Samick Motherlode

December 2014
more... Amp DIYGearDIYJanuary 2009

Everything You Ever Wanted to Know About Preamp Tubes

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Everything You Ever Wanted to Know About Preamp Tubes

The only pentode preamp tube seen with any regularity in amps today is the EF86 (or 6267), which appeared in early Vox amps and has more recently been used in models from Matchless, Dr Z, 65amps, and a few others. Another less frequently seen, but much admired, pentode preamp tube is the 5879, notably used in Gibson’s GA-40 Les Paul amp of the late fifties. Both of these pentodes fit the same 9-pin bottle as the dual triodes but require very different circuitry, and are known for their thick, robust sound. Both have higher gain factors than even a 12AX7, but aren’t prone to distorting the way that dual-triodes can, and instead pass their fattened-up signal on to the next stage. They also have a reputation for handling effects pedals very well. Drive a 12AX7 hard, however, and it will induce quite a bit of sizzling, slightly fizzy-voiced distortion of its own. This can be a great thing if you’re looking for a super-fried overdrive tone that’s cooking at all stages, but not at all desired if you want more headroom and clarity, or the fatter distortion that’s generated in the output stage of the amp when a cleaner preamp signal is driven into clipping at the output tubes (more of which in the next installment).

Some modern high-gain amps are designed specifically to create extreme yet controllable preamp tube distortion by cascading multiple gain stages, one into the other, with gain and master volume controls between them to control the drive levels at each stage. Used in this way, preamp tubes can produce a scorching, harmonically saturated lead tone that sustains all day—what we usually hear as a classic shred or contemporary rock tone—in an amp that really relies on its output tubes just to amplify this sound, rather than to add further distortion to it. When driven into distortion in a simpler, more basic amp with fewer gain stages (a category that might nevertheless include some very high-end, “boutique” tube amps), preamp tube distortion becomes just a part of the amp’s overall distortion character, blended with clipping at the phase inverter and output stages, and often at the speaker too.

Counter-intuitive though it might sound, armed with the above knowledge regarding preamp tube distortion, many players have learned to create a bigger tone by using lower gain preamp tubes. To lower the gain of a preamp stage a little, you can swap a 5751 into any socket that carries a 12AX7. To lower it even more but retain the same performance characteristics (other than gain) you can use a 12AY7. Many players think the last thing they want to do is lower the gain of a preamp stage, but in doing so you can often prevent your signal from dirtying up in the preamp, and thereby pass a beefy, full-frequencied signal along to the output stage when the amp is cranked. This generates more output tube distortion, which results in a fatter, fuller tone in many simpler tube amps. This tip doesn’t usually apply to high-gain type tube amps, whose whole raison d’etre is to generate preamp distortion. This 5751 swap is a trick that was used by Stevie Ray Vaughan, for one, to help generate his signature tone, and it has also been employed by plenty of other great blues players. If you’re trying
Note: the term NOS, which stands for “new old stock”, is applied to tubes manufactured many years ago but never put into use.
to achieve less of what you hear as preamp distortion and more output-tube distortion, you can also try using a 5751 in the phase inverter position, which is usually the last preamp tube before the output tubes.

Even tubes of exactly the same type can sound quite different, depending upon their manufacturer and small changes in their design and production. The fact that tubes distort so organically also means that no two tubes distort or even amplify exactly alike. For one thing, while tubes are manufactured under fairly rigorous conditions, they are still imperfect devices. Every little fluctuation in assembly or materials results in a slightly different sound and performance from each tube that comes along.

That’s why good tube distributors need to routinely test tubes they sell: put even two high-quality NOS preamp tubes from highly respected American or British manufacturers on a tube tester—say, a pair each of Mullard or RCA 12AX7 preamp tubes that came out of the factory on the same day in 1963—and they will most likely have slightly different readings for gain and other factors. Put enough of them up on a tube tester and some will even fail to meet required minimum standards. That’s the way it is. Aside from giving different readings, these tubes will each sound just a little different, and other makes, both NOS and current, will sound different again.

What does this mean for the guitarist? For one thing, it behooves you to get your hands on as many different makes and types of tubes as you can reasonably afford. Try swapping a few around to see which ones help you to best achieve the tone you are seeking. The first preamp tube position usually affects the tone of that part of the amp the most (read your amp’s tube chart or owner’s manual to make sure you know how to change tubes safely, and are changing the right tube, and please don’t touch any hot tubes! Let them cool down first). Try three different makes of 12AX7 or their equivalent in that position, and I’m willing to bet you’ll notice a slightly different voice from each. Search the internet and read up on what other players consider to be the best current- manufacture tubes coming out today (there’s too much detail on that subject to go into here). Also, see if you can find any affordable NOS tubes, or perhaps you can pull some used but functioning examples from old junker radio or hi-fi systems that you find at garage sales and swap meets. Experiment a little, and see which ones work for you. Preamp tube tasting can become addictive, and it’s also a great way to fine-tune your tone.

NEXT MONTH: PART 2 -
Output Tubes
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