Responding to the original post by K8RF, I saw a reference to an MLA-2500
converted to 3CX800s in an article in CQ-Contest Magazine about the 1996
J6DX contest expedition. The alleged owner of the amp was Steve Miller,
N8SM (ex-WD8IXE). I have no details about the conversion, but you might
try to contact him for info.
However, this is not likely to be a conversion for the faint of heart. As
K6GT pointed out, the mechanical cooling requirements are fundamentally
different (axial vs. transverse) between 3CX800s and 8875s. Not impossible
to overcome, but a significant mechanical rip-up.
Although I admire the elegance of K6GT's conversion to a single 4CX800, I
think there is significant advantage to a pair of 3CX800s. He makes the
point that a single 4CX800 has more plate dissipation (800W) than a pair of
8875 (2x300=600), however it is still only rated to put out 750W vs
slightly over 1000W for the 8875s. One possible reason for this is that
the idling current for a 4CX800 is significantly higher than an 8875 or a
3CX800. (The 4CX1600 is even worse in this regard.) While this condition
can be alleviated by using RF bias switching, its my belief that the
4CX800s run hotter than the reasonably equivalent Eimacs. (My Alpha 91b
using 4CX800s appears to run significantly hotter than my friend's Alpha 89
using 3CX800s at equivalent power outputs.)
I bought a MLA-2500 last winter just for contest expeditions. Fortunately
it has two apparently strong 8875s in it. If they ever blow, my amp will
be a candidate for conversion to a pair of 3CX800s.
Seems such a conversion would be a worthy topic for this reflector...
73,
Dave Pruett, K8CC
Home phone: (734) 481-0755
Home email: k8cc@ix.netcom.com
Work phone: (313) 493-2168
Work email: dap14@chrysler.com
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