[R-390] New owner with (probably dumb) questions
Barry Hauser
[email protected]
Mon, 21 Apr 2003 13:06:06 -0400
The nasty metal is beryllium copper. Like the meters, it's OK to handle,
but not to eat. Seriously though, don't grind/sand it or cause to become
airborne dust.
There are two main aspects of the IERC shields -- black surfaces to reduce
reflection and heat sinking. The shields themselves are coated with
something special or black anodized. Even so, you'll notice that the lower
1/2 inch is shiny/uncoated on the inside, so as to make a good thermal
connection to the socket mount. The tube mostly "sees" the heat sink insert
which is blackened. It's actually better if the inside of the shield itself
is bare to maintain good thermal coupling to the insert. It should not be
heavily coated.
I've seed 3 major styles of heat sink inserts -- simple 5-or 6-sided
cylinders, the "finger stock" style with lots of springy contacts stamped
into the insert, and the pleated type, which is a kind of accordion fold.
I'd imagine these vary in terms of effectiveness of heat-sinking vs.
ventilation, with a trade-off. The simplest 5-sided ones allow maximum
direct cooling -- vertical airflow, but minimize the area contact between
the glass and the heat sink. The finger-type improve on the thermal
coupling and still allow for a lot of convection. The accordion pleated
type maximizes the glass to heatsink area, but might limit direct convection
cooling from the glass. However, the pleats run vertically and there's
probably a good deal of convection through the insert as well as heat
conducted away through the metal to the shield.
There's a another variation on the design of the shields themselves. Most
shiny shields have a substantial flange which retains the spring at the top.
This reduce the top opening and probably affecting convection cooling
somewhat. The IERC's usually don't have springs and there is typically a
minimum kerf on the top so it maintains it's shape and the insert has
something to back up against. I've seen some without the kerf, with small
stampings to retain the insert.
Lately, I've come across quite a few shiny shields with the pleated type of
insert. I would imagine that performance is nearly the same as an IERC
type. So, look inside before you cast them aside. I don't know if they
were originally equipped that way or upgraded after the fact.
As for painting -- particularly with barbeque or engine paint -- I don't
think that's a good idea. Regular coatings serve as a thermal insulator, so
the whole shield would tend to retain more heat inside. At minimum, mask
off the bottom inside surface. Best to use some other blackening method,
like gun bluing. Anyone ever try that?
Barry
----- Original Message -----
From: "blw" <[email protected]>
To: "R-390 List" <[email protected]>
Sent: Monday, April 21, 2003 11:32 AM
Subject: Re: [R-390] New owner with (probably dumb) questions
> Found that older reference to tube shields. This was a long thread in 1999
> and 2000. Maybe someone has the info on the pros/cons of cutting on the
tube
> shields??? Wasn't that about berylium, or some other nasty metal? I need
> more coffee before I finish beating horses.
>
> Barry
>
>
> >Bill, KD0HG wrote:
> >
> > I've received several inquiries regarding my post here concerning
> > IERC tube shields and in response, here's a quick elaboration.
> > In ER #66 for October, '94, I described the results of a 1954 ARINC
> > [Aeronautical Radio, Inc] and GE tube life studies that were done using
over
> > 150,000 tubes of 20 different types.
> > I provided summaries and graphs of tube life vs. envelope temperature
> > for several representative tube types- here are a couple:
> >
> > 5654/6AK5W, 200 tube random lots
> > The 80% survival of this type was
> > 750 hours @ 192 C
> > 1500 hours @ 125 C
> > 2000 hours @ 100 C
> >
> > 6005/6AQ5W
> > Over 95% of the 6AQ5 survived 5000 hours of operation
> > at an envelope temperature of 220 C, but at only 17 degrees
> > hotter fewer than 70% survived even 2500 hours.
> >
> > Another chart in the article shows the measured envelope
> > temperature of a 12BY7A dissipating 10 watts [including heater].
> > Under still air conditions, at room temperature, worst to best,
> > >220 C inside a shiny JAN tube shield
> > 190 C inside a black anodized JAN tube shield
> > 165 C inside a black JAN shield with ventilation slots
> > 150 C bare bulb, no shield
> > 82 C inside an IERC T6-1025H shield/cooler
> >
> > The results of these life tests indicate a bare bulb is the way
> > to go for reliability's sake, unless one uses the IERC coolers.
> > Of course, electrical considerations often mandate
> > the electrical isolation of a shield.
> >
> > I once read a reference that around 1960, the shiny tube
> > shields were removed from the military's QPL listings as a
> > consequence of these earlier studies. This does not agree
> > with my observation that many pieces of military gear from
> > around that time are full of the shiny type of shields, so I don't
> > know if this is really true.
> >
> > As of the article's press date in '94, IERC was still in
> > business and making tube shields and coolers. I don't know about
> > today.
> > IERC
> > 135 W. Magnolia Blvd.
> > Burbank, CA 91507
> >
> > Best,
> >
> > -BK
> > Berthoud, Colo.
>
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