Cooling fans are CRUCIAL in broadcast transmitters! Even tube amps
really need at least an exhaust fan(for 6146 class tubes) as otherwise
the plate compartment can get so hot as to create durability problems
for components other than the tubes.
For solid state rigs and small glass tubes, my experience it this: due
to the 100% transmit duty cycle of any broadcast rig, plus the 100%
duty cycle of FM and high dusty cycle of AM as opposed to SSB, almost
any broadcast rig intneded to operate for more thn 15 minutes at a time
needs a fan with the possible exception of less than 1-2W rigs.
Many amp designs that never need fans for, say, ham use will always
need that fan in broadcast use. If a ham used th IRF 510 for a CW/FM
50MHZ rig(not suitable for SSB at this frequency), he could get away
with noting more than a heavy heatsink from an old inverter or so forth
and no fan due to the short transmit times. Not so for broadcast!
Solid state cooling is simple-a heatsink with enough fin area and a
thick enough base to distribute the heat, plus a fan big enough to
distribute the heat. Some of the bigger fans and heatsinks used in
computers can get rid of huge amounts of heat for a very small heatsink
weight(critical for portable rigs) and bulk. They have a very large
number of thin, closely spaced fins and are intended for use with the
attached fan.
Multiple such heatsinks form dumpstered computers could be attached to
a 3/8 aluminum plate with screws and heatsink grease to make a massive
het sink for big power or low efficiency (multi-IRF 510 50W+) rigs.
The only broadcast rig larger than that I've ever sucessfully run
without a fan was an 87.9 MHZ 5W NTE 474 rig that had very good Class C
efficiency and was fine with a Pentium heatsink, getting warm but never
hot with no fan or full cold with the little fan in use. The NTE 474
costs over $13 but has shocking efficiency for a 3-5W output. It
cannot stand more than 12V supply, however, and can break over
non-destructively if there is anything wrong with the tuned circuits(in
that kind of amp).
External anode tubes like the 4x150 and its descendants and big tubes
like 4-1000A will always require a blower no matter what the use, as
they are not designed to ever run without it. Bright filament tubes
like 4-65A and larger, and 4CX800(over 300), etc cannot run even the
filoament by itself without cooling air through the socket.
If you want mega-power in the FM broadcast band, the 4X150 or 4x250 has
a lot going for it. It is one of the largest tubes to still use a
coated cathode with indirect heat, needing little heater power and
allowign directly grounding the cathode.Two of them can output 750
watts(!) and go all day on a good tuned-line amp.
A single 4x150 can be found for as little as $40 on some websites, and
is a hell of a lot more efficient at VHF than the 6146-and will work as
low as 500V or as high as almost 2000, for outputs as low as 50W or as
high as 250 for the 4x140 or 380 for the 4x250, which is essentially
the same tube with a better heatsink on it. The magnum version of this
tube is the 4CX300. All of these tubes rely on forced-air(ducted)
cooling through their special socket.
For a high-power AM rig, you can run 4 MOSFETS in push-pull, with two
on each side with no problems. Four big power MOSFETS(not the little
IRF 510) on a heatsink the size of one side of a cinder block should
give all the power you can ever get away with.
|
