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 bad shape, it had been worked on by someone who left many things in disrepair and modded the speaker circuit incorrectly resulting in the entire bias circuit going up in flames. it needed a full restoration including a cap job, new power resistors, all new parts in the bias circuit, new speaker cable, new handle, and new preamp tubes.

The amp was fully restored and many botched efforts from previous repairs undone and corrected. An almost complete cap job was done excluding the CE brand 40/40/40uf 525v electrolytic cap can, which was kept from a previous repair - all associated parts and wiring to it were redone however. 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 and plates of the phase inverter - this was done due to heat damage and completely burned open 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. A new 4 pin XLR speaker cable was installed due to the original having been modded incorrectly. The death cap was deleted, rendering the polarity switch non-functional.

Some of the original tubes tested good and were left in place - the preamp tubes however were all replaced with NOS units. The tube lineup was: V1-V2 = NOS JAN 6SL7-WGT, V3 = NOS JAN 6U8A, V4-V5 = RCA 7027A Matched Pair, V6 = RCA GZ34/5AR4. The Fixed Bias for the RCA 7027A’s was set to %50 Class AB Plate Dissipation with a B+ of 510VDC. 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 broken handle on the filp-top lid was replaced with a Blue Faux Leather Ampeg style handle. The amp was cleaned, and all the hardware was tightened. Works great!