IERC heat shields...in my Maverick now!

[212Mavguy]

Recently I won an auction on ebay for a military gizmo containing a dozen Bendix 6094's, two CBS 5751's, and a CBS 5814a. Each of these tubes were covered by an IERC heat shield. I scavenged them and the longer ones are a perfect fit for el84's, a very hot running tube. So I stuck a quad on the power tubes of my Maverick, specifically Russian mil spec 5000 hour rated Panno 6p14p-ev, that amp typically runs el84's at a very hot bias. One of the shorter ones worked perfectly for the 12ax7 position in my Frank-en-Champ. In a couple days I will have a Bedrock 651 1/12 boutique amp arriving, it runs a quad of el84's and I will use the shields on it as well. I found this article on the tube shields online...and I apologize in advance for the article length, but in it are a few really nice pearls...like the figures on a 6aq5 tube cited in it, not much difference between one of those and an el84!

HEAT DISSIPATING TUBE SHIELDS


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.

 

[fdesalvo]

Thanks so much for this. Heat is indeed the worst enemy of any electronic device- any device that allows them to run cooler is beneficial.

 

[LerxstLee]

Just a thought mav, I am sure you know much more than I do on this matter yet there are
those who, esp, audiophiles with hifi tube equipment that say they will not use tube coolers
as it takes away from the proper operating temperature, along with their claim they can hear
the difference if their tubes are cooled. I am one to believe there are limits to what people can and
cannot hear.
Yet, most would agree to get a great listen to a tube amp,
you should let those tubes heat up before you judge them.
I am certian cooling tubes would make them last longer, yet I do not believe the military had
any concern on how those tubes sounded. They only cared that they were reliable and they lasted.
I am not trying to mess with you. EL84s run hot. They are usually cathode biased which by
design makes tubes run hotter. If they are not cathode biased they probably have 400 volts on the plate
and -40 volts on the cathode, that is above the MPD .I understand some of the new tubes reliabilty maybe attributed to overheating,but they are probably being ran beyond their design.
My simple point in my opinion is, I would rather get 70% of the tubes life as opposed to 100% or more,
if they sound better. Just my thought.
/cheers- sunglasses engaged.

 

[212Mavguy]

Your thought is a well pondered one...and that is good! But we're nut running HIFI amps, are we? Apples and oranges, guitar amp needs and HIFI amp needs.

The glass bottle gets hot as a result of radiation more than convection and conduction of heat, there is a vacuum inside the bottle, so convection is pretty nonexistent, conduction is blocked for the same reason, that leaves radiation...that heats up the bottle glass from the plates...and the glass becomes more porous as a result of heat than at cool temps...that can allow vacuum to deteriorate over time...and if the glass gets really hot than it might absorb less heat from radiation and start reflecting heat back to the parts that are getting hot in the first place...inside the bottle, possibly increasing their operating temperatures more than if heat were allowed to radiate outward instead. That is what happened under the JAN shields.

The tube coolers absorb heat from the glass bottle, allowing that radiating process from the guts of the tube through the vacuum, through the glass, to the coolers and surrounding air. This process allows excess heat to dissipate and escape rather than being reflaced back from an overheated bottle to the heat sources, remember that cathodes, grids, and plates get pretty get hot anyway, they run in a vaccum...the glass gets most of the benefit from the coolers. The glass protects the parts inside, so if the glass gets hot and become more porous on a molecular level, then vacuum quality deteriorates over time to the direct detriment of the tube's operational lifespan. This is brought up in papers from the Bendix Red Bank company, builder of the highest quality and overbuilt vacuum tubes ever made on the planet. They had to build their tubes the way the did because in the extremely high altitudes that ICBM's operate at traditional radio and audio tube designs readily overheated and failed.

Careful re-reading of the post suggests that the coolers reduce bottle heat by a less than huge percentage amount, like less than 30-35 % of the bottle temps, yet the increase in tube life is more than the percentage you suggest in your post, the sample size is large, enough to be statistically significant. That's why I bolded the text in the original post..."It seems "small decreases in bulb temperatures often result in seemingly disproportionately large increases in tube life"."

I can attest that in my limited experiences there is absolutely no difference in the tones I am getting in my amps, the parts that the tone come from are still getting hot enough...the Panno 6p14p-ev's in my Mav are biased at what is most likely near MPD...tube rec side of the SS/ Tube rectification switch yields no plate glow, but using the SS rectifier side of the switch yields the faintest of red glow from the plates. I repeat, NO DIFFERENCES IN TONES! It still sounds different in a good way 15-30 minutes after adding b+ to the powr tubes just like before the coolers were installed in every one of the amps that I own that I have the heat shields installed in... I also have them in a class AB biased Bedrock 651 running a quad of el84's. Same thing, NO TONE DIFFERENCE! and in my single ended hot running class A cathode biased Frank-en-Champ, whether running el34 or vintage 5881/6l6wgb, NO TONAL DIFFERENCE! In my Bendix 6094 power section Toad Suck Tones 6094 custom boutique amp biased at 75% NO TONAL DIFFERENCE! By protecting the glass bottle from excess heat you protect the guts of the tube where the tone comes from in ther first place, the guts still get very, very hot like before, but not excessively hot!

Right now there are IERC tube shields offered on eBay that are for power tubes, they fit and work on el34's and also work well fitting over my vintage Tung Sol 5881's, my Frank-en-Champ gets 420-ish plate volts with a Bendix 6106 (5y3) rec tube.

For $3.50 each plus shipping they are cheap insurance compared to the cost of scarce vintage el34's and vintage 5881's comparing to the cost of mostly inferior sounding new production tubes in the same designations and bottle diameters. In my limited experience, there is nothing tonally to lose in a guitar amp using IERC heat shields either for 12__7's or power tubes that they fit.

 

[LerxstLee]

I was hoping the old apples and oranges would not start here but,
We sure are not running hifi amps, yet it seems some players are after tubes that would make most hifi
players more than happy. That is where I guess my hesitation starts. Some of the mullards, tele, and amperex
probably do justice to any guitar amp, but no where near the justice you could expect from a hifi amp.
That said, seeing the hifi tubes going into guitar amps, still would at least justify the point of cooling a tube
to save the life over the sound. To the point of apples and oranges, why are people putting oranges where apples
belong, better said why are they using diamonds where cubic Z would work as well, while paying up to more than 10Xs the price in hopes of getting a better sound.
My percent of life had nothing to do with the article it was an of the cuff example.
I would rather get 50% the life out of a tube if that tube sounded better.
/cheers

 

[212Mavguy]

Well said! Got your point about your off the cuff remark on tube life... :wink:

From my viewpoint top end tubes have a noticeable sonic difference in my amps or I would not have spent the money to investigate their sonics for myself. I don't think that using top end hifi tubes in a guitar amp is necessarily like putting a diamond ring in a pig's snout, but it sure seems like it happens, huh? Depends on the circuit, speakers, and transformers in the mix.

My Siegmund Midnight Blues Breaker head has a Radiospares type output transformer that is of HIFI quality, and the circuit has lots of hifi quality PIO caps in it. Cleans out of that amp have the detail within each note that reminds me of a fine quality piano. My Toad Suck Tones one of a kind boutique amps also have very nice parts and OT's, and the Bendix tubes they run pretty much kill any hifi type tubes in similar families for sonic quality.

The trend in high end boutique amps like Siegmund and Alessandro is to use very hifi type and quality components. For instance, Alessandro amps use Jensen copper foil PIO caps and have upgrades to Black Gate caps, as well as super top end pots, and pure silver hookup wiring is available as well, he uses Magnequest transformers in his high end amps, very top shelf HIFI oriented trannys for sure. The difference is that most tube boutique guitar amp circuits are designed to provide musical sounding distortion, while HIFI amps use circuits designed to sound musical without distorting... That is where the apples are different from the oranges, but they both have lots of sweet tastes in them...don't they!

But frankly we all know that I'm a bit bananas anyway! :lol:

peace.

 

[LerxstLee]

Only can agree with that. With at least 3k in NOS and VOS tubes all with test marks, I use my best
where I need them. I can but the finest in my mesa, I hear a difference, but not worth using my best
tubes on. Yet something simple like my '60s ampeg jet, Let the '60s sylvainias and rca's go to work.
If I retube that with new tubes, I am not happy with the sound. Black plate rca 6v6s, and even most
sylvanias sound great in it. It is cathode biased and runs the tubes hot, yet tubes seem to last forever in it.
If I grab a set of new bung sols, I will get less that 25% of the life a nice NOS tube from the '60s. Same with
many others, JJs, last a very long time, yet new tube tone. I will gladly sell matched P.I.s, they fetch an extra
5 bucks, yet with most amps using resistors with 20% I find it of little if any use over a proper testing tube with
a small variance in the test. I always use them in hifi, yet they even have 10% if not more tolerance issues.
If I had one of the first vox amps, the ones that set fires, I would surely use some form of heat shields, the nice ones
you scored or even a fan. No argument here, at least we agree that NOS sound better than new makes. It is
simply a difference of the degree is my thought. Keep up your good work. As in a previous post, I am retiring my road king for a Laney that I could not be happier with. At $900 bucks 60% of list, with the features and sound that this amp has,
made in the uk, into the storage room goes the road king, perhaps time will tell, but with all the experimenting I have
done the clean channel has never really made me smile with the road king , I could not pass this up, it has been a while since I owned an laney and the clean channel is just as the reviews said, absolutely stunning this one is making playing music more enjoyable thus far. I will be around but probably for most thankfully not much.
Best to you
/cheers