1970 AMPEG B-15N

Possibly the most famous bass amp of all time (other than the SVT) The Ampeg B-15 is renowned for it’s all tube tone and medium power output (30 Watts for this model, although other model numbers ranged from 25-50 watts). This amp came in with a blown CTS branded 15” speaker and was in need of a full electronic restoration. The speaker was replaced with an Eminence Beta 15A, and while technically a PA speaker it is the number one recommendation for replacing the square magnet CTS that was original to this amp. The frequency response is as close as possible but with an upgraded power rating of 300 watts. The original CTS speakers are very prone to blowing.

The amp was given a full cap job including the electrolytic cap can, the chassis mounted “fire-cracker” cap, board mounted electrolytic’s, and all the paper/oil signal caps. The carbon comp power resistors were replaced with upgraded metal oxide 2 watt type resistors. The 3 prong power cord was re-stripped and wired in again as the previous tech botched the job - the ground wire was wired short, taught and with a lot of tension pulling on it instead of given a long length compared to the positive/neutral wires. The death cap was deleted (it is a mistake to leave these in place once a 3 prong power cord is installed) rendering the polarity switch non-functional.

Two new NOS 6SL7 preamp tubes were installed (National Electric/RCA brands), and those incredible black-plate RCA 6L6GC’s were Fixed Biased to a factory spec correct %50 Class AB Plate Dissipation with a B+ of 484VDC. Remember this amp is supposed to only put out a very conservative 30 watts of power as that provides maximum clean headroom for bass. Some tech’s feel compelled to bias always to %70 (Class AB Fixed Bias) but that would be a mistake here as the transformers are not rated for that - it would also cause unwanted distortion. The amp was cleaned and all hardware tightened.