>On 2/9/96 you wrote:
>
>> MLA2500 owners, etc....
>> Has anyone tried a 8874 in place of the 8875?
>The mechanical changes necessary to convert the MLA-2500 to use 8874s
>would amount to almost a complete rework of the packaging ... and even
>then the electrical problems (driving impedence, grid metering,
>improper plate circuit Q on some bands) would remain.
--------------------
I disagree completely on the electrical issues (driving impedence, grid
metering, improper plate circuit Q on some bands) and disagree in principle
on the cooling issues.
The ONLY difference electrically is the plate dissipation. 300 watts for
the 8875 and 400 watts for the 8874, which is solely due to the
effectiveness of the heatsink. Electrically they are interchangeable.
Mechanically the 8874 is smaller than the 8875 so should fit the available
space. Then the only issue is the modifications required to cool the 8874.
I don't have one of the amps to look at, but it should (in theory) be
possible to add chimneys from the top of the anodes to the top of the case
(I think the 8875s were mounted vertically). Then the only thing required
is a fan that pulls the air up through the tubes and out the top of the
case. All of those vent holes provide lots of air INPUT area. Muffin fans
do not work well into back pressure, but in this mode where the fan is
pulling through the tube it may work adequately--at least well enough to get
the 300 watts dissipation of the 8875 from the 8874. If the top of the case
is "full of holes" it is possible to mount a square of thin brass the size
of the fan on the underside of the cover, to which the chimneys are attached
to short sections of tubing the diameter of the tubes and located directly
over the tops, which have been soldered to the flat brass. A plenum may be
required on the top of the cover for the fan to be located above the cover
with enough space to allow it to "pull" with full force. (Locating it
directly on the cover might reduce the suction due to turbulence from the
proximity of the tiny holes.) I would probably go for a larger fan than the
standard 4" muffin. Something more like a 6" one. Seems like the 8875s
were mounted very closely to the back of the case, making it more difficult
to mount the larger fan directly over the tubes. This may push one even
more towards the plenum concept, with the fan offset somewhat from the
position directly above the tubes. Probably ought to space it an inch or so
above the cover. Anode temperature is the key issue. Don't know where to
get tempelaq (sp? they say spelling is the second thing to go)
(temperature sensitive laquer that turns color upon reaching a certain temp.
Handy to paint on the anodes to see if they exceed the "turning" temp of
the particular paint used.
(A word is worth a millipicture. Hope this makes enough sense to
extrapolate to the actual amp in question.)
The type of chimneys needed can be fashioned from high temp silicone rubber
sheet the way ETO does it in their amps. If the cabinet top can be modified
(you aren't worried about resale value are you?) then the mechanical mods
should be even easier.
Reminds me very much of all of the fans that I mounted on top of my old
Collins S-line transmitters to keep them cool during the early K0RF contest
operations. They were all held on with cable lacing string tied to the
holes in the top lids in order to eliminate the need for any mods to the case.
Hope this is useful. 73 John W0UN
BTW You might want to confirm that the tube sockets allow some air to flow
around them in order to make sure there is enough air flow to keep the base
end of the tube cool. I would guess that the proposed mod would provide
better air flow than the original design, not worse.
John Brosnahan
La Salle Research Corp 24115 WCR 40 La Salle, CO 80645 USA
voice 970-284-6602 fax 970-284-0979 email broz@csn.net
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