[R-390] IERC type tube shields
Barry Hauser
barry at hausernet.com
Sat Jul 3 07:23:44 EDT 2004
Hi Pete & list
While compiling this body of knowledge, you might want to add yet another
type of insert -- the pleated type of beryllium copper ones. I've seen
these in IERC shields and also in black or shiny conventional shields. Some
might be retrofits - depot, manufacturer or hobbiest.
There is some variation in the style of the shields themselves, even within
IERC ones. Some have a substantial rim at the top, others much thinner and
some have none at all -- made of a flat piece of metal rolled up and
crimped -- maybe spot welded, rather than cut and formed from tubular
stock.. They may have different model numbers, dunno. The wider the top
rim, the more convection is impeded.
I'd guess there are at least three main attributes that determine the heat
reduction effectiveness of the various inserts themselves -- percent of
surface contact area to the glass envelope and inner surface of the shield,
composition and mass of the insert and vertical airflow. It would seem --
using "eyeball geometry" -- that the five or six sided cylinder type would
be the worst, and the many-fingered and pleated type the best. The latter
makes for more contact area and mass, but the finger type may allow for
better airflow.
Another factor is how well the shield is heat-sunk to the chassis. Some of
the heat convects/rises up out of the tube shield, some gets conducted away
through the shield to the mounting base into the chassis. The best of the
IERC's are all black except for the inside bottom which is bare metal,
apparently machined or wirebrushed. Some black shields have the coating in
place where they mount up, so might reduce heat transfer.
All this works "as advertised" on the top half of an R-390, but not exactly
on the upside-down bottom half, I suppose.
One tip: If you have the conventional wide-rimmed shields with either the
five/six-sided cylinder or pleated insert, (or you're rolling your own) make
sure the insert is pushed a bit down from the top of the shield to let the
heat escape. This type of shield is usually missing the crimps in the sides
which keep the inserts in place vertically, so tend to ride up when the
shield is installed -- and fall out when removed.
Barry
----- Original Message -----
From: "pete wokoun, sr." <pwokoun at hotmail.com>
To: <r-390 at mailman.qth.net>
Sent: Saturday, July 03, 2004 3:35 AM
Subject: [R-390] IERC type tube shields
> I can provide this info in a .pdf file if anyone wants it that way.
> I'll soon stick it in my website for future reference:
> www.qsl.net/kh6grt
>
> More than you ever wanted to know about heat-dissipating tube
> shield mil specs...but just the item for those *HOT* 6BF5s in
> Collins equipment.
> (You may need to change your font type to a constant-spacing
> one like Courier for the tables to line up properly.)
>
> MIL SPEC HEAT-DISSIPATING TUBE SHIELDS
> by Pete Wokoun Sr., KH6GRT (6/2004)
>
> We all have heard the benefits of using International Electronic
> Research Corp (IERC) type heat-dissipating shields in the R390A
> and other equipments to reduce tube operating temperatures.
> However, I haven't seen any information on just how how
> much they actually reduce the temperatures. Collins did some
> temperature studies but I haven't been able to find a copy of
> their study, possibly called service bulletin 303. I don't know
> if that study included heat dissipating shields. Searching
> thru the mil specs that these shields were made to I finally
> found some definitive temperature reduction figures. The specs
> are all in degrees C; they have been converted to degrees F in
> this presentation.
>
> The mil spec heat-dissipating shields designated for retrofitting
> to existing equipment come from three mil specs: MIL-S-9372(USAF),
> MIL-S-19786(NAVY), and MIL-S-24251. These shields are designed
> to replace the shiny, nickel plated JAN types. Mil-S-9372 was an
> Air Force spec and MS24233, its mil standard for retrofit shields,
> was implemented January, 1958. MIL-S-19786 was a Navy spec and
> its amendment for retrofit shields was implemented May, 1964.
> Both these specs were cancelled in 1968 and replaced by mil spec
> MIL-S-24251 which covered all branches of the service and was
> implemented March, 1967. Shields made to any of these specs will
> have the mil spec part number on them. Here are those mil spec
> part numbers cross referenced to the well-known IERC numbers:
>
> SIZE IERC # MIL-S-9372 MIL-S-19786 MIL-S-24251
> ------------ ------ ---------- ----------- -----------
> Short 7 pin 5015B MS24233-1 S0761*V00 M24251/6-1
> Med 7 pin 5020B MS24233-2 S0762*V00 M24251/6-2
> Tall 7 pin 5025B MS24233-3 S0765*V00 M24251/6-3
> Short 9 pin 6015B MS24233-4 S0966*V00 M24251/6-4
> Med 9 pin 6020B MS24233-5 S0967*V00 M24251/6-5
> Tall 9 pin 6025B MS24233-6 S0968*V00 M24251/6-6
> Ex-Tall 9 pin 6027B MS24233-7 --- M24251/6-7
> *(X or C)
>
> All the above sizes except the short and ex-tall 9 pin ones are used
> in the R390A. You can get information on how many of which ones on
> many web sites. The IERC numbers are normally used when searching
> for these shields. If someone other than IERC made them, they may
> only have the mil spec number and some other model number. I have
> some made by Waterbury Pressed Metal Company (WPM in the table below)
> that are this way. One I have made by Cinch Connector Company does
> carry the IERC number. I found documentation that the Atlee Corp
> also may have produced some of these shields. Their different model
> numbers are noted in the table below and cross referenced to the
> IERC numbers:
>
> SIZE IERC # WPM # ATLEE #
> --------- ------ -------- --------
> Short 7 pin 5015B RS-215-1 A10041-1
> Med 7 pin 5020B RS-215-2 A10041-2
> Tall 7 pin 5025B RS-215-3 A10041-3
> Short 9 pin 6015B RS-216-1 A10042-1
> Med 9 pin 6020B RS-216-2 A10042-2
> Tall 9 pin 6025B RS-216-3 A10042-3
> Ex-Tall 9 pin 6027B -- ---
>
> BTW, I noticed the last two digits in the IERC number correspond
> to their height in decimal inches. For example, the 5015 is
> 1.5 inches high, 5025 is 2.5 inches high, etc. Anyone know if
> the 50 and 60 designate anything?
>
> Physically, from ones I have seen, the shield inserts (the part
> that contacts the tube) are of two types: a multi-sided cylinder
> (5-sided for 7 pin tubes and 6-sided for 9 pin tubes) or a round
> insert with a multitude of 1/16 inch fingers. I found both types
> on shields from both the -9372 and -24251 mil specs. The multi-sided
> inserts have an open top between the insert and outer shell whereas
> the mini-fingered insert has a top closed. I personally have not
> seen or heard about any shields that have the MIL-S-19786 markings.
>
> Shields made to MIL-S-9372(USAF) (MS24233) were qualified to
> reduce the surface temperature of a test 'slug' by 36 degrees F,
> minimum (a 10-11% reduction). The test 'slug' was an alumimum piece
> shaped like a tube with an internal heater and 3 imbedded
> thermocouples. This 'slug' was heated up to 338 to 356 degrees F
> when the shield was applied. The average reading for all
> thermocouples had to be at least 36 degrees F less than the starting
> temperature. How well this test 'slug' with its greater thermal
> mass related to actual tubes I don't know.
>
> Shields made to MIL-S-19786(NAVY) were qualified using an
> instrumented glass tube called a Thermion. Apparently these were
> tube-sized things containing a heater and thermocouples. It was
> heated to its test temperature when the shield was applied. The
> shields designated for retrofit service were only required to reduce
> the temperature of the thermion between 10 and 25% (symbol 'X' in
> the tables). However, the shields worked so well they were
> qualified to the next higher reduction of 25-38% (symbol 'C' in the
> tables). Specific temperatures for this spec are as follows:
>
> Bare Bulb Shield Temp Reduction (Minimum)
> MIL-S-19786 # Test Temp (X) 10-25% (C) 25-38%
> --------------- ------------- ------------- ------------
> S0761 (short 7) 293 degrees F 27- 65 deg F 65- 99 deg F
> S0762 (med 7) 437 degrees F 41-101 deg F 101-154 deg F
> S0765 (tall 7) 455 degrees F 43-106 deg F 106-161 deg F
> S0966 (short 9) 266 degrees F 23- 59 deg F 59- 89 deg F
> S0967 (med 9) 446 degrees F 41-104 deg F 104-157 deg F
> S0968 (tall 9) 347 degrees F 32- 79 deg F 79-120 deg F
>
> Note: The V00 in the -19786 mil part number refers to a
> vertically mounted shield with no separate base provided.
>
> Shields made to Mil-S-24251 were qualified using actual electron
> tubes. The temperatures were measured from a thermocouple imbedded
> into the test tube's glass at its hottest spot. The hot spot
> location was determined by temperature sensitive paints. Like in
> the previous specs, the test tube was heated to its test temperature
> when the shield was applied. The shield had to reduce the bulb
> temperature by at least the amount indicated in the following table:
>
> Bare Tube Shield Temperature
> MIL-S-24251 # Test Temperature Reduction (minimum)
> ---------------------- ---------------- -------------------
> M24251/6-1 (short 7) 239 degrees F 45 degrees F (19%)
> M24251/6-2 (med 7) 419 degrees F 72 degrees F (17%)
> M24251/6-3 (tall 7) 464 degrees F 81 degrees F (17%)
> M24251/6-4 (short 9) 266 degrees F 45 degrees F (17%)
> M24251/6-5 (med 9) 437 degrees F 99 degrees F (23%)
> M24251/6-6 (tall 9) 446 degrees F 81 degrees F (18%)
> M24251/6-7 (ex-tall 9) 455 degrees F 81 degrees F (18%)
>
> Typical tube operating temperatures I expect are somewhat less
> than these test temperatures which maximized tube dissipation.
> This would lead to somewhat less than the above temperature
> reductions in actual situations. However, I think these tests
> were closer to actual conditions than the 'slugs' and Thermions
> used in previous testing.
>
> The mil spec Mil-S-24251 remains in effect today. However,
> there are no products on its qualified products list. What
> that means is no one currently makes any of these shields
> because the military doesn't have a need for any. Personally,
> I think shields made to any of these mil spec are going to
> perform similiarly because they're not all that different
> from each other.
>
> There are other types of mil spec heat-dissipating shields even
> of improved design but they are not designated for general
> backfitting into existing equipments. These shields and their
> sockets were designed from the start as an integral part of their
> equipment. As such, significant quantities to use in other
> equipments are probably not available.
>
> So, what does all this mean? Here are my thoughts: These
> temperature reductions listed that the shields had to meet are
> all minimums so actual reductions cannot be determined.
> Physically these shields seem to remain pretty much unchanged
> throughout the years; it was the mil specs that were changing.
> And mil specs are sometimes written just to document what is
> normally used and available! From the mil spec 19786 qualified
> products list the manufacturers had test data that supported
> their products qualification of 25-38% reductions in bulb
> temperatures. This range also allowed them to meet the newer
> mil spec 24251 minimum reductions. So I would venture to say
> a typical bulb temperature reduction of 20-25% is realizable
> with the heat-dissipating shields. Having a temperature
> reduction figure only leads to a further question: By
> decreasing the operating temperature of a tube by some amount,
> how much improvement in tube life does this lead to? This
> becomes harder to answer than determining how much cooler the
> tube operates. But one can generalize by saying any increase
> in tube life by lowering bulb temperature is beneficial.
>
> The most informative article I was able to find on-line which
> related tube bulb temperatures to tube life was
> pearl_tube_coolers.pdf on the www.pearl-hifi.com website.
> Although much of the website borders on the more esoteric
> nuances of high-end audio, this paper presents some of the
> earlier works done by GE and IERC on tube temperatures and
> life spans that are difficult to find these days. An example
> from an IERC study in that article: a 6AQ5(6005) tube
> operating near maximum plate dissipation has a bare bulb
> temperature almost 460 degrees F. Enclosed in a bright JAN
> shield its bulb temperature rises to 600 degrees F. With an
> IERC type B cooler installed the bulb temperature drops to
> 365 degrees F. This is a 20% drop from its bare bulb
> temperature and an 39% drop from its JAN shield temperature.
> This related to a tube survival rate after 500 operating hours
> of 35% using no shield, to less than 5% using the JAN shield,
> to over 95% still working using the IERC type B cooler.
> In another example from a GE study: From a batch of 200
> 6AQ5(6005) tubes running at 502 degrees F, 15% were still
> operational after 2500 hours. A second batch running at
> 428 degrees F, 74 degrees cooler or about a 15% reduction in
> bulb temperature, still had 90% operational after 5000 hours.
> It seems "small decreases in bulb temperatures often result
> in seemingly disproportionately large increases in tube life".
> The article is also interesting in that it touches on other
> factors like filament voltage, forced air cooling, and
> temperature gradients that also have an influence on tube life.
>
>
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