[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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