Ampeg B25B Bass Head (Vintage)
The Ampeg B-25 is a 55W all-tube bass amp head released by Ampeg in 1968 and produced until the late 1970s. By 1968, Ampeg was firmly planted as a world class amp builder, having had massive success with their Portaflex “flip top” amplifier series, headlined by the B-15N which was the preferred amp of nearly every Motown act of the 60s. The B-25 was something of a “spiritual successor” to the B-15N, at least in terms of the circuit topology. While the B-15N featured a 6SL7-based preamp mated to a 2x6L6 30W power amp, the B-25 used the nowadays more familiar 12AX7 tube in its preamp but rare 7027A power tubes for a rated output of 55W, the same power amp design used in the V2 and VT-40 guitar amps of the era. While the preamp tubes are different between the B-25 and B-15N, the preamp design is almost the exact same, including identical tone stacks and a very small modification to the “Ultra Lo” switch. For all intents and purposes, the B-25 is the B-15N’s big brother. The vastly increased headroom makes the B-25 a bit less gritty than the B-15N, but both feature a very similar warm, punchy sound that is unmistakably Ampeg.
Designed for a 16 ohm speaker load, the amp has a strange hard-wired 4-pin XLR speaker cable as well as a 1/4″ Extension Speaker jack. Using just the extension speaker jack switches the amp to the 8 ohm transformer tap, while the 4-pin XLR (pinout unknown) would be for use with a 16 ohm cabinet, like the original Ampeg 2×15 cabinets of the era.