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.