Tone Tips

A Player’s Guide To Preamp Tubes

  • By Yoel Kreisler @tonereport
  • March 28, 2017
  • 0 Comments

Anybody’s who’s ever dug into almost any audio application of any sort, will not have to dig far to come across the word “tube”. For many years audiophiles, audio engineers, and musicians (especially guitar players), have all sung praises to the holy vacuum tube, which has garnered obsession from all corners of the both the consumer and professional audio markets. You may have come across the words NOS, black plate, grey plate, long grey plate, O-ring, halo getter, Telefunken, RCA, Brimar, Mullard, and generally, a whole alphabet soup of words and companies that can easily confuse those not versed in the language of glass. However, more and more people outside of the guitar world where tubes were almost always a mainstay are warming up (no pun intended) to our little glass friends. This in turn has filled the Internet with a sea of misinformation, misplaced reverence, and skyrocketing tube prices. As a guitar player looking for the best possible tone at the most reasonable price, it can be difficult to find a quality tube that isn’t hyped up to kingdom come or more expensive than the amp you’re trying to put it in.

Today I give you a bare-bones, baseline guide to the world of vacuum tubes, to help you more easily understand what you’re looking for, and to help you find the tone that you want for your amp. For the purposes of this article, we will be focusing on preamp tubes, which are the first thing your guitar hits after the input. Power tubes and rectifier tubes are a whole different ballgame, and one for another day.

The job of the preamp in a typical guitar amp circuit is to boost the fairly weak signal coming from your guitar, to a louder sound that can be moved on to the output section of your amp. The vibration of your strings hit the pickups, which transduce (or “translate”) string vibration into a low level electrical signal. That signal is sent from your cable to your amp’s input, from which it travels through to the preamp section to be boosted to a healthier level. From there, it goes out to the output section, to the speakers, and into the air for all to hear (especially those pesky neighbors!).

We have established that a preamp is essentially a booster for your weak guitar signal. So why have multiple preamp tubes, if one can do the work? Simply put, what we enjoy most about tubes is their harmonic distortion, which occurs even when we are playing “clean” on our amps. Why is that? Because of the nature of the technology, tubes impart their own character onto an audio signal, and our ears tend to enjoy this. From an engineering standpoint, one would assume that tubes would have fallen out of favor long ago. They keep us from hearing the “truest” possible sound of the audio signal that passes through it, because of the EQ characteristics, compression, and harmonic distortion it adds. However, since our ears are organic material and not computers, many of the frequencies not accurately represented by tube sound, are ones we can’t even hear! The nature of our ears tends more towards softer distortion and clipping, which is part of why we love tubes so much.

Since tubes all impart different sorts of EQ, compression, and harmonic saturation to our signals, we have to better understand the different types. Even two identical tubes that were made on the same day, in the same factory, one after the other, will not sound the same. So it looks like the battle is lost before it is even won, but it is in fact just beginning. You will see on forums, audiophile sites, and magazines something called “tube rolling”, which is essentially taking the same gear and trying a bunch of different tubes of the same type in the same socket, and reporting the results. People will talk about “sweeter highs,” “open mids,” “deeper bass,” and note separation at different volume levels and frequencies.

The earliest incarnation of preamp tubes were of the “octal” variety, which represents the number of pins they had. Some of the most common were (and still are) the 6SL7, 6SN7, 5691, and the ECC35. Many tube amp builders still use octal tubes in the preamp section, as they sound bigger, warmer, fatter, and more pleasantly compressed than their miniature cousins. Octals were slowly phased out a few years after WWII, when the nine-pin Noval miniature tubes we know and love today were introduced by RCA in 1939. These encompass all of the 12A-7 tubes, and a few other tubes that were mainly used in televisions and radios. These tubes stopped being manufactured in the USA for a number of reasons: EPA regulations were getting tighter and the demand just wasn’t there, thanks to the transistor. Tubes continue to be manufactured overseas until this very day, by companies in the Czech Republic, Russia, Slovakia, and China. These new tubes are produced under many different brands, some which are “reissues” of old brands, such as Tung-Sol and Mullard. These are produced in Russia under the New Sensor Corp name (owned by Mike Matthews of Electro-Harmonix). JJ, Ruby, TAD, Svetlana, and Electro-Harmonix branded tubes are all new.

While the new tubes can give you reliable performance (depending on the brand of course) for years to come, there is a reason that the NOS, or New Old Stock tubes fetch such high prices on the used market. There are many conflicting opinions as to why NOS tubes sound better, ranging from better quality control in the “first world” countries where the old tubes used to be manufactured, to “dangerous chemicals” that are now illegal that were once used in vacuum tube production. Pick your poison, but the fact remains that there is definitely a discernable difference in sound between the NOS vintage varieties and the newer offerings. Whether or not it’s positive or negative is up to you, but the fact remains that these old tubes were built to last, and they are still plugging away in countless guitar amps today.

When it comes to preamp tubes, there are a number of options. You can tame the gain in your preamp section by using a 12AU7 or a 5751 where a 12AX7 would normally be, which both have lower gain factors than the ever-popular 12AX7. You can switch out your new tubes with NOS ones, to improve the sound quality and change the character of your amp. Preamp tubes do not need to be biased like output tubes, but if you have two or three preamp tubes in your amp, make sure that they are reasonably close in gain factor.

If you decide to buy new, I personally enjoy JJ, Tung-Sol, and TAD tubes. They are almost always of consistent sound and quality, and are inexpensive as well. If you want to go NOS, you can never go wrong with Mullard, RCA, Amperex, vintage Tung-Sol, Raytheon, Sylvania, Phillips, or GE. Most vintage tubes can be found at reasonable prices online, but make sure they have been tested before you buy. These tubes are of excellent quality in build and sound, but again being quite old they can be fickle sometimes. Some tubes are more hyped than others, such as blackplate RCAs, Mullard ECC83s, Amperex “Bugle Boys,” Telefunken ECC83s, and their prices reflect that. Certain Sylvanias, Phillips, and GE tubes can still be had for reasonable prices, but make sure you understand the risk before taking the leap.

Well, that’s all I’ve got. I hope you enjoyed our excursion into the world of vacuum tubes, and taken something with you to help better understand this wide, beautiful, and confusing world of glass powered sound. Until next time!

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