The Ampeg Portaflex series of bass amps are iconic bass amps for the studio. They are found in many, if not most, of the top studios in the world and are also popular for gigs that do not require a high powered amp.
This review concerns the original tube models. There was a solid state version of the B-15 that is not covered here.
The classic amp in the series is the B-15N. 30 watts from 2x6L6GC tubes, 3X6SL7 (preamp), 2 channels with treble, bass, and volume controls, 1x15" speaker (Altec 421A optional). No frills, just good sound. There were more B-15Ns made than any other Ampeg amplifier.
The flagship of the series was the B-18N (the model I currently own, although I've also owned a number of B-15s). Same controls, 60 watts from 2x7027 power tubes, 2x6SL7, 1x7199 preamp, 1x18" speaker.
The junior member was the SB-12 with only one channel, 25 watts (power tubes were 7591 or 7868 on older versions, newer versions used 6L6GCs), 2x12AX7 preamp, 1x12" speaker.
All versions used the 5AR4 tube rectifier.
All these are excellent bass amps for the studio. The notable visual characteristic of these amps was the "flip-top" design, in which the amp was mounted on a board that formed the top of the speaker cabinet and flipped over inside the box for transportation or storage, making a quite compact package. In use the chassis with exposed tube cage sat on top of the cabinets with the glowing tubes (and edge lit plexiglass name plate) cutting a very striking image.
These amps are also the first I know of to incorporate a line out jack so an additional amp may be added for more power.
There were also 2x12, 6x10, and 1x18+horn guitar amp versions made with reverb and tremolo, but these are quite rare.
Sadly, the current owners of the Ampeg brand decided that these amps were too costly to produce and they're no longer made. The last new (reissue) B-15N was sold about a year ago by a dealer in San Francisco for significantly more than the list price.