[R-390] IERC type tube shields
pete wokoun, sr.
pwokoun at hotmail.com
Sat Jul 3 03:35:24 EDT 2004
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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