Many vintage Soviet valves have obscure OTK symbols on them. Sometimes these symbols are in blue or green paint, less often red or silver paint is used. Sometimes, but not always, after the letters OTK are numbers 5, 7,9, 11, 71 or others.
Sellers of valves on Ebay take advantage of the lack of information from Western buyers and call OTK valves military and on this basis increase the selling price.
But is it true? Let's look into it.
Abbreviation OTK is translated from Russian into English as "Technical Control Department".
A valve that passed quality control was labeled OTK. Very often, to save time, a single inscription could be applied to a paper label inside the package in the honeycomb box. If the valve was delivered in an individual paper box, the stamp could be placed on the cardboard box and not on the valve cylinder.
Valves labeled OTK approximate the Western standard of Industrial Grade. Immediately after quality control, 99.9% of such valves were good functional. However, these valves were not tested according to military standards and we can only speak about the average quality of such valves. The quality of valves without OTK and with OTK was approximately the same until the mid-80s. But after the beginning of Gorbachev's perestroika at the end of the 80's the quality of the valves fell and the quality of OTK-printed valves became much higher than the quality regular valves.
For military purposes in the USSR used valves passed the control of the military department. Such valves were stamped OTK-5, which corresponds to the military “5 acceptance (standart)” in the USSR. These valves are much better than the OTK stamped valves and undoubtedly indicate the original military purpose of the valves.
After a valve from the manufacturer's factory with OTK-5 stamp was delivered to a military hardware factory, an incoming quality control was performed.
Such valves were stamped with a diamond-shaped stamp with a small inscription ВП (russian) and an asterisk.
Of course, not all "military" valves have such a rhombus, but the presence of a rhombus indicates a high level of testing valves.
If a military valve came to the warehouse of a military plant but was not used immediately or during the first year of storage, after a few years such valves were additionally tested and stamped with the date (in Russian, ПЕРЕВЕРЕН).
Very often for military purposes were used special varieties of valves, which were forbidden in the USSR to use in household equipment. Such valves were labeled after the name with the symbol B, E, DR. In 1970, the USSR state standard came out, which replaced the symbols B or E with the symbol EV. But while valves with the symbol B or E indicate a purebred military origin, valves with symbols EV were used for military equipment and for responsible industrial equipment.
Also, tubes with the DR index after the name were almost always used in military equipment. In the USSR there was a legal ban on the use of tubes with gold leads in civilian industry and household equipment.
But testing of valves according to military standards (OTK-5) did not satisfy the space and nuclear industry of the USSR.
Therefore, for the space industry, testing according to military acceptance standards OTK-9 was used. There is a difference in the quality of OTK-9 valves compared to OTK-5 valves.
Among the valves with OTK-9 symbols there are a lot of valves with 100-200 operating hours of anodic voltage pre-aging. Such valves have dark or mirror spots inside the cylinder and can be mistaken for used valves. This is especially noticeable for 6E5P, 6J5p, 6n6p, 5C4s, and other valves
For special purposes, for example for the use of valves in naval equipment, a special OTK mark was placed together with the ship's anchor symbol.
For the nuclear/power industry, the USSR produced special analogs of octal tubes. (with different tube names).
So the analog of the 6p6s valve was the 1515 valve (in a transparent cylinder!)
So the analog of 6n9s was tube 1579.
For the tube 6n8s was produced analog tube 1578
Note: the peculiarity of the valve 1578 was in the maximum symmetry of triodes, minimum microphone effect, smaller Ua max = 275V and smaller Pa max = 1,5W.
The same differences between conventional and nuclear tube modifications existed for all tube types.
OTK 7, 11, 71, 90.... stamps were put on tubes that had been tested in accordance with the requirements of a military industry, such as the Navy Submarine Fleet or the Air Force. But I don't know the names of branch customers and numbering after OTK symbols (7, 11, 71, 90 others), but undoubtedly these valves have better quality factory execution and tested according to more complex programs.
If you have any questions for me, please post in the comments. I will be happy to answer you.
Very often tubes for military equipment were painted with a bold dot on the cylinder head or painted with red or yellow paint on the top point (tip) of the cylinder rather than stamped OTK. This was done if the tubes were packed in honeycomb packaging.
The USSR produced dampers in tropical version intended for operation in friendly countries of Africa (Angola), Cuba, Asia (Vietnam, Korea, China, Bangladesh). Octal valves for tropical version always had brown color of carbolite base.
In 1967, the USSR began to use the "Quality Mark of the USSR". Tubes labeled with this mark met the highest standards of the USSR, but it is not necessarily the case that these tubes had a military purpose. In most cases, the "USSR Quality Mark" was put on products for household and industrial purposes.
I am sure that the main difference between tubes with OTK and tubes without OTK is reliability and durability, not the difference in sound.
In the last years of the USSR, it was very common for military warehouses to store tubes in bulk, because in the 1980s, tube production had all but stopped, and warehouses did not have enough space to store tubes in their factory packages.
If you see a tube with a shallow scratch on the glass, it was often stored in a crate in a warehouse for decades and this may indirectly indicate the industrial or military origin of the tube.
You can always buy a variety of tubes in my Ebay store. About 30% of the tubes sold in my store have OTK stamp:
https://www.ebay.com/str/kievchurchshop
I also mark tubes that have passed a particularly careful inspection with my own red labels.
If you want to get tubes that have been especially carefully inspected, please let me know in the comments of your order.
