a higher output than the older Eimac 4cx1500B and is more readily available but don't buy on eBay.

Emtron purchase from the factory direct re the FU-728F and will supply all Emtron owners if needed in the future.

If you check the all new Emtron Website and read up on the ranges and read about the FU-728F and the equivalent 4CX-1500B you will see that it plainly tells you that these tubes are interchangeable and what changes needed.

Here is an example from the DX-3SP range and take note on the FU-728F and 4CX1500B: 

The "Big Gun" is getting even bigger, with the new DX-3SP! With over 4000 Watts CW and more then 4500 Watts SSB, this QRO muscle amp is not a joke! It is an amplifier with gravitas capable of satisfying the serious and most fastidious station operator. All specifications and features, except output power, are the same all throughout the Emtron amplifier product range, and the DX-3SP comes complete with QSK module as standard. The design deploys two durable FU-728F tubes (improved Chinese mil-version of the 4CX1500B) in parallel, with a total plate dissipation of 3000 Watts, comfortably delivering the expected output power and durability. These robust FU728F tubes are manufactured in present production and are available directly from the manufacturer, or from Emtron, exclusively for Emtron amplifier owners. We believe that this new DX-3SP is physically the smallest 4000 Watt desktop RF amplifier on the market today, and the most popular as well.

Please note: This new DX-3SP comes with two FU-728B tubes, which are directly interchangeable with Eimac 4CX1500B tubes! Only the filament tap on the transformer must be changed from 9 volt to 6 volt, if Eimac tubes are used!


The above PEP rating is very conservative as tested into a Dummy Load of 50ohms and showing 5000watts yes 5000watts PEP.

So this Emtron DX-3SP is no slouch comparing to other brands that are much dearer and not as sophisticated nor with the same Engineering and Safety features as the emtron range of amplifiers.

Even the DX-2SP tested into a Dummy load of 50ohms gives a whopping 3000watts PEP.

I have been building my own 50ohm Dummy Loads for quite some time using Carborundum Resistors. The later one using 3 of 150ohm x 3000watts each with 450cub Mt's of forced air cooling, with a total of handling 9000watts of power. I have tested the above to the maximum into these loads.

Others using these High Power Emtron Amplifiers can agree with me on what you can use and do with these Superb Amplifiers that nobody and that is nobody has been able to match in Quality / Engineering / Safety Features along with Support for these highly sought after amplifiers.

I hope this sorts out the issue that some have about the different basses for the FU-728F and the Eimac 4CX1500B.

Check the all new Emtron Website for the latest information and all the new features:
http://www.emtron.com.au/

I welcome others inputs confirming what I have just written.

73's Chris
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G3RZP
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Posts: 4393




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« Reply #14 on: April 05, 2013, 04:25:09 AM »

Think of the electricity bill with that DX3 SP amplifier!

You might be able to afford to buy it, but could you afford to run it?
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