DISTRIBUIDORES

One of my favorite old guitar amps is the all-tube Fender Champ“Tweed” model, shown in Figure 3.2. In addition to the sweet tone, the beauty of this amplifier is the simplicity.

Figure 3.2 Fender Tweed Champ amp.

© 2015 Doug Hubble Music.

It has three vacuum tubes, one speaker, and one knob for volume. Where is the on/off switch? Look more closely at the volume knob (see Figure 3.3). Yes, the whole amp functions from one knob!

Figure 3.3 Fender Tweed Champ control panel.

© 2015 Doug Hubble Music.

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A guitar amplifier is really part of the entire electric guitar instrument. But amplifiers don’t seem to get as much respect as guitars. A wise man once said that a nice electric guitar played through a piece-of-junk amp will sound like doo-doo, but a junky guitar played through a nice amp might not sound too bad. Even the most sophisticated modeling amps, amp simulator software, and rack-mount amp simulators are designed and judged by how well they emulate the sounds and behavior of actual tube amps. This is one reason why it’s important to explore the tube amp sound in its essence.

Let’s look at some of the basic components of the Fender Tweed Champ.

w Input jack for accepting cord from guitar.

w Volume knob that goes to 12.

w On/off switch. Wait, it’s part of the volume knob!

w One 12AX7 preamp tube. This is the first tube to amplify the guitar signal.

w One 6V6GT power tube. This further amplifies the signal, enough to power a speaker.

Figure 3.4 Fender Tweed Champ amp rear view, showing all three tubes.

© 2015 Doug Hubble Music.

w One output transformer to properly match the output signal from the power tube to the speaker input.

w One power transformer to convert the incoming wall electricity to voltages usable by the amp’s circuitry.

w One 5Y3GT rectifier tube to convert the incoming alternating current to direct current. It’s worth noting that most modern tube amps use a solid-state rectifier.

w A fuse to protect the amp in case of an internal failure, such as a tube failing.

w One guitar amp speaker. Speakers used in guitar amps are special and are quite different from full-range hi-fi speakers. The main difference is that guitar amp speakers have a deliberately limited high-end response.

In other words, if you plugged your tube amp into hi-fi or PA speakers, you would be aghast at how bright and harsh your amp sounded. Don’t try doing this unless you’re sure your hi-fi speakers can handle the power from your amp; otherwise, you’ll fry (burn up, destroy) your hi-fi speakers. Guitar amp speakers warm up the sound by cutting out or rounding off most of the high frequencies above about 5 kHz.

If you distill the components and operation of the most sophisticated tube amps, you’ll find that in their essence they aren’t too far off from this little guy. Guitar amps can appear in a number of different configurations, but two are by far

the most common. The first is the combo amp—a single box containing the electronics portion of the amp as well as the speaker(s). The other common configuration is the head-cabinet amp. With this setup, the speakers are in a separate box from the electronics.

Within certain parameters, it’s fine to mix and match amp heads containing the electronics with different speaker cabinets. As long as the speakers are rated to take the power of the amp, and the output impedance of the amp speaker jack matches the impedance of the speaker cabinet, you should be good to go. A simple rule of thumb is that if two speakers are wired in series, the impedance doubles, so two 8-ohm speakers wired in series would be 16 ohms.

Figure 3.5 Circuit diagram illustrating two speakers wired in series.

© Ken Silva.

If the speakers are wired in parallel, the impedance is cut in half, so two 8-ohm speakers wired in parallel would be 4 ohms.

Figure 3.6 Circuit diagram illustrating two speakers wired in parallel.

© Ken Silva.

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Figure 3.7 Circuit diagram illustrating four speakers wired in a series/parallel combination.

© Ken Silva.

Usually speaker cabinets are marked with the proper impedance, so you don’t even have to do the math. The output jacks of amps sometimes have an impedance selector, usually with 4-ohm, 8-ohm, and 16-ohm settings (see Figure 3.8).

Figure 3.8 Trillium speaker output impedance selector.

© 2015 Doug Hubble Music.

Figure 3.9 Trillium 7-watt guitar head. The Trillium is an amp that can accept a 6V6 output tube or a 6L6 without any adjustment.

© 2015 Doug Hubble Music.

Figure 3.10 I’m so lucky to have Kevin Silva as a friend and as an incredible amplifier technician. Kevin comes from a line of tube amplifier designers. His father and grandfather both designed tube audio gear.

Among other things, his dad designed the amplifier used in the Sears Silvertone guitar with the amplifier built into the case.

© Ken Silva.

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Figure 3.11 Mesa/Boogie Mark series speaker output jack. A single 8-ohm speaker can be plugged into the 8-ohm jack. Two 8-ohm speakers should be plugged into the two 4-ohm jacks (2 × 8 ohms in parallel = 4 ohms).

© 2015 Doug Hubble Music.

If you’re using a vintage amp that doesn’t have the output impedance clearly marked, then you need to do a little research. The output of almost all vintage Fender amps can be determined as long as you know the model and year the amp was manufactured. However, it is beyond the scope of this book to compare all the varieties of guitar amp speakers.

From a gigging musician’s standpoint, there is one other factor to consider besides the tone of the speaker: the weight.

I have replaced a few great-sounding speakers because they added too much weight and made the combo or cabinet burdensome to me. Recent advances in technology have enabled a number of companies to produce speakers with neodymium magnets, which dramatically reduce the weight.

Over the years there have been a few other amp configurations worth mentioning:

w The Sears Silvertone guitar with a little tube amp built into the guitar case.

w The 1960s-era Realistic combo tube amp with a built-in fold-down record turntable in the back of the amp.

w Rack-mount tube amps.

w Separate rack-mount preamps (containing the smaller preamp tubes and tone and gain control) and power amps (containing power tubes).

One of the beautiful things about tube amps is that 9 times out of 10, if you’re having a problem with the amp, you can fix it by simply replacing a bad tube. It used to be that you could go into almost any appliance store or even drugstore and use their tube tester. That’s because back in the day, not only guitar amps, but also TVs and radios used vacuum tubes. These days, it’s hard to find a tube-testing machine, though most amp techs have one nearby.

One simple test for preamp tubes is to turn the volume up pretty loud on the amp and gently tap on the tube with the soft, eraser end of a pencil. If the tube goes ping or Ping or PING!, then it’s microphonic and should be replaced.

Microphonic tubes will ring or feedback if there is too much mechanical or sonic vibration.

Guitar amps—especially combo amps with built-in speakers—can really shake up the insides of a tube. I always carry at least one spare preamp tube with me whenever I play a gig with a tube amp. I’ve had them fail at gigs. If the amp has multiple preamp tubes, I’ll use the eraser trick (or my fingernail) to track down the culprit.

You don’t have to replace a tube with the same brand of tube, but it’s important to replace it with the same type. Tubes have stenciled markings that indicate the type. Your amp likely has a chart affixed to the chassis with a layout of all the tubes (see Figures 3.12 and 3.13).

Figure 3.12 Fender Tweed Champ tube layout chart, none the worse for wear!

© 2015 Doug Hubble Music.

Figure 3.13 Fender Pro Junior tube layout chart. Note the absence of a rectifier tube. Most modern amplifiers use solid-state rectifiers.

© 2015 Doug Hubble Music.

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In general, stick to the chart when replacing tubes. I’ll qualify this by saying that there are a few modern specialty amps, such as the Trillium, shown in Figure 3.9, that allow you to swap out a power tube with certain other types of

designated power tubes. These amps come with precise instructions about what substitutions are possible. If you’re replacing a set of power tubes (you wouldn’t replace just one power tube in a set), you may have to have the amp biased at a technician’s shop. With preamp tubes there is room for a little experimentation—for instance, a 12AX7 can be replaced by 12AY7 or a 12AU7, both of which have different gain factors. Gain factor is the factor by which the tube amplifies the signal it“sees” at its input stage.