Amp Anatomy: How Power Tubes Affect Your Tone

Date:2020/3/3 17:08:04 Hits:

How does one get better guitar tone? It’s a question that will instantly set off a minefield of opinions, conjecture, legend, lore, and even a few wrong turns. But one thing most guitarists can agree on is that much of the glory of a great tube amp comes from the little glowing glass bottles that supply its power.

Because there are so many conflicting opinions about the effect different types of tubes will have on your tone, we decided to go to the most knowledgeable source we could think of, Sweetwater’s resident amplifier guru — Electronic Repair Technician Greg Bowers. After years of designing, building, repairing, and modding tube amps, he knows a thing or two about what makes each one sound unique, and what makes the great ones truly great.

Here, Greg dishes on the most popular types of guitar tubes, why they’re popular, and what you can expect from each, and he even dispels a rumor or two about these magic sonic capsules.

But first, a bit more about Greg Bowers.

Greg has been working tirelessly at his profession for over 20 years. In fact, it goes all the way back to servicing old tube radios when he was in high school in 1979. Over the years, he’s had the pleasure of servicing amps for countless Sweetwater customers and famous players alike who were looking for repairs and modifications. And as he says, “If you do this long enough, you also get ideas for amplifiers yourself,” which is why guitarists all over Sweetwater proudly play amps designed and built by the man himself.

Sweetwater: What is it about the sound of tube amps that makes them so much more sought-after than solid-state circuits?
Bowers: There are some solid-state circuits that do an impressive job for tone, but the reason guitarists gravitate to tube amps is the harmonics. The secondary harmonics of a tube are additive to the harmony of the note, where the secondary harmonics of a solid-state amplifier are dissident to the tone.

Does the type of power tube have a massive effect on an amp’s overall tone?
This depends on if the power amplifier is driven hard. The secondary harmonics of an EL34 sound different than the ones from a 6L6GC or others. Each have a distinct tone and shape. At lower volumes, most of the extra “juice” we like to hear is not happening.

EL34, EL84, 6V6, and 6L6 are the most common power tubes. Why do you think they rose to the top?

6L6
It started with the 6L6GC. These were invented back in the 1930s. There were other notable power tubes at this time, but the 6L6 was common and easy to get. I assume this is why Leo Fender chose them.

6V6
The 6L6’s little brother is the 6V6, which also is very commonly used. It worked well for smaller guitar amps because it has a unique “chime” tone to the harmonics. Fender used them in his classic Deluxe and Princeton amps. Since many American groups of the 1950–’60s used Fender for their trademark sound, they set the definition for this tone.

EL34
When Marshall was making their first versions of Fender’s amps in 1959, they did not have the 6L6 available, so they went with another favorite power tube, the EL34. Because it has an extra grid in it for more power, they provided a unique distortion tone that became the very distinct Marshall sound! They were also lucky that major rock icons like Pete Townshend and Jimi Hendrix were their first customers.

EL84
VOX’s designer Mr. Denney decided to use the little brother to the EL34, which is the EL84. This tube breaks up into distortion early, and he found this distortion was very pleasing to the guitar tone. By biasing them very hot, it also gave them a very nice “ring” to the harmonics. Denney also got his amps in the hands of a little English band called the Beatles. And when the Beatles got famous, everyone wanted to use what they used.

6550s and KT66s, KT77s, and KT88s are also popular tubes. What do they bring to an amp’s sound?
As rock music was demanding louder and larger amplifiers, makers started using larger power tubes. Here come the 6550, KT88, KT66, and KT77. They are known for more headroom and a harder edge when they eventually distort.

Understandably, bass amplifiers also needed more watts to keep up with the guitarist. So Ampeg and others began to use six or more of these larger tubes — like in the iconic SVT.

Later in the 1970s, EL34 tubes became hard to get. Marshall had to go with other tubes, like the 6550 or KT77 (and then even back to the good old 6L6). They made the Marshall amps sound different. Today, people commonly swap these tubes around to tailor the Marshall tone to their liking — with bias correction of course!

Does where tubes are manufactured make a difference for the quality of tone?
The tube-making business dried up in the 1970s. We are now left with only three primary places to make tubes in the entire world: China, Russia, and Slovakia. But a more important factor to consider is tube construction. The same part number tube from different companies can have the same technical capability but be made physically different.

In the example of the 12ax7 preamp tube, it can have short plates or long plates used in it. Long plates sound wonderful but have the side effect of sensitivity to microphonics in portable gear. Therefore, a short plate tube is better for guitar amplifiers because we want low microphonics.

The tone signature of the harmonics also is slightly different from one brand of power tube to another. Therefore, one brand of tube may be more to your liking. It is recommended to use brands that carefully screen for quality from these available sources.

Are there any myths about power tubes that you’d like to dispel?
One that comes to mind is that if you see a blue glow it means the power tube is bad. Sometimes a power tube may glow blue inside. It has no effect on the sound. This phenomenon only happens in a tube with better-than-average vacuum (pure). Therefore, the tube is actually a better-quality tube!

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