1966 Ampeg B-15N

The amp that many people associate with James Jamerson and his Motown/LA Session sound, the B-15 is an all time favorite tube bass amp. Released in 1960 by The Ampeg Company Inc. it quickly took the bass world by storm and became a staple seen on TV performances (like Dick Cavitt) and heard on tons of famous recordings. The B-15 was initially designed in 1958 by Jess Oliver, who is perhaps more well known as the man that put the Spring Reverb effect into guitar amplifiers for the first time. It has a preamp section made with all Octal 6SL7 type tubes, an output section with a pair of 6L6GC power tubes at it’s heart, and is rounded out by a 5AR4 rectifier tube that gives it just a slight amount of sag when pushed. Early models had Cathode Bias and were hand-wired on eyelet boards, later models went on to PCB construction and Fixed Bias output sections (like the one we have here). These amps are notable for their carefully engineered double baffle cab design, the “Portaflex” hide-away head that tucks into the cab during transport, and of course the large single Jensen or CTS brand 15” speaker that they are paired with. Notable Players include: Rick Danko of The Band, John Paul Jones of Led Zepplin (recording mostly), Andrew “Blueblood” McMahon who played with Howlin’ Wolf in his later years, Duck Dunn, and Chuck Rainey.

This amp came in to my shop having had been worked on quite a lot in the past. The amp already had a complete cap job (both electrolytic and film), re-tube, and a power transformer that was restored by Mercury Magnetics. The cap job used a cheap counterfeit part however which ham-stringed its performance (lots of blown fuses), and a good amount of preventative maintenance was left undone. This one needed a partial restoration including a partial electrolytic cap job in the filter/bias section, new power/misc resistors, re-wiring of the heater circuit/adding an artificial center-tap, new half wave rectifier diode, and a new speaker.

A partial cap job was done excluding the CE brand 40/40/40uf 525v electrolytic cap can, and the lone bypass cap, which were kept from a previous repair - all associated parts and wiring to the cap can were redone however. The chassis mounted “fire-cracker” cap, which is supposed to have at least a 600VDC rating had been replaced with a MIEC brand electrolytic, these are counterfeit or fake parts that despite having a 600VDC rating on the sleeve are actually a low quality 450VDC rated capacitor which is extremely under-rated for this position. Amp techs and amateur builders take note, do not use MIEC branded parts! This is the third or fourth time I have seen this, and you can tell they are not rated at 600VDC because of their small size, smaller than standard 500VDC electrolytics available everywhere. This was replaced with a CE brand 25uf 800v “fire-cracker” unit which are specially made as replacements for Ampeg amps. All other electrolytics used were CE brand with upgraded temperature and voltage ratings. The carbon comp and wire-wound power resistors were replaced with upgraded 2 Watt Metal Oxide and 10 Watt Wire-Wound type resistors respectively. New 5 Watt Cement, 2 Watt Metal Oxide, 2 Watt Metal Film and 1 Watt Carbon Film type resistors were used to rebuild the bias circuit in it's entirety - this was done due to heat damaged original parts. A 1N4007 1A 1000v diode was installed in the bias circuit for the same reasons. The higher wattage resistors mean better reliability and a lower noise floor. The death cap was deleted, rendering the polarity switch non-functional. There was no ground reference or center-tap for the heater lines so an artificial center tap was installed using 2 Watt Metal Film resistors.

The customer wanted the speakers changed to a model that was more in line with what would have been in the amp originally but with a higher power rating. A new Eminence Beta 15A 15" 200 Watt Speaker (8 Ohm) was installed to replace the previous Peavey branded unit.

All of the original tubes tested good and were left in place. The tube lineup was: V1/V3 = SYLVANIA 6SL7, V2 = JJ 6SL7, V4-V5 = JJ 6L6GC Matched Pair, V6 = JJ GZ34. The Fixed Bias for the 6L6GC’s was set to a factory spec correct %50 Class AB Plate Dissipation with a B+ of 513VDC. The pots were sprayed out with De-Oxit and the amp was cleaned inside and out. All hardware was tightened. Amp sounds really good with the new Eminence speaker and it no longer blows fuses or suddenly shuts down when playing loud.