History of the Fender Bassman

High Times for Low-End
History of the Fender Bassman
’61 Bassman: VG Archive. This ’61 Bassman head-and-cab in blond Tolex marked a major change of format, but is also an enduring classic.

If they could have just one amplifier, many guitarists – from bar-room grinders to arena megastars – would choose a Fender Bassman. One of the most lauded and influential amps of any sort, for nearly 70 years it has been the go-to for players seeking robust, gutsy, dynamic, gigworthy tones. But, which Bassman? After all, there is a lot to love in the many circuits and formats that have worn the name.

A serious look at the birth of the bass amp begins with the instrument intended to plug into it – the electric bass. Fender showed serious consideration with the solidbody Precision introduced in late 1951; some music historians argue it had a bigger impact on popular music than either the Telecaster or Stratocaster. Being more amplifiable than preceding basses also meant it was more versatile, and just as Fender’s solidbody guitars strove to eliminate feedback and improve sustain, the Precision delivered punch and presence while enabling a fleeter, guitar-like playability. However, when it arrived, there was no dedicated amp with which to pair it; most often, the Fender Pro was called to service, its 15″ speaker initially believed adequate to propel the new instrument. But it soon became clear that bassists needed a dedicated amp.

’52 Bassman: Ron O’Keefe. This “TV front” ’52 has been revived with replacement tweed covering and grillecloth.
Its wiring is rudimentary, and the control panel hosts dual inputs, Volume, and Tone controls.

Outwardly, the first proper Bassman looked a lot like the Pro, with its similar-sized “TV front” cabinet. But, in most ways it was very different; though it used an octal 6SC7 preamp tube (like the Pro), it employed a 6SL7 phase inverter with 6L6 output tubes, and a 5U4 rectifier. With just a single channel, it needed one fewer preamp tube and carried single controls for Volume and Tone. These and the dual inputs were mounted on an upper panel connected by what collectors call the “umbilical cord” running from a socket to the lower chassis that housed the bulk of the electronics. The back was enclosed by a panel with two round ports – a clear intention to entice a fuller, more bass-heavy response from its 15″ Jensen.

A schematic exists for what is believed to be a first-iteration Bassman (circuit 5A6), but no examples of the amp have surfaced in this format. The diagram shows a circuit with two octal 6SJ7 tubes in the preamp, a 6N7 phase inverter, two inputs for one channel and one for the other, individual channel Volume knobs, a shared Tone control, and two output transformers feeding the two speakers. This is the exact configuration of the V-front Dual Professional from 1947, which seems to indicate someone doctored and re-labeled the schematic for that amp.

In ’53, the Bassman migrated to Fender’s new “wide-panel” tweed cabinet but retained its split-chassis construction even though it was the same physical size as the Pro and new Bandmaster (both of which were 1×15″ configurations). And while it seems odd that the Bassman didn’t share its siblings’ upper-mounted chassis, it did retain the ported back panel, which was good since there likely would have been heat issues from upper-mounted tubes suspended so close to that piece of plywood. As it was, though, heat from the bottom-mounted chassis was able to rise and partially vent through the ports.

A look at the back reveals its bottom-mounted chassis, 15″ Jensen, top-mounted control panel, and (when installed) dual-ported rear enclosure.

In any case, this short-lived back panel was seen as important enough to low-frequency reproduction as to warrant the odd constructional format. During the wide-panel years, the Bassman also transitioned from octal preamp tubes to more compact nine-pin tubes, using a 12AY7 in the first gain stages and a 12AX7 phase inverter.

’53 Bassman: Patrick Healey.

As the largest speaker commonly in use at the time, the 15″ Jensen would have seemed the obvious choice for full, deep bass. But, its large cone was prone to flubbing out when hit hard with a low E at high volumes, and the travel required of the underpowered voice coil proved problematic. Fender surmised that dividing the load between four smaller speakers would be the way forward; less individual cone surface – but more overall when used in multiples – required less voice-coil excursion, therefore applying less stress to each speaker.

This ’53 Bassman has the wide-panel tweed cabinet, still with top-mounted control panel.

This theory played out in the next incarnation of the Bassman, which not only introduced several upgrades to the platform, but launched the journey toward one of the most-revered tube amps ever created… for guitar.

Arriving in late ’54 (more substantially in ’55), the first 4×10″ Bassman appeared in Fender’s new “narrow-panel” combo, so named for the narrower upper and lower strips at the front to hold the speaker baffle in place. The short-lived 5D6 and the 5E6 that followed had circuits much like larger Pro, Super, Bandmaster, and Twin of the era.

Oddly, the early narrow-panel Bassman had only one input for each of its two channels, labeled Normal and Bright (the latter differentiated internally by a capacitor on its Volume pot), whereas similarly sized amps had two inputs per channel (four total). They otherwise shared most aspects. It was also given independent Bass and Treble controls governing a cathode-follower tone network with its own 12AY7 preamp tube and a Presence control enabled by tapping the feedback loop around the output stage. Also new were fixed-bias output stages – a significant development for the Bassman (and guitar amps in general) that offered more headroom and tighter/firmer low-end response. As those were qualities demanded by bass amps more than mere guitar amps, the Bassman was also equipped with two 5U5GA rectifier tubes, where its guitar-intended siblings had just one. While such was clearly the way forward for bass amplification, it was clear that guitarists also needed louder, stouter amps, and the Bassman was having plenty of crossover success (hence the Bright channel).

’54 Bassman: Nathan Fasold. Tufts of thread-bare tweed grace this character-laden ’54 “narrow-panel” Bassman – first version of the 4×10.
The Jensen speakers are period-correct replacements.

Combined, these elements showed the road forward for guitar amps, but the narrow-panel Bassman had yet to reach its zenith.

For the 5F6 Bassman of 1957, Fender scrapped the cathodyne (a.k.a. split-load) phase inverter design shared by its medium and larger amps at the time in favor of the long-tail-pair topology developed by Mullard, a feature that helped define the biggest and most-powerful amps in the lineup. Feeding the output tubes with this more efficient stage encouraged even greater headroom and a clearer, more-balanced overall signal reproduction. Notably, only the Bassman and Twin of 1958-’60 were given the long-tailed-pair PI. At the time, a Middle control was added to the Bassman’s cathode-follower tone stack, and the dual rectifiers were abandoned in favor of a single 83 mercury-vapor rectifier tube. The latter was short-lived, swapped in ’58 for a more-common GZ34 tube rectifier.

Note the the unusual dual rectifiers, two inputs, and lack of Middle control.
The tube chart lists the two 5U4GA rectifier tubes.

This is perhaps the most notable change to have ushered in the 5F6A, which is not only the most legendary of Bassman models, but today is a highly-regarded “amp for all seasons” tone machine. Other alterations might have seemed minor, but did play a part in the culmination of a classic design. Among these was the removal of the 1.5k-ohm resistors between the coupling caps and the grids that served as the inputs to the dual 5881 output tubes (a high-grade variant of the 6L6GC), a change that helped make the amps a hair livelier.

In ’59, the Bassman also gained the rubber “dogbone” handle in place of the leather that was prone to wear. Otherwise, the ’58 would be the final state of the narrow-panel tweed Bassman until its deletion from the lineup in ’60. By this time, Buddy Holly and other major stars had made it their amp of choice, and a near-endless list of greats would discover its toothsome charms in years to come, from Buddy Guy to Bruce Springsteen, Jimmie and Stevie Ray Vaughan to Mike McCready of Pearl Jam, and near-countless others.

While “the Fender tweed tone” is lauded in all of its forms, the Bassman’s success as a creative tool comes from the fact it takes that tweed je ne sais quois to its optimum point of expression. Push it past it’s delectably full and rich clean tones and it retains admirable clarity amid a juicy, euphonic breakup. Hit it harder and it veritably wails with the perfect blend of speed and articulation from the four 10s and touchy-feely dynamics from the tube rectifier, the no-nonsense gain stage, and the cathode-follower tone stack. And most guitarists know the legendary tale of Marshall’s homage to the 5F6A Bassman, and how by basing the JTM45 (and iterations including the hallowed “plexi”) on that schematic, the British maker assured this tweed beauty would become a rock legacy.

After having created the legend, in 1961, Fender reconfigured the model into an entirely different thing. And that should have been the end of the story – draw a line under the Bassman, we’re done. Yet, Leo and company miraculously created yet another classic in the process. Go figure.

The ’61 Bassman was part of Fender’s Professional Series that also included the Showman, Bandmaster, and Tremolux, and represented several significant advances in design. Outwardly notable were the rough-white Tolex covering (usually referred to as “blond”) paired with oxblood grillecloth, the forward-facing dark-brown control panel with white knobs, and “piggyback” format with heads and cabinets split into separate, stackable units. The first version, the 6G6, is odd in many ways; its Bass channel still used a cathode-follower stage to drive the Bass control, but placed this stage immediately after the input with the Volume control after the first tone stage and Treble following only after another two gain stages provided by another dual-triode 7025 preamp tube. Otherwise, the Normal channel displayed Fender’s new preamp topology with an EQ stage sandwiched between two gain stages. This would be seen more prominently in the brown, white, and black Tolex-covered amps to come, and play a big part in defining the “blackface sound” of amps made by Fender and others. For the time being, the Bassman retained its GZ34 tube rectifier.

’58 Bassman: Gary Dick. King Bassman – the late-’50s narrow-panel 4×10 5F6A combo is the amp’s legendary iteration.

Early Pro Series Bassmans were paired with a cab carrying a single 12″ speaker. This might have seemed a step backward except that Fender was putting a lot of thought into cabs at the time, and the closed back brought significantly increased efficiency. Even so, Fender upped the ante to two 12″ speakers for the 6G6A and 6G6B models in ’62 and ’63. Along with a few other minor circuit changes, these saw the tube rectifier replaced by solidstate diodes.

Though the tweed 5F6A Bassman combo got kudos when guitarists later “rediscovered” the glories of vintage amps (to be fair, many were aware all along), the early blond Bassman with 2×12″ speaker cab was pressed into service making a vast quantity of classic rock and roll. Despite talk of their use of Vox amps, one of The Beatles’ most-used amps was an early-’60s Bassman, and it was the choice of Pete Townshend for many early Who recordings and performances. Later, rockabilly supremo Brian Setzer would fly the flag for the prowess of the piggyback blond Bassman, and it proved a favorite for Tom Petty and Mike Campbell of the Heartbreakers.

In later ’63, Fender changed the formula yet again as part of a segue to the new “blackface” lineup. As with a few other models, the Bassman made its way gradually to what we think of today as blackface. Initially, white knobs remained on a black control panel that retained the Presence control and fronted the same circuit within the chassis. Most amps were still covered in white Tolex, but toward the end of the transition, many were given the new black.

Midway through ’64, Fender changed the circuit considerably for the new AA864 and AB864 Bassman, successively. While outward indications included skirted black knobs with numbers around their edges and elimination of the Presence control, internal changes were even more dramatic. The Bass channel was given a preamp circuit similar to that on the Normal channel from a couple years before but voiced for the instrument and with an added gain stage. This channel had a new Deep switch, the Normal channel had Bright, and while the preamp was made up of three 7025 dual triodes, one triode of the third tube in the signal chain was left unused. The phase inverter was changed from a 12AX7 to a 12AT7, which has less gain, and therefore encourages later onset of clipping at the output stage. A few other components and values were changed in the output stage.

These changes were made in the name of making the Bassman a better bass amp, yet it continued to be popular with guitarists. Heads from the mid/late ’60s established an even greater niche as utilitarian tube warriors with rock, punk, and garage players, in particular, who discovered the favorable tone-per-dollar ratio of these amps on the used market in an era when 50 watts no longer cut it for bass (but was eminently crankable for guitar). For many years, a blackface Bassman was the go-to of Social Distortion’s Mike Ness, and was purportedly used for many of Kurt Cobain’s guitar tracks on Nirvana’s Nevermind. Plug into the Normal channel, and it offers the punchy cleans of the archetypal blackface Fender, but with a little more wallop thanks to the robust output stage, which includes the heavy iron of an output transformer intended to reproduce ample low-end.

After Fender was purchased by CBS, the blackface Bassman evolved into the AA165 iteration; last of the highly desirable Bassman circuits, it was produced for only a few months. Though its designation varied little, the AB165 exhibited several changes. Ever conscious of damping distortion and artifacts perceived as flaws – even if that meant stamping out desirable sonic characteristics – engineers added negative-feedback loops and other squelching networks, which made the amp an underwhelming performer for guitar. The AB165 and circuits that followed also abandoned the traditional bias-level network for one that enabled a balance of bias level between the two output tubes, but not a broad setting.

’58 Bassman: VG Archive.

While this rendition of the Bassman and other less-inspiring models of the ensuing years might have denoted the end of an era, they still provided the templates of potentially great tube-amp tone for countless guitarists. Silverface iterations of ’68 onward have become prized “utility amps” in part because they’re affordable, powerful, and can be easily modified back to blackface status… or just about anything else. Want a Fender-style preamp on one channel, Marshall on the other? Easily done! Such has been the craze for maximizing their potential. In fact, it’s rare to see a guitarist playing a silverface Bassman head that hasn’t been at least moderately altered, and it’s widely accepted that thoughtful modification by an experienced tech will only improve these models rather than diminish them.

In other developments, the silverface styling (denoted by a silver control panel and blue-sparkle grillecloth trimmed in a silver “drip edge”) came in early ’68, atop the larger speaker cab introduced with the last blackface Bassmans of ’67. Midway through ’68, that cab gained two 15″ speakers in place of the 12s.

In ’69, Fender’s first major-league bass amp was added to the lineup – the 100-watt Super Bassman II, with two 2×15″ cabs. In this guise, the Bassman was arguably becoming a real bass amp. But, as it evolved through changes and formats – tube, solidstate… the more it became a “real bass amp” – the less it shone in the pantheon of guitar amps.

By the ’70s, the Bassman as a classic tone machine was a thing of the past. Yet, the amps created before that time and a few humble silverface heads produced a few years past continue to give great service to guitarists – and, yeah, pound out the low-end for bassists, too.

After a handful of small-shop makers offered well-received clones and homages to the late-’50s tweed Bassman combo, Fender caught on and in 1990 released its own reissue. One rendition or another of the legendary 5F6A Bassman has since remained in the catalog, and it continues to be one of the most revered and referenced guitar amplifiers.

This article originally appeared in VG July 2018 issue. All copyrights are by the author and Vintage Guitar magazine. Unauthorized replication or use is strictly prohibited.

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