The ECC83S is a modern re-working of the classic 1954 double triode design and is essentially a direct equivalent.
The anode is made along high frequency lines in that the central (active) anode region is placed close to the grid with the extra metal further away. The darker area in the image is where the pressing forms the section that is close to the grid. Note also the use of double micas. In the past this technique was employed to reduce the damage caused by vibration.
The robust construction, internal screen and shaped anode is in direct contrast to the original ECC83 shown below.
The original ECC83.
The shaped anode and internal screens.
The thin glass tube envelope is 21 mm in diameter, and excluding the B9A base pins is 51 mm tall.