Portrait of a Tube

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Description of the 6072 (12AY7) tube

6072 12AY7 is a not so well known member of the 12A..7 series. Sorted by gain here, this family of tubes, 12AU7, 12AY7, 12AT7, 12AX7 are double triodes, each specialised on a special function. As my late friend Roger Modjeski always pointed out, the best design results from using the tubes the INTENDED way. Roger based his entire product portfolio on this philosophy, being contrary to what newcomers do. Some think the more "interesting" a circuit looks, the better it sounds. Only to find out, they get quality problems with the tubes.

So not say, I need a gain of 25...50, so I could use 12AU7, 12AY7 or 12AT7. This is not so! Only one of the three will work best.

NOS tubes that are 'the best of all' or 'hard for find', are a lot more expensive. The price difference is always such, that it balances the market. It can be no other way.

Like when only 0.1% of the NOS tubes is five stars, that is one out of 1000 tubes, and so they may as well be 10x more expensive. At just 10% price adder, everybody would grab for the five stars * * ** * Series. Probably at 2x the price as well. That is why such are 10x more expensive rather. As a proof for sellers, price and demand is balanced when such items do not sell immediately for the asking price. If you do not know, what exactly is the five star series, the WRONG place to look is the internet talk. The right place to look, is the manufacturer data sheet. Believe it or not... they put that on paper. So what it says there, is facts.

This is the situation with all 12A..7 tubes:

12AT7: Excellent standard tube. High gain, high plate current. A lot of NOS is for sale. Easy to find and not expensive. E81CC, 6201 are the better versions, with lower noise, longer life, some have gold pins etc. They may easily cost 3x more. Though noise is perhaps just a little bit less and not 3x less. ECC801S is a balanced tube, per data sheet. And only 1 of 10000 NOS tubes may be an ECC801S. Very few are Telefunken, and not all Telefunken have a diamond.

With 12AX7 the situation is another. Not as many were made, but these were used often in applications for high gain, and along with gain comes always noise. So any low noise types, virgin tubes, finest brand, NOS, new in the box, these are really expensive.

12AY7 is the low noise tube, high gain tube from this family, and even less were made than 12AX7. At least 12AX7 was a commercial tube, but 12AY7 was found in few products only. Even rarer was the 6072. This may sound unexpected, but really, the best place to read about tubes, is such a boring document like a data sheet, and not forum babble.

6072. Let me quote from the RCA 6072 data sheet:

"For use in industrial and military applications, critical as to microphonics, and in which dependability is paramount".
"Shock Rating: 600g". Do you know what that means? It means an item of 50 gram, impacts with a weight of 30kg.
Fatigue rating. "2.5g applied for 100 hours with 25 Hz".
Heater Cycling: "2000 cycles". But not just that, they specify that at a heater if 7.5 Volts. So that is 19% (!!) too high.

CV4068 / 6158 / 13D3

CV4068 is the British military specification for an 6158 with extra testing. Already 6158 is a high reliability tube for critical applications. These tubes comes somewhere near to 6072. Pin connections are the same. Though this is not a direct replacement for 6072, some people use it as such on their own risk.

CV4068 British air force data sheet

Brimar 6158 data sheet

I read this mess on a forum: (Link here)

'Also, I am unaware of any additional testing (torture test, expected life measurements, whatever) that went into the military version. As far as I know, they are the exact same tube, just labelled JG-6072/JG-6072A/JAN 6072A. GE produced tubes for a number of different clients and the military was no different; they just put whatever brand needed to be printed on the tube. Again, if someone has documented information and not just internet hearsay that counters this, please post and correct me'.

So this talker suggests the military just buys whatever they can get, and brand it 6072 because that's what they do. Now that is really silly. Even more confusing, people even write "Thank you, thank you". The author writes there, he is "not aware", and "as far as he knows". Well yes I think so too. Indeed, he is not aware, and he is far from knowing it. I won't explain it here in fine detail, but roughly it is like this: JAN Specifications force 500 hours abusive testing on a certain number of samples from the same lot. Then, it requires a 90% survival rate of the tested samples, and after that comes a documented destruction of the abused tubes (date, quantity, operator), and the manufacturer must allow unannounced audits. These 90% untested tubes from this lot, have a certain mathematically proven failure rate, , which can be calculated from the 10% that were abused (and destructed). In reverse, with that failure rate under excessive conditions, the probability on NOT having a failure (=reliability), during a specific number of hours, and normal use, can exactly be calculated with a special formula and data tables. To my personal opinion no other program can replace JAN testing, because it gives the exactly that probability in percent. From this may result such requirements, to replace all tubes of vital electronics, after a given number of hours, even when the tubes are all still testing strong. JAN Testing was introduced by the USA Army and the Navy, in order to deal with tube defects in a predictable way. This method is still used today, with semi conductors.

From General Electric, I see on Ebay both GE6072 and GE6072 ***** can be found.

"Five stars" is a general quality program by General Electric.

FIVE STAR Construction REQUIREMENTS

Bottom and top mica are double for better mechanical stability. (That is four micas)
Additional getter shield mica at the top (That is the 5th mica) to keep the getter flash away from the functional mica. This mica can be clear (none eliminated) because it has no electrical function. Those double micas (nr 1...4) have more leakage by itself, because they are each two plates attached together. This defines the next requirement:
Anti leakage construction of mica (extra slots and holes). Which again is possible without compromising the stability, because each is made from two plates. The inner mica however has larger clearance holes, so only the outer mica touches the metal parts of the triode.
If the heater wire sticks out of the cathode sleeve, the isolation material must be electromechanically removed, to avoid migration of this material inside the tube. Alternatively, the ends can also be additionally plated. Here is an interesting picture of a special 6SN7 by General Electric for radar applications, showing this in great detail. (I have 8 pcs or so for sale!) WIth this 6SN7, this method results in less welds, and in the end higher reliability. Yet it requires removal of the coating. So the connecting end glows very bright. They might have done so with the five star tubes as well, but what is inside now is probably better.
Anode cage is shorter, to be more sturdy. (Forget the "long plate" forum babble). This also makes the grid shorter, for lowered microphonics as well.
Gold or silver plated grid (to prevent grid current, and have better bias stability).
Connect getter with two welding joints (vibration proof)
Heater wires first mounted on a bar, and this bar is mounted to the tube bottom.
Ends of the grids bars are polished, to prevent scratching / sawing of the mica. Such sawing would cause grid rattle and microphonics.
Some more small details, unlisted here

FIVE STAR General considerations

Not all tubes have all construction requirements. For instance, such that are meant for lower voltage will not have additional anti-leakage slots, because low voltage tubes don't have that kind of leakage anyway.
The tubes are dropped in hot water, to detect beginning glass cracks along the pins. Such tubes would basically crack the base diagonally, leading to a white getter.

It should be obvious, the 6072 five star, is an almost ideal candidate for RIAA amplifiers, or and microphone amplifiers. For such applications, users want to reduce the noise at almost any cost.

I Was send this tube just for my interest, for this web page, send to me by a customer from Hungary. By mistake he broke the glass. The tube inside is the same as GE five star.
	<----- YOU CAN CLICK ON EACH PICTURE, TO ENLARGE IT
	You can see, the bottom and top Mica are made by putting two standard pieces on each other.

	The top mica is a clear one, these have lower voltage per millimeter. It's function is only to keep getter flash away from the tube system. The mica parts that hold the plates together are treated with Aluminium Oxide, this makes them opaque, and this process increases the isolation properties. It is normal for most miniature tubes anyway, but what is nice here is to see both mica types here in one tube.

	The so called "pearls" are drops that are manufactured as such and nice of them these are melted into the glass base. Such glass bases were sold as "materials" by glass factories.
	View from the bottom

	Pull out the filaments. It is amazing how full the cathode sleeve is with those. I wonder how they get those in. They did not use the more expensive spiral filaments here.

	The grid looks like gold plated, I am not sure. It has a yellowish color, but not as yellow as gold. Also it has some blueish burn color on some wires.
	The Cathode coating is quite corny, probably fully normal since these are Macro pictures, you see anything. So I do not know how corny this is, or is not. A corny cathode is a source of electrical irregularities as well.
	Hopefully you like these pictures. It was fun making those.

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