Phono preamp tube rolling.

@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.

@lowrider57 

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

You right about passing through quality control… but too optimistic about highest specs…all Soviet NOS has to be carefully checked before using…we have to be specifically careful with tubes made in USSR in 80s - 90s cause quite control of this period was much lower than 60s - 70s. 

@surfmuz , you are so right. I  mentioned on another tube thread that I'm done with Melz. I purchased a pair of 1578 OTK from a dealer in Russia, 30 day returns. I loved them, ordered 2 more pairs. Long story short, 4 tubes had crackling issues, the remaining pair will reside on my shelf. I'll save them for a rainy day.

I found out later on a different forum about the poor QC after 1970s. Mine are 1980 vintage and can be restored by resoldering the connections inside the pins. A few owners who also bought the 1980s have done it successfully.