The Chinese FU-728F first intruduce in 2005 or 2006 by myself in amps
reflector. Before that year , as far as know,NEVER non-chinese had ever use
and know it .
In past years ,some homebrewers and amplifier manufacturer( EMTRON) begain
to use it.But many people did not know it at all.
The FU-728F develop in end of 1960s or early 1970s for Chinese TYPE 70/76
1600W ISB/SSB transmitter, this equipment widely use in
land forces , navy and air force in 1970s to 1990s in China.FU-728F design
baseed on FU-100F( similar 4CX1000A) by Chinese, so without any
tube can replace it direct. it can regards as longer( taller) FU-100F ,
longer anode and cathode, so it with bigger power rating.
There are three version of FU-728F, please view the picture here:
http://www.zmdz.com/bbs/incomefiles/201112/2113423610426.JPG, and
http://www.zmdz.com/bbs/incomefiles/201112/2113425923319.JPG right: is the
oldest version, it was built before 1987 even end of 1970s,
this version usually have shorter tube life, the oxide is easy flake
away from cathode if over drive a bit,
this led to the cathode and grid shorted;the left( mark “J”), it is a
newer and better version, it built in the end of 1980s to 1990s; the
middle , it is
best version and made in 1990S to now, this version's quality better
than Russian GU-84 and near Eimac's.The left and middle seems same , but
look
at fin carefully, the left with dense fin , if you still not
see it clearly, please look here
http://www.zmdz.com/bbs/incomefiles/201112/2113431253493.JPG. te top is te
best version of FU-728F .
These are three version of FU-728F and their difference.
About the manufacturer of FU-728F, There is only a manufacturer in China
today, it located in Jiangxi Province.You can see the left tube what mark
“J”
with difference logo,red star like US air force’s logo, it is BOE Groupe’s
logo , although BOE groupe havd ever made FU-728F ,but duo to the quality
problem,only
few tube were produced. Most of BOE’s tube actualy OEM by jiangxi Planet.
How to use this tube? first, it must use shunt screen regulator, because
this tube often have larger negative screen current, if you use
serial regulator like ARRL Handbook “Suny Vale”/Saint Petersburg
Kilowatt-Plus”, you will damage the screen power supply.
If you use a pair tube , you must use same version tube, because
difference fin will led one of tubes over heat.
about the socket, FU-728F’s sockets can not fit 4CX1000A’s but 4CX1000A’s
socket can fit FU-728F.Why? I do not know.I know US force rife's
caliber is 5.56mm, Chinese is 5.8mm, maybe it is the eastern trick.LoL! You
can modify its socket easily, jut replace centering pin,
so te socket can fit both.In past few tyears, I have supply some modify
sockets to EMTRON.
Finally, the price, the newer version NOS tube usually quote $250 to
$300, but old version is cheap, from $50 to $220 .Please remember, no free
lunch in the world.
I do not recomded old version, very easy to damage. The factory new tube
quote $450 orso. Now, My amplifer use factory new only.
Merry Xmas and Happy year!
73!Hsu
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