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Gibson Post-WWII

Amps
 
Amps

Introduction
In its first 40 years of corporate rule, Orville Gibson’s lutherie developed into a manufacturing giant, expanding to meet the needs of mandolin orchestras popular before World War I, creating banjo and ukulele lines during the 1920s, enlarging the flat-top and archtop offerings following the depths of the depression, joining the fast-growing electric instrument market in the late ’30s and finally, embracing the violin maker’s family in the early ’40s. The acoustic line was made up of eight carved-top, six flat-top, two tenor, two Hawaiian and two gut-string guitars, five mandolins, one mandola, one mandocello, two sizes of ukuleles, 14 banjos, eight violins, two violas, two cellos and six bass viols. These joined by the electrics; two mandolins, three banjos, one tenor and three Spanish guitars, three Hawaiian lap steels, one doubleneck console and one pedal steel, plus three AC and two AC/DC amplifiers. If you include the Kalamazoo and Mastertone lines, there were close to 100 major items being built in the Michigan factory just prior to WWII.

During the war, the factory was transformed to produce military goods, employing more Kalamazoans and using most of the backup supplies, not to mention destroying or misplacing a great deal of important tooling. No wonder the owners sold out to Chicago Musical Instruments before they had to switch back to producing musical instruments. CMI, however, was confident in their purchase, funding an addition to the factory before being released from government work, and resuming instrument-making.

For all the promise Gibson represented to CMI, the new owners were extremely conservative in the breadth of the post-war line. Of the mandolin family (Gibson’s pride for years), only one A model survived, with no F styles, mandolas, or mandocellos available. Also excluded were banjos, ukuleles and the previously ballyhooed violin-family instruments. Guitars made up the rest of the acoustic line, with three good-sized, long-established archtops and three flat-tops (two dreadnaught-shaped and one Little Guitar) having to suffice; no Super Jumbo, tenor, gut-string or Hawaiian guitars for CMI.

The electric line wasn’t much bigger, although it was apparent CMI was in tune with the heightened demand for these. They produced a new catalog, two revamped Spanish guitars with laminated bodies and P-90 pickups, plus three new Hawaiian lap steel/amplifier sets, as well as the BR-6, BR-4, and BR-1/Ultratone. That was it for Gibson in 1946; nearly 85 percent of the instruments gone, the remaining acoustics basically unchanged, and the electric line receiving major changes.

Gone from the amps was pre-WWII builder Lyon & Healey models, and the long-standard tweed, replaced by stylish new coverings, grillcloths, handles and cabinets (rumored to have been designed by Barnes Reineke, who was responsible for the stylish new lap steels).

The initial post-war circuits are credited to bar pickup (a.k.a. Charlie Christian pickup) designer Walter Fuller, licensing basic circuits from Western Electric. Without getting too technical, let’s look at the late-’40s amplifiers from the mighty Gibson.

All the amps discussed here had rear-facing control panels on the backside of the cabinet, with all but one having the chassis mounted at the bottom. This is a world of field coil speakers, output transformers mounted to speakerframes, cathode-biased outputs and an interesting selection of metal tubes (don’t let this scare you away, Gibson used 6V6 and 6L6 power tubes, 5T4, 5U4, 5V4 and 5Y3 rectifiers, plus 6SC7 (high-mu twin triode), 6SJ7 (sharp cutoff pentode), 6SL7 (hi-mu twin triode), 6SN7 (medium-mu twin triode), 6SQ7 (twin diode, hi-mu triode) and 6J5 (medium-mutriode) tubes to perform preamp, phase inverter and tremolo oscillator functions).

Part One – Initial Series, ca. 1946

BR-1
At the top of Gibson’s post-war amplifier line was the short-lived BR-1, which featured a perforated aluminum grille with the prominent, new CMI “G” stencil. The brown leatherette cabinet was taller than it was wide and housed the chassis at the top – still a relatively novel concept in the world of guitar amps. Instead of the kitchen cabinet handles of the smaller amps, the BR-1 came with a heavy-duty leather handle similar to those Fender would eventually use on their tweed amps. A monstrous 12″ field coil speaker by Jensen mounted to both the front baffle board and to a wooden brace at the magnet structure. It’s difficult to tell whether this back brace was to help support the weight of the speaker or to stop acoustic vibrations. The back was sealed, with a large port at the bottom.

Getting to the tubes required removal of both the back panel and the chassis, a major design flaw. The steel chassis itself was covered in a three-dimensional, brown, textured paint and provided minimal air circulation through a small (approximately 11/2″ tall running across the back of the cabinet, another major design flaw) ventilation strip near the top. Only the controls section was exposed, and it was tipped up at a 45-degree angle for ease of viewing. A cream-colored aluminum panel contrasted nicely with the three large, brown knobs, each with a small, clear plastic/red lined pointer underneath (and seen only on this amp). Individual volume controls (for Microphone and Instruments) were followed by a master treble roll-off Tone. A vertical row of 1/4″ inputs to the left allowed for the use of a single mic and two instruments, while the vertically aligned pilot light, fuse cap and on/off switch to the right added symmetry to the layout.

Rated at 15 to 18 watts, the BR-1 was the successor to the Pre-WWII top of the line EH-185. Twin 6L6 power tubes were driven by a 6SN7 phase inverter, preceded by a 6SC7 preamp. One side of this twin triode preamp tube took care of the Instrument channel, while the other side was employed by the Mic channel, preceded by a 6SJ7. Power was supplied by a 5U4 rectifier.

At $190 (later dropped to $175), the BR-1 was considerably more expensive than Gibson’s second-in-line BR-4, not to mention top-of-the-line offerings from National ($125 for the No. 500), Rickenbacker ($115, M-12) and upstart Fender ($159.50, Professional). Only Epiphone’s Dreadnaught was comparably priced.

For collectors, this amp is a must to go with a single P-90 ES-300 or 350, a post-WWII L-5P with Charlie Christian pickup, or the fabulous Ultratone Hawaiian guitar, in the same way an EH-185 amp complements the ES-250 and pre-WWII ES-300, or the GA-50T goes with the two-pickup ES-350 and early ES-5, L-5CES and Super 400CES.

BR-4
While the BR-1 surpassed the best of the pre-WWII amps, the $125 BR-4 could easily pass for a late-’30s model. The two-tone brown/cream leatherette covering and swirly pattern cloth grille call up images of console radios, as do the twin wooden accent/protective strips over the speaker opening.

The catalog model was fitted with a kitchen cabinet handle, although some used the standard leather fare. Other pre-WWII characteristics include the thinly constructed upright cabinet, a field coil speaker (12″ Utah) and the bottom placement of the chassis.

Control panel layout was similar to the BR-1, with three 1/4″ inputs in a vertical line to the left of the mic volume, instrument volume and tone control knobs (black pointers). The pilot light, fuse and on/off switch added symmetry to the right. These electrical components were mounted directly to the smooth, black-lacquered chassis, without the use of an extra control panel plate. Twin 6V6 power tubes fed by a 5Y3 rectifier put out 12 to 14 watts, otherwise the circuit design was almost the same as the BR-1, save for a 6SL7 in place of the 6SC7 for the preamp tube.

BR-6
At the bottom of the Gibson amp line sat the BR-6, which, like the BR-4, harkened back to the floor radios of the ’30s. Three speaker openings on the front of the cabinet exposed the lovely pre-WWII-style grillcloth, contrasted by the smooth brown covering with decorative piping inserts. On the backside, a small panel at the top added structural integrity, as the BR-6 lacked the protective panels of the BR-1 and 4. This left the bottom-mounted chassis and tubes exposed to the dangers lurking in transporting an amp without a cover or case. The 10″ field coil speaker was also exposed and, along with the speakerframe-mounted output transformer, provided a total of seven wires with soldered connections outside the safety of the chassis.

A single volume control acted on the two instrument inputs to its left and right were the fuse and on/off switch. Missing from the larger amps were the pilot light and the separate mic input. This saved a tube in the preamp section, which used a twin triode 6SL7 (but could have gotten by with a single triode, since the input signal was split to feed each of the two grids, then combined following the two plates). The rest of the circuit was almost identical to the BR-4, using a 6SN7 phase inverter, twin 6V6s for power, and a 5Y3 rectifier. Output was specified at 8 to 10 watts, putting it out of the student model class and into the lower end of the professional market. Price was $77.50.

Part Two – First Series Additions, ca. 1947

GA-25
An all new amp was released in late 1947, to replace the mid-priced BR-4. At $132.50, the GA-25 was in the same price range for the short time both were available, but it exuded a definite post-WWII design. The cabinet was noticeably wider than it was tall, and completely lacked ornamentation, other than the new company logo. This isn’t to say the amp was unsightly. Rather, it projected a sleek, mid-century modern image, with uninterrupted surfaces and two speaker openings, of different sizes. Nothin’ but a box covered in “…rich brown Keratol” with a handle on top and two holes for the sound to get out… simplicity in industrial design. On the backside, the panels covering the top third and bottom third exaggerated the stretched out lines of the new cabinet, offering protection to (but without impeding ventilation for) the tubes, which stood straight up from the bottom-mounted chassis. Even the input jacks, control knobs, pilot light, fuse and on/off switch were in a single, horizontal plane.

Using two Jensen speakers of different sizes (12″ and 8″) was a novel approach, offering a little better high-end than a single 12″, without sacrificing much in the bass frequencies. These new speakers were permanent-magnet Jensens instead of the field coils used on all previous Gibson amps. One lost benefit of the field coil was the smoothing action on the DC plate current (ahead of the filter caps), which was conveniently used to charge the electromagnet. Without running the pulsating DC through the windings of the field coil, the addition of a choke (inductor) to supplement the filter caps was neccesitated (the choke is the little transformer-like device protruding from the bottom of most amp chassis and is basically one side of a transformer).

Just about everything else in the new circuit was different, too, starting with the inputs. Three instrument jacks were run into the 6SJ7 preamp tube formerly reserved for the mic input. A small difference in the value of the input resistors was now all that differentiated the mic input from the instrument inputs. Missing was the 6SL7 preamp tube that had added a second stage to the mic channel and made feasible a separate mic volume control. A single volume and a tone were now the only controls.

In the phase inverter section, the BR-4’s 6SN7 twin triode tube was replaced with a pair of 6J5 tubes, possibly so they wouldn’t have to call the GA-25 a five-tube model. The number of tubes used was often equated with quality, as seen in the BR-4’s original catalog description “6 tubes, two of which are dual purpose, providing the equivalent of 8 tube performance.” The GA-25 offered six-tube performance from six tubes, including the unchanged 5Y3. The twin 6V6 power tubes, however, yielded a “…full 15 watts” according to company literature and coupled to twin speakers, should have provided a louder amp.

BR-9
By 1948, CMI was expanding the Gibson line both up and down, with the new BR-9 lap steel and amp aimed at the huge student market for Hawaiian music.

The portable amp, originally available only with the instrument at $99.50 for the set, was a throwback to the pre-WWII circuits, having a field coil speaker (8″) and a transformer for the phase inverter. Twin inputs allowed for student/teacher sessions, with a single volume control the only variable. An on/off switch and fuse completed the rear-facing control panel layout.

Tubes included a 5Y3 rectifier, with a 6SN7 providing two gain stages, feeding twin 6V6s for a screaming little amp when compared to most single 6V6 student amps. The cabinet was covered in “…washable ivory leatherette accented by a brown plastic trim” and had a trapezoidal shape from both the front and side views.

The BR-9 amp stayed in the Gibson line until about 1955, when it was replaced by the slightly larger GA-9.

Part Three – Second Series, ca. 1948

BR-6 Style 2
By the end of 1948, the initial line of Gibson amplifiers was history. The mid-priced BR-6 stayed on in name and price only, with a new cabinet being the most obvious change. Like the GA-25, the new BR-6 had a stretched out shape, horizontally, almost as if they had chopped off the old box at the top speaker cutout, then stretched the bottom part out. Gone was the decorative piping and the anachronistic grille of patterned cloth, replaced by plain, monochromatic tan. New was a “G” monogram on a gold medallion.

A 10″ “dynamic” speaker replaced the old field coil and the twin-triode 6SL7 preamp tube (that had both sides run in parallel) was replaced by a single-circuit 6SJ7. Otherwise, not much was new. Although two inputs were specified, most of these amps seem to have three, all for instruments and electrically identical. In about 1955, the BR-6 was replaced by the larger GA-6.

GA-50
Replacing the BR-1 at the top of Gibson’s amp line, the GA-50 has become one of the most sought-after non-Fender vintage amps around, particularly in the jazz community. There were actually two models, the $175 GA-50 and the $195 GA-50T, among the first amps with tremolo.

Both versions used 6SJ7s for preamp functions, one for the mic channel and a second for the three instrument inputs. Twin 6L6 power tubes produced approximately 25 watts with juice coming from a 5V4 rectifier. A third 6SJ7 and a twin triode 6SL7 were used for the tremolo oscillator and related circuitry, which worked only on the instrument inputs. While the GA-50 used seven tubes to operate, the 50T needed eight, and to get by with only one extra tube required the change from twin 6J5s for the phase inverter on the 50, to a twin-triode 6SN7 on the 50T.

Like the GA-25, the 50 and 50T were equipped with two permanent-magnet Jensen speakers, a 12″ and an 8″, housed in a brown leatherette-covered cabinet. Whereas the speaker openings on the 25 were circular, with nothing but the grillcloth to protect the cones, the face of the 50 had five vertical slats for the 12″ and three for the 8″. A removable back panel gave the amp a Samsonite suitcase-like appearance. Removal of the back panel helped cool the tubes (which protrude up from the bottom-mounted chassis) during operation and offered protection when installed.

For the first time since the early EH-185 amps of the late ’30s, Gibson used separate bass and treble controls. A Gibson first (and possibly a first in the musical instrument amplifier field) was the inclusion of a grounded power cord, using a clip for ground connections instead of the three-prong plug (and outlet) that came into popularity in the ’60s and ’70s. Shock-mounted tube sockets were another Hi-Fi-influenced feature. Starting with the release of the GA-CB in about 1949, a series of other high-end Gibson amps kept the GA-50 dethroned from its original prestigious position until its retirement, circa 1954. All the other amps, however, were short-lived and expensive, making the GA-50 a good, long-running seller, generally obtainable today with a little searching. The catalog description calling the 50 the “ultimate in amplifiers” may have been a bit exaggerated, but the phrase “…produced to meet exacting demands for tone, power, portability and beauty” stand true to this day.

GA-30

Gibson’s midline amp was changed for the third time in three years, as the short-lived GA-25 (which replaced the short-lived BR-4) was superseded by the identically priced GA-30. Cosmetically, the porthole speaker openings were replaced with a large, rectangular picture frame-style with a Gibson logo across the middle.

The 1949 Electric Guitars catalog showed the GA-30 with a two-knobbed control panel, as on the 25 (this was a new photo, not a reprint of the 25 from the ’47-’48 catalog), although most GA-30s had separate volume controls for the mic and three instrument inputs. An extra 6SJ7 preamp tube was assigned to the mic input to accommodate the extra volume control, in turn requiring the twin 6J5 phase inverter of the 25 to be replaced with a single 6SC7 to maintain a six-tube chassis. The two 6V6 power tubes and 5Y3 rectifier were basically unchanged.

A bass “Tone Expander” switch inserted/bypassed a low-frequency blocking capacitor in the negative feedback loop of the Instrument channel circuit; on the two-knob version, the switch was located on the control panel between the volume and tone controls, on the three-knob version, the mic input was moved into the row of instrument inputs and the Tone Expander was installed directly below, in its place.

The deeply textured “dark brown leatherette” covering of the 25 and early GA-30s was short-lived, replaced after about a year by a smooth, light brown material. This model would run until the change to the more powerful top-mounted chassis two-tone model in 1954.

GA-CB
No, Gibson didn’t offer a citizen’s band radio. The CB stood for Custom Built, and at $395 (more than double the price of the GA-50T), it was out of the price range of all but the most professional of professionals (as in those making very good money). Only the L-5C and Super 400 carved top guitars cost more. Described by Gibson as having “…tremendous volume and tone qualities found only in the finest public address broadcasting systems,” the CB was obviously meant to be more than just a guitar amp.

A coaxial 15″ Jensen speaker offered a 50 to 15,000 Hz frequency range from the built-in tweeter and crossover network. A recessed four-position “Frequency Selector” switch was mounted to the back panel of the amp’s cabinet, offering roll-off points at 5,000, 7,500, 10,000 and 15,000Hz (the author assumes it was a roll-off and not a crossover point, as this range is too high for crossing over a cone speaker; unfortunately, the “dividing network” is only shown as a black box in the schematic).

Having the selector circuitry installed outside the chassis implies there was no interaction with the tone controls, although a working example could not be found to try out this feature; know anybody with one of these?

Compared to the full-range 10″, 12″ and 15″ speakers most travelling bands sang through (even into the late ’60s), the 1949 GA-CB was light years ahead of its time for portable PA. Oddly enough, the designers equipped the full frequency range combo with only one microphone input to go with the three instrument inputs. Gibson claimed “30 to 40 watts – Class A” power from two 6L6s, a possibly exaggerated figure, but a spec that showed a serious HiFi ethic from the engineers. The bottom-mounted chassis housed eight more tubes, a 5T4 rectifier, two 6J5s for the phase inverter, two 6SJ7s for the preamps (one each for the Mic and Instrument channels) and three tubes for the Instrument channel tremolo, two 6SQ7s and a 6SJ7. A foot switch activated the tremolo, which was adjustable for intensity and frequency. Separate controls over the Bass and Treble ranges effected both channels.

Mottled brown leatherette, as seen on the GA-50, covered the cabinet, and like the GA-50, the GA-CB had a detachable rear cover. The six cutouts for the speaker opening were also styled after the GA-50, with similar luggage qualities. The Gibson Custom Built Amplifier was offered on pricelists until 1953, although it was only pictured in the ’49 catalog.

The circa 1951 Electric Guitars catalog offered a brief mention of its availability and the circa ’53 and ’54-’55 catalogs made no mention at all of the most expensive and low production GA-CB.

It’s doubtful the “…years of electronic and acoustical research by Gibson engineers” that resulted in the GA-CB paid off directly, but the acquired technology would be put to use in the early ’50s on the more reasonably priced (but still expensive) GA-75, 77 and 90 amps. Gibson’s prestige over Epiphone, National, Rickenbacker, Vega and the up-and-coming Fender remained intact with an amp that provided “…discriminating professional artists truly superlative electronic sound reproduction.”



Gibson’s mid-1940s amplifier line, the BR-4, BR-1, and BR-6. Photo: Bob Fagan.

This article originally appeared in VG‘s Dec. ’97 issue. All copyrights are by the author and Vintage Guitar magazine. Unauthorized replication or use is strictly prohibited.

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