Max Headroom

February 1, 2011

gpnp_shop_nr_3996IF THE ELECTRIC GUITAR HAS ANY socially redeeming qualities, volume must certainly be among the most important. The guitar and amplifier have evolved quite a lot since Charlie Christian’s first solo flights, with a major part of that evolution being directed to the amplifier’s ability to deliver distortion-free power. There are many factors governing this quest for fortissimo (that’s Italian for really loud), so let’s look at a couple of obvious yet commonly overlooked points.

One of the axiomatic truths of physics is that the lower the frequency of a sonic wave (electronically or acoustically), the more difficult it is to reproduce accurately. Whether we’re talking about the capacitors, tubes, speakers, transformers, or the instrument itself, the lower the note is, the more difficult it is for a system and its components to translate it without distortion. This reality is important in dictating the performance of any guitar and amp. Early musical instrument amplifiers— and this includes most any amp made before 1960—were designed to reproduce a wide bandwidth, with the goal being accurate frequency response across the entire audible range.

Fender and Gibson amps of this period are typified by a warm, rich tone, with early breakup being a defining characteristic. A look at the input designations on a amp of this type will reveal why: Amplifiers of this period were expected to amplify everything from electric guitars to accordions to vocal mics to phonograph players, and the wide bandwidth was needed to adapt to whatever was requiring volume.

With this harmonically rich bandwidth coursing through the circuitry, it doesn’t take much to push these amps into distortion, with the lower frequencies hitting the ceiling first. At some point (probably at the behest of Dick Dale), audio engineers discovered that by filtering the offending lower frequencies out entirely, the amp was able to reach higher volumes without distortion. The archetype of this design philosophy is the classic “blackface” AB763 Fender circuit, which has tone control circuitry designed to limit the amount of low frequencies available. This can be easily demonstrated by setting a tweed Bassman next to a Super Reverb and setting the controls flat. Relative to the Super Reverb, the Bassman has more bandwidth and sonic information available to the speakers, and thus will be perceived to be fuller and harmonically richer. The yin of this yang is that these lower frequencies will go into clipping sooner, inhibiting the clean volume of the Bassman. As a general rule, the warmer and fuller an amp’s character is, the sooner it will break up.

Not that the circuitry is solely responsible for an amplifier’s headroom. Perhaps the most overlooked aspect of the volume formula is the cabinet’s contribution to an amplifier’s capacity for headroom. The distinction between pre- and post-1960 amp design is most obviously typified by the “tweed” and the “blackface” examples. Though the color of the Tolex varied from year to year, after 1960, the benchmark Fender amplifier platform was changed forever by the introduction of an ultra sturdy, surf band approved cabinet. In addition to being ergonomically superior to its tweed brethren, its architecture features some major construction differences. A key difference is the baffle board. Instead of the relatively rickety tweed-style “floating” baffle, with its thin plywood and Spartan fastening arrangement, the blackface-style baffle is made from beefy 3/4" cabinet-grade plywood or ultradense MDF (medium density fiberboard), which is fastened to the cabinet by big cleats around its entire perimeter. This true industrial- style construction fixes the speaker baffle to the cabinet extremely well, and in the process eliminates most of the vibration and rattling that is endemic to the comparatively lighter-duty tweed units.

Relative to a tweed, the blackface cabinet has almost no transfer of vibration from the speakers to the cabinet. This allows the speaker’s vibrational energy to be used more efficiently, pushing the air in front of it with waves of musical information rather than wasting this energy rattling the cabinet itself. Sure, the tweed’s lightweight, thinner cabinet elements produce gobs of personality, but they rob the system as a whole of volume potential. For a startlingly revealing demo, try plugging a Super Reverb chassis into a Bassman’s speakers and vice versa. Big difference. As always, there is a yin/yang factor to consider: The tweed’s natural resonance is largely what accounts for its characteristic, “I don’t miss the reverb” dynamic. Tweed cabs already have an organic inherent “reverb” to them.

Turn the ’verb off on a Super Reverb or Twin Reverb and the tightness, stiffness and directionality of the package becomes very apparent. As such, the reverb is quite a necessity on these amps; it really softens this trait and makes them more enjoyable to play. Ultimately, many variables account for a guitar amplifier’s performance. But the next time you’re wondering why your vintage Bassman 410 seems to getting buried by the reissue Deluxe Reverb sitting next to it, consider the contributions that basic speaker cabinet physics bring to the bandstand.


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