Portrait of a Tube

Description of the 6072 (12AY7) tube

607212AY7 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 friend Roger Modjeski always pointed out, the best design results from using the tubes the INTENDED way. So not say, I need a gain of 25..50, so I could use 12AU7, 12AY7 or 12AT7. Only one of the three will work best.

With NOS tubes, you always what you pay is what you get. Those that are 'the best of all' or 'hard for find', are a lot more expensive. The price difference is alway 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. Even at just 2x the price, everybody would grab for the five stars * * * * Series. As a proof for sellers, price and demand is balanced when such items do not sell immediately for the asking price.

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:

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

He suggests the military just buys whatever brand needed on the tube. Now that is really silly. It is very confusing, people even write "Thank you, thank you". The author writes there, he is "not aware", and "as far as I know". Well yes I think so too. He is not aware, and he doesn't know: what JAN Specifications are. I won't explain it here in fine detail, but roughly it is like this: JAN forces 500 hours abusive testing on a certain number of samples from the same date code 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. The untested tubes from this lot, have a certain mathematically proven reliability level, which can be calculated. In reverse, with that reliability level, the probability on a failure, during a specific number of hours, can exactly be calculated. 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.

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 PROGRAM REQUIREMENTS
  1. Tubes have higher heater current, to avoid burn out
  2. Gold or silver plated grids
  3. Set up to withstand Shock and vibration
  4. Requires a separate factory for these tubes only.
  5. Requires special (no stress) pay system for the operators.
  6. Use thyratron welding.
  7. Inspection must be done by binocular microscope.
  8. Heater cycling on a daily sample base.
  9. 50 hours aging.
  10. Specification of end-of-life ratings.
FIVE STAR Construction REQUIREMENTS
  1. Bottom and top mica are double for better mechanical stability. (That is four micas)
  2. 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:
  3. 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.
  4. 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.
  5. 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.
  6. Gold or silver plated grid (to prevent grid current, and have better bias stability).
  7. Connect getter with two welding joints (vibration proof)
  8. Heater wires first mounted on a bar, and this bar is mounted to the tube bottom.
  9. Ends of the grids bars are polished, to prevent scratching / sawing of the mica. Such sawing would cause grid rattle and microphonics.
  10. Some more small details, unlisted here
FIVE STAR General considerations
  1. 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.
  2. 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.