The Supro Thunderbolt S6420

Despite their catalog-grade status, Supro amps have been used by several noteworthy guitarists. For many, the sturdy Thunderbolt is the preferred workhorse.

It’s been a long time since Supro amps were any kind of secret find or hidden gem; players have long recognized the eccentric splendors of certain mid-sized examples, with their thumping tremolo and gritty-but-gutsy tube tone. Also, reissues by the revitalized Supro company have brought the name back front and center.

One of the ubiquitous models among the lineup – and an amp often thought of as a Supro cornerstone – the Thunderbolt arguably delivers less of the quirky signature Supro circuit characteristics while somehow remaining in step with the legend.

Introduced in 1964, the Thunderbolt was promoted purely as a “bass-guitar” amp, as stated on the combo’s control panel. Like other low-powered early offerings for the four-string, however, it has been used far more often by guitarists who quickly discovered that many of the qualities contributing to an underpowered all-tube bass amp can equate to just right with a higher-register instrument plugged in. Riding atop the tone, of course, is the perpetually cool Supro styling, comprising a black “rhino hyde” covering, silver grillecloth with black pinstripes, and that iconic logo with the lightning-bolt swoosh. And it’s fitting that the image speaks to an ageless form of hip in the guitar universe.

Supro’s origins date to the days before the electric guitar and the formation of the National Resophonic guitar company circa 1926, with its resonator guitars. Supro’s parent company, Valco, was founded in ’35 by Victor Smith, Al Frost, and Louis Dopyera (the company name derived from the first letters of the first names of each, plus “co”), who had bought the remnants of National Dobro for that purpose.

Smith and Frost had been employees of National, while Dopyera was co-owner, having helped establish National and then Dobro resophonic guitars alongside Adolph Rickenbacker and George Beauchamp in the mid/late ’20s (Beauchamp and Rickenbacker went on to produce some of the first commercial electric guitars and amplifiers under the Rickenbacker brand by the mid ’30s, as the quest for loud evolved from aluminum resonator cones to tube-powered pulp paper). Just a year after founding the company, Dopyera, Smith, and Frost moved Valco from Los Angeles to Chicago, which was then the hub of the musical-instrument industry in the United States. It was also a central location from which they could easily service the many house brands that would form the core of their clientele, including Gretsch, Kay, Harmony, Airline, Oahu, and a few others.

Supro’s amplifier line of the early/mid ’60s carried modern styling, though its circuits were still basic, as was its construction, with the chassis mounted in the bottom of the combo cabinet with a slanted, rear-facing control panel – a format that had faded from popularity a decade or more before. The ’64 catalog “boasted” of the Thunderbolt: “No frills – no fancy circuits – no extra weight – just fine quality and clean, true bass response. For the mobile Bass player and those ‘one-night’ stands.”

Indeed, the circuit uses just about the bare minimum of stages and components to get signal through the preamp to the output stage. One lonely 12AX7-derived triode carries it through the Volume and Tone potentiometers in the amp’s single channel – and a coldly-biased one at that – with half of that tube remaining unused, while the phase inverter employs the second 12AX7. A pair of 6L6GCs generate approximately 35 watts, cathode-biased with no bypass capacitor in place, presumably to avoid swamping the stage with unflattering low-end.

This catalog listed the amp at $179.50 – $10 more than a Princeton Reverb made the same year, and we know which one most guitarists would take today! Our featured example remains with its original owner, VG reader David Figie.

“In 1964, I was 11 years old and playing in a band called The Rising Sons, in West Allis, Wisconsin,” he said. “I played a Gibson EB-0 and my dad bought me this Thunderbolt. It was not good for bass, so Dad bought me a Gibson Titan.”

Many agreed, apparently, and Valco attempted to firm-up the Thunderbolt’s performance through the decade, adding further bracing to the baffle to try to help the 15″ Jensen C15PS – and the cabinet in general – stand up to the low-end onslaught. In ’68, they revamped the design. Even before, however, guitarists had begun discovering what the stock Thunderbolt S6420 could do – Jimi Hendrix among them, who toured one with Little Richard and the Isley Brothers before heading to London and gaining massive solo fame.

Figie’s Thunderbolt sat unused a lot longer than that, though he never thought to sell it.

“Occasionally, I played my brother’s Rickenbacker 366/12 convertible through it. In 1989, I bought a Stratocaster and have been using the Thunderbolt ever since. I replaced all the tubes; I still have all the originals, and everything else is original. This amp sounds great for guitar!”

Designed to get a bass signal from input to output with minimal flubbiness or distortion, the Thunderbolt is predisposed toward a warm, rich, and slightly woolly tone, which sounds best when pushed to drive those 6L6GCs into a little breakup. There’s a whole unused triode stage sitting there in the preamp’s lone 12AX7, which can be used to add another gain stage or an active EQ stage, but most guitarists who dig the Thunderbolt love it for what it does naturally. That might not be thunderous by nature, but it’s a juicy, throaty, edge-of-breakup drive that epitomizes a certain breed of vintage tone, and has a lot to offer on its own.

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This article originally appeared in VG’s April 2025 issue. All copyrights are by the author and Vintage Guitar magazine. Unauthorized replication or use is strictly prohibited.