On 8/23/2011 4:24 PM, Manfred Mornhinweg wrote:
> Roger,
>
> I see that Bill posted some of our private interchange here! No problem
> with me. But now I feel that I have to reply to your comments!
>
>>> I just looked up values for solders: Common 60/40 solder conducts
>>> heat about one fifth as well as copper,
>> This is the reason we use heat transfer compounds like Artic Silver
> But Arctic Silver conducts heat about ten times worse than 60/40 solder!
>
> That's not to say anything against Arctic Silver. It still seems to be
> the best thermal compound available. But it's a grease with metal and
> oxide filling, which just cannot come anywhere close to the thermal
> conductivity of 100% metal, like solder is!
Agreed. I use Artic Silver when I can not solder. The problem with
solder is the temperature compared
to the critical temp for the device and the need to make sure the entire
area between the device and
heat sink is wetted. The extra time to make sure the entire area is
wetted can some times destroy the device.
OTOH there are "solder pastes" available that can be applied to the
mating surfaces and then melted by a brief induction
pulse so not a lot of heat or at least not enough to be destructive is
transferred to the internal, temperature sensitive parts
of the device.
>
>>> In a catalog from an aluminium supplier I found some extrusion
>>> stock that looks much like an Omega sign, but closed. Like a round
>>> tube
>> I'd stick with solid copper with a machined water way and a cover
>> silver soldered on rather than one with a removable cover.
> I will see what I can do. Certainly teh specially machined copper block
> should be better. But if I can buy that aluminium stock at very low
> price, and use it almost without modifications, that's a powerful advantage.
I'm replacing my drill press with a used Bridgeport mill. 3 axis power
feed with digital read out.
There are a lot of machine tools available at fire sale prices if you
can find them. Next is a toolroom lathe
with a hole through the headstock of 3 to 3 1/2" and a bed of at least 40".
> The core issue is that I need to calculate this stuff! If it turns out
> that the cheap aluminium extrusion is all I need, why should I bother
> with machining a copper block? On the other hand, if the aluminium
> extrusion proves to be unable to cool these MOSFETs well enough, while a
> machined copper block will do, then of course that's the deciding factor.
In my case I only need find the piece of copper large enough.
>>> m here? That's no issue for me. It would go into a closed container
>>> under the desk, holding maybe 20 liters of water.
>> Find for casual operating, but what about contesting.
> True, for contesting you need to place a coil of copper tube in that
> container, and run some tap water through it. But I'm not into
> contesting. I'm a technical ham, I like building equipment, and my
> operation on air is not intense. I calculated that such a container with
> 20 liters of water would warm up only by 15 degrees, during the wildest
> operating sessions I ever have!
I like building as well, but that extends well beyond the amplifier.
I'd choose a closed system using a
modified air conditioner to cool the tank of coolant for the device. I
spent many years working with this
stuff at the industrial level and have had too many bouts of , "it it
can go wrong it will". Hence the desire for
the closed system. A bucket under the desk would be a disaster for me
the fist time I put my feet under the desk. <:-))
>> You are looking for ohms per CM^3 or were you just measuring the
>> resistance between two probes?
> I filled a 1 meter long hose with water, and measured end-to-end using
> just the multimeter's probes.
What you need are two 1CM square plates, separated by 1 CM which should
give a close approximation
of the resistivity in ohms per CM^3. I used a piece of plexiglass
machine out so it held the two places with 1cm
spacing. That insulated the back side of the plates.
I think the limit we set was on the order of 3000 ohms/CM^3. Of course
that was
with 6 to 8KV, but we used about 12 to 15 feet of hose for isolation.
That was also well over
>> It's rare to find tap, well, or spring
>> water with more than a few hundred ohms per cubic CM.
> Even that might be good enough. If not, I can still buy some de-ionized
> water. And then watch it eat ions out of my cold plate over time!
IF you only have water running through, or in the device when it is in
operation
(IOW it drains back into the bucket when not running) eating metal ions
will take a very long
time to become a problem as in many years..
Here's a thought...put the coolant exhaust 6" to a foot above the device
or higher (what ever is handy)
with a quart container (or what ever is handy) in the line.
Take the return from the top of the container. When power fails to the
pump, you will have a quart (or what ever
size container you use) for a safety reserve that will drain back
through the device, giving plenty of time to shut down
before lack of cooling becomes a problem.
>
> ;-)Anyway, first I have to decide on the more electronic aspects of my
> amp! I'm still waiting for the samples of a few candidate MOSFETs,
> ordered a month ago. With international shipping and VERY slow customs
> processing, one needs to have patience...
I'd really like to build a SS 160 through 6 meter amp capable of 1.5KW
CCS with enough overhead
to comfortably run that legal limit. Many commercial tube amps
advertised as a legal limit amp just don't have the power
supply or components for sustained operation at the legal limit in SSB
or CW let alone digital. Some of them even push
the tubes way beyond their ratings to get the power they do and that's
at a greatly reduced tube life. IOW I'd
want an amp capable of at least 2KW continuous and preferably 2.5KW that
should loaf along at 1500 watts
without breaking into a sweat.
I had a Tokyo Hy-Power HL-1.5Kfx that ran an honest KW out without the
cooling air even feeling warm. it was
one of the nicest amps I've owned.
The current project is a 4CX3000A7 that I want to run class A. The
problem with the higher power tubes is their size.
Plus they use more filament power than the average ham station uses in
total power. (Bout ~375 watts) One amp on HF and one for 6-meters.
It' d be nice to have one for 2-mters as well. I have the tubes and
sockets but not enough components to build complete amps.
73
Roger (K8RI)
>
> Manfred.
>
>
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