All About Classic Tube Amp Circuits

July 30, 2008

For the purposes of this article, I’ll break down these classic circuits into five categories: Single-Ended Practice Amp, Small Tweed, Big Tweed, Brit Class A, and Blackface Twanger. The list reads a little like a generic amp menu from the control panel of a digital modeling amp, and for good reason. While I don’t mean to imply that every amp fits into the boxes these circuit categories define, nine out of ten amps on the shelf today will exhibit archeological traces of these five classics—which is pretty impressive longevity from a handful of 50-year-old tube concoctions.

Single-Ended Practice Amp

Many petite ampettes such as the Fender Champ and Princeton, Vox AC4, Gibson GA-5, and a host of others by Danelectro, Airline, Kay, and Valco were originally marketed as beginner, student, or practice amps, but have come to be appreciated by players of all stripes as killer recording amps. What these low-wattage designs all have in common are their simplicity and inherent single-ended, pure class A performance (see “All About Class A Amps” in the April ’08 issue of GP). There just isn’t much of anything to clutter the signal path in these amps—just one preamp tube, one output tube, one rectifier tube, and a whole lot of magical superfly mojo dressing—and your tone thrives for it. Each of these models exhibits the jangle, bite, and chime associated with class-A circuits at lower volumes, and can ease smoothly into sweet, harmonically rich distortion when cranked up. The small, single-ended tube amp has become popular again in both reissue and modified formats, from Epiphone’s uber-affordable Valve Junior at one end of the scale, to hot-rodded single-enders such as the THD UniValve, Dr Z Mini Z, Bad Cat Mini Cat, and Cornford Harlequin at the other. Many amps that follow the practice-amp template do have handicaps, however, such as limited headroom and a lack of oomph to cut it in the gigging environment.

Small Tweed

This title most often hints at one hallowed amp—the 5E3 Fender Deluxe of 1957-1960—but many other 15-watt amps of the era follow a similar template, and fit perfectly into this category. Design-wise, this compact monster of greasy blues and vintage-rock tone has cousins in the Gibson GA-20 and GA-30, some Silvertones and Ampegs, and dozens of other “poor man’s Deluxes” of the era. While the narrow-panel tweed Deluxe might look something like a Bassman or Twin, only smaller and quieter, it is actually entirely different from input to output in just about every way that counts.

Although the big and little Fender tweeds of this era both used 12AY7 tubes in the first preamp stage (which have a lower gain factor than the more familiar 12AX7), from there on out things change completely. The 5E3 Deluxe’s preamp is an extremely simple configuration, with a simple treble-bleed Tone control shared by the two channels, and a swift and largely unencumbered ride straight through tone valley to the phase inverter. This lack of hurdles in the preamp stage makes for a hot signal—even with just one stage of mild-mannered 12AY7s in there—and these tweed Deluxes drive like all get out when wound up to just 10 o’clock. Also, the two channels are coupled in such a way that their Volume controls interact, and each can affect the saturation of the other—even when you are plugged into only one channel.

Up until the early ’60s, Fender was using a simpler split-load phase inverter on medium-sized amps such as the Deluxe. This circuit—which splits the signal into two and flips the phase of one leg before patting it on the butt and sending it along to the output tubes—is a little more basic than the one used in the Bassman and high-powered Twin of the day. When driven to clipping, these circuits introduce their own element of distortion. Finally, the output stage is cathode biased with no negative feedback. This configuration—and the dual 6V6s that power it—are run at fairly low DC voltages (around 350V), and when combined with the squashy, sag-prone 5Y3 rectifier it makes for mucho dirt and bountiful compression when you hit it hard. Of course, with dirt coming at you from preamp, phase inverter, and output stage, there isn’t a whole lot of headroom to be had here—although a healthy Deluxe can usually hack club gig levels.

Big Tweed

Fender’s 5F6A tweed Bassman of 1958-1960 is one of the most revered amps of all time. While it might look like a traveling salesman’s tweed suitcase with a pilot light and a few knobs on it today, it was one of the most advanced amplifiers of its time, and key elements of its circuit have been retained in more major amp models through the years than those of any other design. Despite being designed for use with the electric bass guitar, the attributes attained in a bid to fulfill that objective—firm lows, low distortion, and a bold, punchy presentation—also made for one bellowing, lust-worthy big-gig amp for 6-stringers. Crank it up, and, yes sir, it does indeed distort—and in the most comely, infectious manner imaginable. Keep the volume down, on the other hand, and its clean-with-bite tones are also to die for. Loud, proud, and extremely versatile, it’s the original amp for all seasons.

Like the tweed Deluxe, the Bassman uses a 12AY7 in the first gain stage of each channel, but its preamp is far more involved from there on out. Both channels share a three-knob active/interactive EQ stage that has come to be known as the “cathode-follower” tone stack because it is driven from the cathode of the second side of another preamp tube, this time a 12AX7. The cathode-follower has become famous not only for providing an effective EQ stage for guitar, but also for yielding a tactile, touch-sensitive playing feel that greatly contributes to the dynamic range of the amplifier.

Having used the split-load inverter for not only the Deluxe, but medium-sized amps such as the Super, Bandmaster, and Pro (and even the low-powered Twin of the day), Fender sought a leg up for the Bassman in terms of phase inverter elegance and performance, and adapted a Mullard circuit that has become known as the “long-tailed pair.” This phase inverter passes a truer, less-distorted signal on to the output tubes, and therefore helps the amp’s full voice to be expressed. The output tubes were a pair of 5881s (ruggedized 6L6 types), with their bias fixed in efficient class AB with a little negative feedback to keep performance just shy of freak out at full volume. Everything was pumped through an output transformer that was larger and of a more fully rendered design than anything Fender had yet put into an amp this size, and the result was 40 or 45 watts of muscular tone.

The Bassman was so impressive in its day, that Jim Marshall selected it as the template for his own JTM45 of 1962, and roughly the same circuit lived on right through the plexi and metal-panel JMP50 and 100, and even the JCM800. Multiple gain stages, channel switching, and master volumes have come along to fill out the picture, but, at the center of it all, you can still trace the foundations of the 5F6A circuit. Peak under the hood of many a high-gain amplifier of the modern era—such as models from Soldano, Rivera, Mesa/Boogie, and others—and you’ll find elements of the old Big Tweed.

Brit Class A

The two amps that were paid tribute to more than any other by the boutique boom of the 1990s were the 5F6A Fender Bassman and the Vox AC30 with Top Boost (a “deluxe” EQ option available as an add-on from 1961, and as a factory standard from 1964). The Vox’s jangle and chime was to vintage British pop and rock what the Bassman’s succulent roar was to seminal American rock and roll and Chicago blues. We have already covered some of the major ingredients of this amp, but Vox designer Dick Denney combined them differently in his groundbreaking designs, yielding amplifiers that are perceived as being totally different from the Yankee tweeds.

The AC30’s preamp is similar to the Bassman’s, except for its use of a hotter 12AX7 (a.k.a. ECC83) as standard. And, although it is rarely called out as such, the man behind the curtain of that famous two-knob Top Boost EQ section is our good friend, the cathode-follower tone stack (albeit with a preset midrange network in place of the Bassman’s Middle control). The phase inverter is likewise the efficient long-tailed pair, but Leo Fender had borrowed that one from the Brits in the first place.

Hell, that’s three stages of extremely familiar geography, but inject it all into four cathode-biased EL84s with no negative feedback—nominally a class A output stage—feed it through a pair of alnico Celestion G12 speakers, and it all comes out sounding nothing like a tweed Bassman. Or perhaps just a little like a tweed Bassman, if you crossed it with a tweed Deluxe and changed the tubes. See how incestuous it all gets? As the other big impetus for the boutique boom, the AC30 has been paid homage by the Matchless DC30 and Bad Cat Black Cat 30 (both with the rare, original EF86 channel), the TopHat King Royale, and models from Dr Z, Bruno, 65 Amps, Mojave, and others.

Blackface Twanger

Headroom was the name of the game for amp manufacturers in the golden years of tone, and while we often lust after the overdrive sounds that gush out of these old tubesters when cranked toward oblivion, the designers of the day were mostly trying to get them loud, clean, and bright. Fender took a big step toward that goal in the early-to-mid ’60s with its brownface and blackface amps. These designs featured a cleaner, brighter, more hi-fi sounding tone stack that sandwiched the controls between two stages of a separate preamp tube on each channel, rather than using the juicy, touchy-feely cathode-follower tone stack of the larger tweeds. Fender also ramped up the voltages at many points to get more drive and less squash out of both preamp and output tubes, and tweaked other factors in the name of volume, definition, and cutting power. It worked. The blackface Deluxe Reverb, Super Reverb, Twin Reverb, and others are the epitome of twang. Crank them up higher than the boys in R&D ever intended, and they are also outstanding rock and blues amps. They are plenty dynamic and touch sensitive, but a little less so than the tweeds. They also tend toward a wide, slightly scooped frequency response compared to the raw, mid-forward grind of a tweed (or a Marshall). Elements of this higher-fidelity performance would surface in big amps from Traynor, Selmer, Hiwatt, and plenty of others.

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