[R-390] Re: Tube shields? We don’t need no steenkin' tube shields!
Barry Hauser
barry at hausernet.com
Tue Dec 4 03:19:26 EST 2007
2002tii wrote:
>
> I would have thought that most boatanchor afficianados would know all
> about IERC shields by now. There is a military study that was pretty
> widely available on the net a few years ago, and probably still is.
> It details the temperature of tube elements with dozens of types of
> shields and other variables. The study shows convincingly that IERC
> tube shields with proper bases (see below) are the best way to reduce
> tube element temperature.
>
Yup, Don, I'm pretty sure I read the same study, but I don't recall
dozens of types of shields -- more like the IERC vs. standard (stock)
shields, which were of the shiny, top spring, bayonet, no-insert type
and vs. no shield. Also maybe black painted standard shields. Was
there another study? Chuck R. refers to the Collins study which showed
tube life increased 53% with the IERC's over standard shields.
> Most of the heat loss from tube elements is by radiation. This is
> true with or without tube shields. Without them, the radiative sinks
> are the walls of the chassis, the transformers, etc. With them, the
> radiative sinks are the insides of the shields. So, IERC tube shields
> do not work by convection, but by conduction and radiation.
Might be so, but Les is getting relatively small differences in
temperature, so the convection differences could account for it.
> The liner in an IERC shield does two things -- cools the tube envelope
> (glass) by conduction and, much more importantly, absorbs heat from
> the tube plate(s) by radiation. (Note that the liners are flat
> black.) All of this heat is passed to the outer shield can, then
> conducted into the chassis through the shield mounting base. Thus,
> you need the appropriate, rather high shield mounting bases for them
> to work correctly. Without the right bases, IERC shields are worse
> than nothing (i.e., the tubes run hotter than they run bare). With
> them, IERC shields are way better than anything else (including
> nothing). Most other tube shields are worse than nothing.
There have been discussions about the realities of radiation. It's not
that cut and dried. As for the "right bases" -- most of the IERC
variants we consider are backward compatible types, not those with the
proprietary chassis mounts. I have some equipment that was made for
special IERC shields with special "slot & blade" bases. I have a set of
IERC special designs with those bases with yet another variant of the
insert -- more of a horizontal latticework that does not completely
cover the tube envelope. The sides of the shield are cut out -- it's
mostly a framework with that particular design. Apparently, emphasis
was given to direct venting on this more sophisticated designs -- rather
that maximizing radiation absorption or heat sinking the glass envelope.
The most preferred IERC style seems to be the minimum flanged, "finger"
insert type -- the one that yielded the best results in Les's
experiment. The bottoms of these are machined out and shiny where the
shield contacts the standard bayonet mount. The bottom forms tabs with
punched holes which (hopefully) match up to the nipples on the bayonet
base. They snap into position, they don't twist 'n lock, like the
original shiny shields. The area of metal-to-metal contact is
relatively small -- even less than with the standard shield. So it's
arguable as to how much of the heat conducts out through the base into
the chassis vs. into the air from the main part of the insert and
shield, which is many times the surface area of the base metal contact area.
Les, being one of the few authorized boatanchor historians of the 20th
and 21st centuries, is probably aware of that tube shield study. He may
have well been the one who unearthed it. So, I suppose he didn't
consider it to be the final word on the subject. Either that or he just
wanted to get some use of his Raytek laser-guided thermometer gun. ;-)
BTW -- the variation in top flanges seems to be very deliberate -- as if
the designers were trying to minimize blockage of the airflow. Some of
the IERC's are made of welded or extruded tubular anodized aluminum with
slightly rolled top edge. There are some made of a flat piece of
aluminum, rolled into a tube and crimp-finished, and those have no
lip/flange at all - just a raw edge. I consider this to be a clue.
Even if the convection aspect were a subset of the overall, apparently
it was considered worthy of tweaking.
Then there are the black bayonet WPM's which appear to be painted rather
than anodized and otherwise the same as the shiny shields. They usually
have the five or six-sided insert made of what appears to be black
spring-steel rather than beryllium copper, and may also still have the
top retaining spring of the standard shiny model. They would have
better contact area at the base. They typically do poorly and are
considered to be worse than nothing. Is it because the liner has
insufficient absorption mojo? Or is it because of other
conduction/convection/heat-bottlenecking factors overriding any
radiation absorption benefit?
Some have questioned the notion of radiation of heat through the vacuum
of the envelope and through the glass into the flat black insert. The
attempt to capture/absorb or envelop/conduct/sink involves an immediate
and tricky trade-off. First, you have to enclose the thing. There are
a number of guys on the list, past and present, that run their '390's
24/7 -- many with no shields and get quite a bit of mileage out of their
tubes. At least that's what I've read here.
>
> Airflow through the liner/shield is not a significant source of
> cooling, as shown by the net rise in tube element temperature without
> the proper bases (though undoubtedly some marginal convection cooling
> happens).
>
For the small differences Les came up with, it could be a factor. He
got 104 for the finger type shield which was 14 degrees cooler than the
next best one which used the pleated style -- I'm ignoring the five
sided one in the WPM shield for the moment. The thing is, the pleated
type maximizes flat black absorption area AND area of contact with the
glass and inside of the shield. The finger type maybe makes contact
with 30% of the surfaces -- I'm guessing. Both are made of beryllium
copper alloy -- very nasty stuff - do not ingest particles. So, it
would seem the finger type allows for more air circulation around the
glass. The whole notion of heat reducing shields is a tricky
proposition. You first have to bottle things up, so you're starting two
steps back before you begin. Any small design mishap can blow the benefit.
While the elements are in a vacuum and the primary way the heat gets out
initially is by radiation, that radiation first hits the glass and much
is absorbed in the glass. What passes through gets to the shield. If
it's a shiny shield, much would be reflected back. If it's a flat black
insert and shield it would be prevented from reflecting -- but still
needs to shed that heat to the surrounding air. If it doesn't, or it is
impeded, the insert and shield will continue to heat up, the glass will
be hotter, etc and an oven is created. The elements run hotter one way
or another -- because the shield/glass is hotter and can't absorb as
well? Of course, it's not really a true vacuum. There's quite a bit of
material supporting the elements which eventually pass through the
bottom of the glass. The elements are not floating in space. Tube
designs also vary. Some may radiate heat out directly more readily than
others.
> Remember, it is the temperature of the tube elements (grids and plate)
> that is important, NOT the temperature of the glass. Sure, the glass
> can get too hot, but by the time it does, the plate and grids are long
> past fried. And cooling the glass won't do much to cool the elements
> -- there is a vacuum between them -- so fans don't really do much good
> directly. By reducing the temperature of the radiative heat sink (or,
> in the case of IERC shields, reducing the chassis temperature), fans
> can have a secondary effect.
Other than longevity, what we have is the glass temperature to measure
as indicative of the operating temperature of the tube. Again, we're
looking at 10-15 degree differences in glass temperature as _indicative_
of running temperature differences. Again, heat has to go somewhere and
once you get "your hands on it", grab it and toss it out the door. It's
more like the second step rather than secondary. Keeps the heat flow going.
I tried to find that study on the WWW, but no luck. Do you know where
it is?
Would be interesting to take some infrared photos of tubes in action --
can anyone do that?
regards,
Barry
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