The history of modded Marshalls

As the saying goes, necessity is the mother of invention, and in the late 1960s and beyond, repair shop engineers began to tweak guitar amplifiers to increase dependability and tone. Paul Rivera Sr., an industry icon, gives an insight into the early days of the hot-art. rodder’s

The acoustic transparency and reactivity of early Marshall amps were appealing, as was the tremendous power of a 100-watt EL34 head put into a 4×12 Celestion-powered cabinet.

It was, and still is, the classic rock guitar tone, a collision of components and voltages that produced a distinct voodoo. However, as guitarists progressed into the new era of progressive rock led by Jimi Hendrix and Cream-era Eric Clapton, the early Marshalls’ simplicity – and the unreliability of circuits pushed to their limits – became more of a hindrance for some players, resulting in a growing demand for modifications as well as repairs.

The amp-modding industry had not yet truly begun in the late 1960s and early 1970s, but amplifiers frequently needed maintenance, and it was repair shops that pioneered concepts in response to artist needs.

Early Marshalls, such as the 1959 head, were utilitarian clean powerhouses that only began to distort when turned up loud, at which time overworked power valves began arcing, therefore many improvements, aside from adding additional gain or managing output power, were aimed at improving dependability.

Later, as features like preamp gain controls and master volume became more prevalent, they finally made their way back into new factory designs like the Mk2 and JCM800.

Repair shops pioneered innovations in response to artist demands in the late 1960s and early 1970s.

Meanwhile, amp repairers were raising their game, particularly in the United States. Paul Rivera is one of the original tone gurus and a founding member of an elite fraternity of engineers responsible for launching the amp modification and boutique amp industries.

Paul is best known for his involvement with Fender in the early 1980s, but he also worked on several Marshall amps during his 70s time at the famed Hollywood shop Valley Arts Guitars and in the years before that. Paul Sr. continues the story…

Rockers and mods

“I cut my teeth on early Marshall amps at my first shop in NYC in the late 1960s, mostly warranty repair work, which allowed me an opportunity to learn from the resident Eastern European engineer who worked at their NYC distributor,” he explains.

“Due to overly high anode and screen-grid voltages, those early amps were temperamental.” Furthermore, due to the impedance swings of their speaker loads, the output transformers were severely stressed on peak-to-peak AC voltages. There are numerous blown output transformers and arced octal sockets!

Built-in attenuators and automatic biassing began on the desk of an independent amplifier designer.

“In 1968, a chat with a Dynaco transformer engineer gave me faith that we’d discovered a superior output transformer, and I began installing Dynaco’s A-431 in 50-watt models and the A-451 in 100-watt models.” I quickly realised that using the ultra-linear screen-grid taps would necessitate a big filter choke to reduce the ripple, so I stuck with the default DC wiring.

“In retrospect, if Marshall’s Ken Bran had built a split DC supply with the screen grids set to 50% of the anode DCV, he could have gotten 600 volts on the anode and substantially boosted reliability – assuming he used modern Mullard or Philips EL34s.” Those Dynaco transformers were significantly more dependable and sounded incredible.

“I came to Southern California in 1972, where I established contacts in the music industry.” I continued to build Marshall changes in phases to accommodate varying budgets, using input and comments from the studio engineers, producers, and performers with whom I worked.

“Around the same period, Marshall’s Mk2 amps hit the market, with significantly lower DC voltages and greatly improved stability.” When 6KV+ diodes became available in the 1970s, we were able to utilise them on the anodes as ‘flyback’ diodes, thus controlling the arcing and blown transformer difficulties.

“While most Marshall players preferred the cranked tone, master volumes were required to make their amps usable in smaller venues.” With a master volume, you could add greater gain without worrying about deafening your first row of listeners – this was Stage 1.

If you listen to Eddie Van Halen on Michael Jackson’s Beat It, you’ll hear a Stage 3 I did for a journalist who loaned it to Eddie for the session.

“The first Stage 2 mods included playing with the mids and bass frequencies, as well as inventing a rotary switch to do so.” Stage 2 added master volume, switchable gain boost, a fat switch, an output stage rewired using flyback diodes (changing the octal sockets if necessary), and beefed up filter caps to prevent ripple and hum.

“Along with all of the Stage 2 mods, Stage 3 includes an additional preamp tube and associated circuitry.” As time passed, another possibility emerged: an active effects loop.

“On a Marshall 1959 model, Stage 4 was full channel switching with two completely independent channels, requiring dual-concentric potentiometers and stacked knobs to retain the stock appearance, additional preamp tubes, a bespoke motherboard PCB with Vactec (silent optical) switching, and a multi-function footswitch to control the effects loop bypass and channel switching.”

“If you hear Eddie Van Halen on Michael Jackson’s Beat It, that was a Stage 3 I did for a journalist who loaned it to Eddie for the session.” My customised Marshalls were used on the majority of Steve Lukather’s tracks on the first four Toto albums.

“Many additional tracks, including ones by Eric Johnson, add to the legacy of those modified Marshalls I built.” So it was the pinnacle. After Fender, I had less time and inclination to continue customising other amp brands by the time I started producing my own amps in late 1985.”

They are in a league of their own.

Paul Rivera is still travelling with Tool nearly five decades later, having recently completed a restoration on a flood-damaged Marshall Super Bass 100.

Reinhold Bogner, who arrived in Los Angeles from Germany in 1989 and worked on amps for Steve Stevens and Jerry Cantrell before unveiling his own designs, is another well-known Marshall hot-rodder who followed Rivera’s path. Metaltronix’s Lee Jackson helped Paul Gilbert and Steve Vai’s tones, and the late Jose Arredondo and César Daz were also known for their work with Stevie Ray Vaughan and Eric Clapton.

These extremely brilliant designers contributed to the advancement of the Marshall sound and shaped the tones employed by the next generation of musicians. Bogner’s three-channel Ecstasy established multi-channel heads as the norm, and features like built-in attenuators, automated biassing, and several other enhancements all began on the workstation of an independent amp designer somewhere.

The early to mid-1970s amp hot-rodding movement in the United States offered a platform for Rivera, Soldano, THD, Kendrick, Mesa/Randall Boogie’s Smith, and many others to eventually manufacture and sell their own designs. It never quite made it to the United Kingdom.

While there is no shortage of talent in the UK, most engineers have tended to limit themselves to servicing and repairs, with some eventually working for top brands such as Marshall, while others, such as Victory’s Martin Kidd and 633 Engineering’s Cliff Brown, have made the leap into manufacturing their own designs.

It’s a difficult job for both large and small producers; procuring components and subcontractors has become especially difficult, with typically plentiful goods like diodes and capacitors now having lead times of 60 weeks or more.

Nonetheless, Marshall continues to progress, with current JVM flagships featuring many capabilities we now take for granted, courtesy to the pioneering effort by an army of hot-rodders in the past.