What does it really mean when an amp is "fast" or "slow"?

Here at the laboratory, we’ve been having some deep discussions about "fast response" types of guitar amplifiers. At the end of last month’s column about my first meeting with Alexander Dumble, I promised we’d take a closer look at this very concept of "fast" and "slow" amplifiers, so let’s dig in.

Frankly, I think this concept began with the original Vox AC30. If you ever get the chance to play a real AC30, you will notice right from the very first notes played through those lovely sounding, original 15-watt Celestion "Blue" speakers that there is something very special going on. At first, the notes seem to pop from the speakers in a way that’s almost psychic—it’s as if the notes come out before you actually play them. One of the sweet spots found in the AC30 is when the amplifier is set precisely to where the notes or chords start to distort when you strike the strings a bit harder. The intro riff to the Foo Fighters’ song "Summer’s End" (on 2007’s Echoes, Silence, Patience & Grace) is a great example of this incredible sound.

Most of you understand what tube compression is and how a tube amplifier reacts when you play a guitar through it, but an AC30 has several other factors that make it sound unique when compared to other amplifiers. For starters, there’s the extremely high efficiency of the original speakers. Their sensitivity is a whopping 103 decibels. Compare that to the mere 96 dB rating of many other speakers, such as a Celestion Greenback. This difference becomes very clear when you hear the two speakers driven by the same amplifier. When comparing the two speaker models side by side, the Greenback’s 96 dB sensitivity rating seems downright paltry. In fact, the 103 dB rating of the original Celestion G12-M makes it almost twice as loud as the Greenback.

The combination of those highly efficient speakers, coupled with the EL84’s sweetly compressed, overdriven signal being sent through the larger AC30 cabinet gives you a perception of a much louder amplifier than its rated 33 watts of peak music power. Standing in front of an AC30 that’s set to its halfway point sounds more like hanging with a 50-watt amp—easily!

Additionally, the sheer physical size of the AC30’s cabinet produces an effect that sounds very much like "natural reverb." This is another sonic trademark of this particular guitar amp— it has a bouncy, springy feel. The AC30 is a ton of fun to play because it’s so lively.

The AC30 holds another surprise, hidden in the circuit schematic itself. This amplifier has a huge choke that is much bigger than that of most other guitar amps. For example, a Fender Bandmaster—a 40-watt amplifier— has a 90-milliamp, 4 Henry choke (Henry is a measurement of inductance). Another example is found in the Marshall JTM45. This amp sports a 150-milliamp, 5 or 7 Henry rated choke. In contrast, the AC30 wields an absolutely oversized choke that weighs in at 100 milliamps and 20 Henrys—gigantic for a conservatively rated 30-watt amp.

A choke, for those not familiar with the term, is an inductor—in this case a coil of wire spun around a laminated ferrous metal core—that sets up an electromagnetic field when current is passed through it. According to the Radiotron Designer’s Handbook [4th edition, 1952], when the current is varied, voltage is induced in the coil. In essence, the choke stores electrical energy as an electromagnetic field and gives up energy when the current going through it starts to sag. This means the choke has a large effect on an amplifier’s dynamics.

Here’s an important point about fast amplifiers: One thing common to most models we’ll be discussing in this and future columns is that fast-type amplifiers have raised voltages in the power supply. This gives these types of amps the ability to get more reserve power quickly when they need it. This translates to an amp that will feel a bit "stiff" when you first dig into the strings with your pick.

Furthermore, when talking about fast or slow types of amplifiers, it is worth mentioning that the perception of these fast dynamics (or the opposite "slow" dynamic effect) may be due to a lot of parameters. However, the power supply and output circuitry need the ability to deliver their power quickly, and the speakers must be able to reproduce these dynamic peaks accurately.

Interestingly, the AC30’s fast response is also a bit of a paradox. AC30s use a GZ34 rectifier tube, which is not as fast as some guitar amplifiers produced just a few years later. Hiwatts are a great example of fast response amps that use a solid-state rectifier in place of the slower GZ34 tube rectifier. Original Vox AC30s also actually possess lower overall power supply voltages, but the EL84 output tubes require different operating parameters than the octal tubes in the other amps I mentioned a moment ago.

That’s a lot of information for this month, but to summarize simply, the power supply, output circuitry, and speaker choices are indeed the crux of the biscuit! Many thanks to my dear friend Bjorn "Mad Professor" Juhl and my personal amp technician, Charles "Grumpy" Lyons, for their wonderful contributions to this discussion. We’re only getting warmed up, so see you next month.

Dean Farley
Dean Farley is the chief designer of "Snake Oil Brand Strings" (snakeoilstrings.com) and has had a profound influence on the trends in the strings of today.

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