Tech's Files: Need Tubes?

Jul 9, 2010 4:11 PM, By Eddie Ciletti

ADVICE OF A THERMIONIC NATURE

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WHAT MANUFACTURERS DO
We all know that buying in quantity can reduce per-unit cost. When combined with the need for consistent performance (and the luxury of time), the lot can be sorted, graded and even matched. Geeks need quantifiable evidence, hence the need for test equipment. In some cases, it’s even necessary to build custom test fixtures. This is especially true for vacuum tubes because test equipment manufacturers don’t see “tube users” as a viable market.

Figure 2: The performance curves of a 6386 twin-triode (red) and a 6BA6 wired as a triode (green) are nearly identical—closer than samples of the same type might be—even if this image is too small to reveal subtleties.

Figure 2: The performance curves of a 6386 twin-triode (red) and a 6BA6 wired as a triode (green) are nearly identical—closer than samples of the same type might be—even if this image is too small to reveal subtleties.

For mission-critical tubes, people like Oliver Archut of TAB Funkenworks will tell you that no modern tube can match NOS, and this may be especially true for the tubes required by the V72-style preamps that his company manufactures. Larry Janus of Tube Equipment Corporation relies on a quartet of tightly matched NOS 6BA6 tubes to replace the two 6386 tubes (per channel) in his Fairchild 670 re-creation, the SR-71.

In both cases, being a geek on a mission requires some prescient urge to hunt and gather, plus a lifelong research quest for documentation, NOS alternatives, custom-made parts and the ability, desire and tenacity to D.I.Y. on a grand scale. Archut learned to make/rewire transformers while Janus modified/augmented a tube tester to improve the speed and quality of the matching process. (See Fig. 2.)

Tube matching is essential in Fairchild-type compressor designs. “In remote-cut-off [variable Mu-style] compression circuits, there’s a need to match tubes at multiple bias points,” Janus explains. “Any imbalance between the tubes allows the control voltage to leak into the audio, which reveals itself as ‘thumping’ during gain reduction. Unlike a fixed-bias circuit [such as a power amp], the remote cut-off stage is shifting bias during gain reduction and the pair of tubes needs to track at many bias points. In addition, both channels of a stereo remote cut-off compressor must track equally well, or image shift will occur during gain reduction.”

Although their products are very different, EveAnna Manley (Manley Labs) and Mike Spitz (ATR Magnetics) have similar D.I.Y. stories. “We have loads of experience with specific tube types—especially brands and lots that work exceptionally well in our circuits,” states Manley in her Website’s FAQ. “We have thousands of them in stock so that, after testing, we get a good enough yield to be able to sustain production for many, many years to come. Not all tubes are created equal. You can get a 1,000-piece lot of 1960s Phillips 12AT7s that are absolute trash. Brand-new Ei’s from Yugoslavia will whoop ’em performance-wise and sonically.” Manley also cautions against shopping for “highly prized” tubes on eBay, instead recommending www.upscaleaudio.com for pre-tested replacement tubes. Upscale’s site also contains a lot of useful general information on tube technology.

THE HUMBLE SERVICE BENCH
When a tube is not the problem, a tired, old carbon resistor or capacitor might be. If a high-gain circuit is noisy, then the plate resistor is suspect and should be replaced. If the sound is thin or oddly distorted, the coupling capacitor between stages—from plate to grid—may be the culprit and should be changed.


For more fun with Eddie Ciletti, visit www.tangible-technology.com.






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