Hi
You could always liquid cool the tubes. That would keep the glass and
seals at a nice constant temperature.
There is reason to believe that reducing the temperature of the glass
to metal seals at the base of the tube has an affect on the long term
performance of the seals. If you have tubes mainly failing due to gas
then this is something to be concerned about. The tube socket
probably heat sinks this end of the tube pretty well.
At the temperatures you find in a receiving tube the temperature of
the glass envelope by it's self doesn't have much of an affect. If
you get the temperature up a lot higher the envelope will have some
problems. If we start to see tubes collapsing in use then this is one
to worry about. I have seen pictures of transmitting tubes that have
failed this way.
The real question is what makes receiving tubes fail. Transmitting
tubes have been studied quite a bit. Audio power tubes and rectifiers
have been studied to a lesser extent. Low power / small signal tubes
have only been studied in a fairly cursory way. You can look at it as
tubes are tubes, but we don't stress them all the same way. Plates
and grids in receiving tubes simply do not heat up the same way they
do in power tubes. Filament power is the main source of heat in a
normal receiving tube.
Cooling tubes down helps reduce the temperature of things like plates
and grids. Cooling also helps seals. I doubt that external cooling
has any big affect on the filament. Most receiving tubes I have seen
go bad die from filament related issues. Some simply do the light
bulb thing and stop glowing. Others get to the point that cathode
emission drops below the level needed to keep working. Certainly
things like shorted grids do occur, but they are not very common.
If what I swap out here is any indication emission is the main (>
80%) issue for dead receiving tubes. Open filaments make up almost
all of the rest. I can probably count the number of receiving tubes I
have seen fail for other issues without taking off my shoes.
Black tube shields look cool. That alone is a good reason to use
them. They definitely cool down the tube (though the radio is just as
hot). Cooler is always better. They sometimes are easier to get on
and off - never a bad thing. Given the way we use the radios these
days I would not put them in the same category as the capacitors.
They are a nice thing to swap out, and the radio is better with black
shields. I don't think the radios are enough better with black
shields to run out and spend hundreds of dollars re-shielding all our
radios to make them work better.
Take Care
Bob Camp
KB8TQ
On May 4, 2005, at 11:08 PM, Flowertime01 at wmconnect.com wrote:
> Scott,
>
> You may not need that many shields.
> Recommendations were to shield the RF, 1st, 2nd, 3rd mixers, VFO, BFO.
> Just the RF mixers and Osc need shields. Recommendations were to
> uncover
> every thing else.
>
> We know the good shields will run cooler than the plain cans.
> I do not know if the good shields will actually get you a cooler
> glass bulb
> temperature than no shield with ventilation being the same.
> So it still may be best to run most of the tubes naked.
>
> Just because we used those crummy old shields is not reason to
> continue.
> We put in better caps today, so why not better shields.
>
> Round up as many as you can. One never has enough spare parts.
>
> Roger KC6TRU
>
>
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