1966 Ampeg B-18N

The Ampeg B-18N was first introduced in 1963 as the larger and more powerful brother to the already very famous B-15N that had taken the bass world by storm. The B-18 comes with an unusual Cleveland brand 18” speaker that has thunderous low end available and makes the amp look massive. With 50 Watts of power it had almost twice the output of the B-15 - it did this by using the 7199 phase inverter which produced more gain to drive the 7027A power tubes. 7199 tubes are an interesting combination of a small signal pentode coupled with a 12AU7 like tridoe in the same glass envelope. Having a pentode line driver in the phase inverter allows much more voltage gain and also produces a different tone than a standard triode (like the B-15’s 6SL7) with more high end definition and 3-dimensional mids. The 7027A power tube is essentially a high grade 6L6GC with a slightly different pinout - this allowed Ampeg to drive these tubes harder resulting in more power. The B-18 has 3 input jacks going to two independent preamp channels - “Guitar/Bass” and “Instrument”, these each have different input impedances and gain providing a plethora of unique tones. The 6SL7 preamp circuits are essentially the same as the B-15 meaning that the PI and Power Section + the speaker choice is what sets these amps apart. Despite a cool circuit design and great tone the B-18 never gained the popularity of the B-15 and was discontinued in 1967. Famous users include: Roger Waters who used this amp on recordings for The Wall in 1979.

This amp came in to my shop in mostly original electronic condition. It already had the power cord and speaker cable replaced, along with a re-tube, but nothing else. This one needed a full restoration including a cap job, new power resistors, all new parts in the bias circuit, cleaning, and new preamp tube. This was the second 1966 Ampeg B-18N which had come in the shop in under two weeks. These are not that common, so I was grateful to be able to hear two fantastic examples back-to-back.

The amp was fully restored. A complete cap job was done including the CE brand 40/40/40uf 525v electrolytic cap can, which was replaced with a new USA made CE brand replica with upgraded voltage ratings for the filter section. The chassis mounted “fire-cracker” cap was replaced with a CE brand 25uf 800v unit. All of the paper/foil signal caps were replaced with Mallory brand 150 series polyester film capacitors. 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 2 Watt Metal Oxide and 1 Watt Carbon Film type resistors were installed in the bias circuits, plates of the phase inverter, and plates of the first preamp stage - this was done due to heat damage, noise, and drift of the original parts. The higher wattage resistors mean better reliability and a lower noise floor. A 1N4007 1A 1000v diode was installed in the bias circuit for the same reasons. The death cap was deleted, rendering the polarity switch non-functional. Two new #47 indicator lights were installed, and the sockets adjusted and cleaned, as they were not functioning upon arrival.

Most of the original tubes tested good and were left in place - the Phase Inverter tube however was replaced with an NOS unit. The tube lineup was: V1-V2 = JJ 6SL7, V3 = NOS JAN 6U8A, V4-V5 = JJ 7027A Matched Pair, V6 = JJ GZ34. The Fixed Bias for the JJ 7027A’s was set to %70 Class AB Plate Dissipation with a B+ of 543VDC. The 6U8A is basically the same tube as the 7199 but with a different pinout. This was chosen because production of the 7199 stopped 40 years ago and almost all of the NOS supply has been used up. This has resulted in very high prices for extant examples, and very low quality of the remaining tubes, having been picked over for decades. An adapter was installed allowing plug-and-play for the high quality JAN (Joint Army Navy) 6U8A of which there remains a large supply of military grade quality tubes. The amp was cleaned, and all the hardware was tightened. Sounds good for either bass or guitar!