JJ/Tesla ECC83s / 12AX7 - Dual Triode, Spiral Filament
These tubes are immediately discernable from other 12AX7's. They have a construction that is similar to a frame-grid tube - similar to the Telefunken ECC803s. This construction makes them very rugged and reliable. It is very musical - rich with great harmonics.
These are the best preamp tubes I have ever used. I have bunch of expensive NOS tubes on the shelf that I replaced with these.