The 6688 (European: E180F) is a miniature high-slope VHF/UHF pentode tube with a B9A (Noval) miniature 9-pin base, primarily intended "for use in wideband amplifiers in telephone carrier systems, radar equipment and measuring equipment".
It is used in:
- the staircase generator circuit of the 3T77
- the P80 cathode-follower probe
- TR-4602, Hungarian 551 clone
Tek part number: 154-0215-00
P80 probe internal view