Phono preamp tube rolling.

@mulveling 

Melz 1578 OTK, Russian military

The stamp of “OTK” is not about the brand or any tech spec… at old Soviet and Russian manufacturing “OTK” means Department of Technical Control passage. Usually checked and ready to ship products was stamped with OTK stamp. 

Totally agree about a Cap' change having a profound effect.
The Phonostage I am using has been auditioned in various circuits for the end design to be produced.
When the prototype was almost complete and identified as being a very attractive performer.
Curiosity was still with myself and a couple of Cap' and Resistor changes were encouraged to be undertaken that had been discussed and not as yet trialled.
A Brand change of a Copper Cap with a different value and a few Z Foils was a remarkable change and the one I have maintained in my owned model.     

@lowrider57
I have a very good friend, younger and with better ears than mine - he says the exact same things about that Soviet 1578 tube. Yes the vintage/make is supposedly very specific and very important. Something abut round plate holes too, etc. I’d like to try them someday but understand they will be very hard to find (unless I can pry a pair off my friend), and my main system is not currently married to 6SN7 tubes.

The OTK stamp meant it went through quality control and met the highest specs.

@mulveling
There are a couple dealers in Russia and Ukraine selling various Russian tubes. A 1578 stamped on the glass with the multiple holes on the supporting rod is the real deal. Many sellers call their tubes Melz 1578, you just have to see the internal structure to know they are not.