The Most Powerful Tube Amp of All Time: Behind the Fender 400 PS

Fender set the rules for the game with two amps in 1952: the Twin and the Bassman. With rock and roll just taking off, these amps became popular with musicians who needed louder guitars and with amp designers who wanted to start building for their own markets.

Jim Marshall found that Bassmans were selling like hot cakes in Britain, but couldn’t keep up with demand just by importing alone. So he cloned and modified its circuit and launched Marshall Amplification with the JTM 45 in 1962.

This kickstarted a trend toward incredibly powerful amps as the 1960s came to a close. Fender, with all of its capital and cred, was poised to dominate the game once again.

By way of legendary amp engineer Ed Jahns, Fender would produce the most powerful production tube amp ever made, the Fender 400 PS. Debuting in 1969, the 400 PS was a behemoth conservatively rated at 435 watts RMS.

This is a story about the making of that monster.

Louder and Louder

As musical tastes transformed from big band to swing to jump blues to early rock and roll, guitarists needed more powerful amplifiers, and they needed them quickly. The story of popular music evolving in the middle of the 20th century is also the story of the guitar getting louder.

Fender was the trailblazer in this market when it updated its Twin with a cutting edge 80–watt version in 1958. That iteration of the Twin was met with very little competition until 1965 when the 100–watt Marshall stack — originally provided with a single 8x12 cabinet — was put into use by Pete Townshend, following the theft of The Who’s tour van full of Vox amplifiers.

In 1967, the Marshall 200 (soon to be called the Marshall Major) was introduced. Early Majors were plagued by the massive high–voltage power supply shorting the output transformer.

Marshall solved this problem by placing the then space–age Teflon insulation between windings. With that, the Marshall Major could be viably reproduced and would remain one of the most powerful guitar and bass amplifiers in production until 1969.

NAMM ‘69: Fender Gets Upstaged

At the Chicago NAMM Show in 1969, Ampeg introduced the new 6146B–equipped Super Vacuum Tube (or SVT). That amp pumped out 300 blaring watts into its accompanying pair of 8x10 “fridge” cabinets. Fender, on the hand, was just unveiling a brand new 100–watt Super Bassman head (named the Bassman 100 in later years).

Fender lost a wattage battle at that show, but Ampeg’s new SVT was far from perfect. For all of its power, there were severe design flaws, including fragile output tubes.

Such problems led to Ampeg’s own Rich Mandella travelling with The Rolling Stones for the duration of the band’s 1969 tour that would culminate in the ill–fated Altamont Free Concert. His responsibility was to watch over the then prototype SVTs and to frequently swap out amps that were overheating or blowing tubes during the actual concerts.

Around the time of NAMM in 1969, Fender’s Ed Jahns was hard at work on Fender’s own high–output amp. Jahns’s own design at that moment had faulty output tubes as well, but he and Fender were not as ready to release a design with that particular flaw as SVT was. So Jahns continued toiling at what would become the 400 PS.

Jahns’s solution to the output tube issue was using specially selected Tung–Sol 6550s in the circuit. In order to make the amplifiers even more reliable and increase output to a documented 475+ watts RMS, Ed Jahns and other Fender engineers pushed General Electric to create the new 6550A tube.

By using special explosion–forged plate material, GE was able to increase the 6550As rating from 35 to 42 watts of dissipation, matching the British KT88. This meant that each tube could output more power before risking meltdown and, more importantly for the circuit, could operate more efficiently.

In 1970, Fender charged $750 for the head and $500 each for the three speaker enclosures. That’s a total of $2,250 for the whole set–up, which equates to $14,500 in today’s dollars. That means that the head alone would cost a consumer the equivalent of $4,800 today.

The Monster Comes Alive

Upon introducing the Fender 400 PS in December of 1970, buyers were informed that, “A single speaker enclosure large enough to utilize 435 watts without damage to the speaker would be too large to be portable. For this reason, we have divided the available power into three separate speaker output jacks.”

That’s right. Fender recommended using three individual speaker cabinets to fully harness the 400 PS’s power.

For use with bass guitar, each of the 400 PS’s outputs would connect to individual folded–horn cabinets standing at four feet tall and equipped with an 18–inch Cerwin Vega speaker. Each speaker cabinet would theoretically receive 145 watts for a total of 435. The actual output was closer to 480 watts RMS.

It was possible (and encouraged) to use the amp with one, two, or all three speaker cabinets for 145, 290, or the full 435 watts of output. Removing a speaker cable from one of the jacks would isolate a pair of the tubes, effectively putting that pair on standby.

All of this power was possible in an amp weighing the same amount as an SVT by using a 6L6GC as the driver for the 6550As. This put the amp into more efficient Class AB2 operation, less like a guitar amp and more like a HAM radio.

What About the Tone?

The 400 PS plays like Fender’s greatest hits. On the normal channel, Fender recycled the lush reverb and throbbing tremolo circuits from its so–called Blackface Twin. The amp’s bass channel has the same circuit as a Blackface Bassman.

Both channels enjoyed the same clarity and roundness, thanks to a hi–fi style power amp that can output full power at 40 Hz. The SVT preamp, on the other hand, had a bass roll–off starting at 90 Hz. Plugging into the 400 PS is like strapping yourself into a high end sports car.

The clarity of these amps is nothing short of amazing when they are properly maintained. Imagine the Blackface Twin Reverb of bass amps. It’s perfect for thumping Motown, scooped punk or reggae, or bright punchy slap tones.

The front control panel of the Fender 400 PS

One of these amps was used by bassist and founding Allman Brothers Band member Berry Oakley on the live recording of “Mountain Jam” from the legendary Eat A Peach album.

This same clarity is even more prominent with guitar when played through the normal channel. When playing through the bass channel, guitar tones range from thick jazz tones to raging Keith Richards–style midrange.

This same tone is available at volume levels going from whisper quiet to earth shattering. At full tilt with efficient speakers, these amps can theoretically achieve over 148 decibels of peak SPL at a distance of three feet. That’s louder than the deck of an aircraft carrier during take–off.

Most players will likely never need this much power, but the same can be said about having 800+ horsepower in a street car. It's as unnecessary as it is exhilarating.

Well Why Didn’t They Catch On?

Countless reasons can be attributed to why the 400 PS was not as popular as the SVT. For one, it was only marketed for bass guitar use, even though it featured reverb and tremolo. This seems correlated with the fact that Fender didn't make guitar–focused cabinets suitable for use with the 400 PS.

The Fender 400 PS on its rolling stand

The 400 PS also had an uglier aesthetic than its direct competition. Stacks were quickly gaining popularity at the time, and the SVT made for a good–looking stack. The 400 PS, on the other hand, came with its own chrome rolling stand. It would save the head from ventilation and vibration issues, but made the amp look more like a piece of hotel furniture than a piece of road equipment.

Of course, Fender also introduced the 400 PS a full year and a half after the Stones first toured with a backline of SVTs, meaning that Ampeg’s design had a significant head start. Perhaps if Ed Jahns had not waited for the much more reliable 6550A tubes to be developed, the 400 PS would be the SVT of today.

Using One Today

To build a 400 PS from scratch would likely cost several thousand dollars. Fortunately, these amps are a fantastic under–the–radar specimen from the days of yore that are truly built to survive doomsday. With quality output tubes, these amps can give years of service, just like any other Fender from the era.

GE 6550As were the original tubes, but modern premium KT88s do almost as well in these amps, both for maximum headroom and tone.

Many have claimed that these amps are just a fire waiting to happen, but the reality is that these amps are almost bulletproof once properly biased and checked out by a proper tech with the simple Fender 30 step factory procedure.

This sounds daunting, but it is only a small inconvenience comparable to similarly powered amps, some of which require use of proprietary bias rated tubes.

Thankfully, the late Richard Koerner of Time Electronics made the schematic and Fender documentation on these amps widely available online.

Lugging around three huge cabinets isn’t fun. However, it is very easy to use any cabinet, such as a 3x10 or 6x12, as long as it can be reconfigured as three separate 4 ohm speaker loads. A set of three 2x10 cabinets would also work nicely. The amp also does not need all three speaker loads to be used to operate.

If that’s too complicated, the 300 PS (the 400 PS’s replacement) weighs in at 76 pounds and puts out 300 watts, as compared to the SVT’s 85 pounds. And the 300 PS does so with the same clarity as the 400 PS, all into a single 8 ohm speaker cabinet.

The build quality, tone, and clarity of these amps is incredible for either guitar or bass.

In the realm of Hiwatt, Sunn, and Mesa Boogie powerhouses, the 400PS is still an amazing alternative, if not the king of all tube high–output tube amps.


Bryan Parnell owns and operates Retro Sound Works in Philadelphia. He services everything from guitars to synthesizers, but his passion is repairing vintage Fender amps.

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