Guitar Amps and Tubes

Jul 1, 2003 12:00 PM, BY EDDIE CILETTI

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I have more than a passing interest in guitar-amplification technology, specifically the tubular variety. Here are a few tips that range from safety to sonics, and noise immunity to tube testing. As you know, the inner workings of tube amps can expose you to hazardous voltage. Proceed at your own risk: Keep one hand in your pocket and wear shoes, please! No bare feet!

NO MORE HOT LIPS


Anyone who's been “zorched” by a nasty shock when your lips touch a mic while playing through a miswired guitar amp will appreciate this: All guitar amps with a two-pronged AC power plug will most likely have a ground switch. The switch must be removed and a three-conductor cable installed, initially without attaching the ground (green wire). Unplug the amp before soldering. There are two possible ways to connect the black and white wires (hot and neutral, respectively) from the new power cable. Determine correct orientation first by connecting an AC voltmeter from the chassis to a known ground: the third “hole” or plate screw of the AC outlet. Of the two possible voltages — the “wrong” one will be in the neighborhood of 60 volts — choose the connection that yields the lesser of the two and then connect the green wire to the chassis. No more hot lips!

MAGIC TRICKS


Guitar amps and other “classic” audio gear are all about nuance. One fairly simple explanation for this magic is simply to count the number of “active devices,” tubes or transistors. Keep in mind that a typical IC operational amplifier may have two-dozen transistors. We all know what op amp clipping sounds like: It's nasty and symmetrical, but from noise floor to fuzz, it's linear-clean all the way. This is a design goal; an op amp is not supposed to have character, just like digital without plug-ins.

The magic, which just about everyone is looking for, originates from a single tube or transistor gain-stage running at full-bore. Between linear-clean and full-overdrive is the region that I refer to as “the sonic air bag,” that classic richness full of lovely, inherent flaws (distortion). Some designs have an inherently larger air bag.

A guitar amp with four output tubes and four dual-triodes becomes a baker's dozen of devices if a tube rectifier is included. While the actual number of gain stages in the primary signal path is less than this total, each device makes it mark. Changing the tube rectifier to solid-state will reduce the amount of “sag” in the power supply when the power amp is overdriven. Choosy guitarists utilize tubes even in the power supply.

While vacuum tubes do have unique sonic characteristics, they can be configured to deliver linear performance like an op amp. Whether solid-state or “valve,” an op amp benefits from a greater number of active devices, capable of incredible amounts of gain, but operating far more comfortably in their linear region. Add some negative feedback to reduce gain (as required by the task at hand) and distortion is reduced, as well — to nearly unmeasureable levels across the dynamic range — until the AC signal hits the DC power supply rail(s). There's no nuance in hard clipping; lots of gain equals gobs of sustain, aka, instant fuzzbox.

TUBE E OR NOT TUBE E?


For circuit designs that are all about character, every component plays a role, from the amplification devices to the resistors and capacitors. And when vacuum tubes are considered, heat not only accelerates the aging process of all components, but also changes their properties as temperatures shift from cold to hot. This vacillation is good to keep in mind if a tube replacement yields a temporary fix or when you're specifically looking for “masked” or sporadic problems.

Everyone knows to scrutinize plate resistors for noise, but another source could be the cathode resistor, although its noise may be “hidden” by the bypass cap. If disconnecting the cap reveals some “rocky” noise, then that resistor should be replaced. In addition, a funky/thermal cap might be responsible for intermittent noise and frequency anomalies. The figure shows an input preamp schematic from a Fender Twin Reverb. The white section indicates the cap in question.

PRE-STRESSED


Some tube vendors specialize in selling “glassware” that is pre-tested and selected for high performance. Audiophiles pay a premium for this type of service, though it generally does not apply to studio and guitar types who require a better value for their dollar. Tube performance varies among manufacturers — especially now — and even compatible tubes like the 6L6 and 5881 are very different, the latter being far more linear.

It is common to match power tubes so that each equally shares the load. These days, I recommend that all tubes be tested before purchase, even in the most basic way, at minimum, to screen the clunkers. All equipment manufacturers (hopefully) put their valves to the test, selecting the best for preamps: delegating the noisier “bottles” to driver duty, with the rest ending up in the trash.

PRE-TESTING THE 12A?7 DUAL-TRIODE


Many circuits rely on several 12A?7-type tubes, the “?” may be a “U” or a “T” or an “X,” just a few of the letters that refer to performance characteristics, the most obvious of which is gain. (In tube lingo, that's amplification factor. For semiconductors, it's called Beta.) Military tubes have only a four-digit number instead. Tube and transistor parameters have a tolerance window that varies during production; the differences between new-old-stock (NOS) and more recent products can be extreme.

Most tube-testers measure only “static electron emission,” enough to determine that a tube is functioning as displayed on either a numeric or a bad/good meter. A tube could test “good” and also be bad if it suffers from “gas,” a condition that can cause extremely nonlinear amplification. Because just about everything tubular uses a 12A?7, I began by testing each triode section for “equal emission.” The booklet included with my “Supreme” TV-7/U military test set specifies a “minimum value” of 32 per triode. (This number is specific to the tester and does not relate to any other unit or specified parameter.)

It is easy for military-grade NOS tubes to meet this spec and have nearly identical triode sections — off-the-shelf — but it is far less common among modern tubes, hence the need for pre-testing. Variable-mu compressors and some Pultec amplifiers prefer “matched” triodes, while most preamps do not. I use this test only to weed out the junk.

Okay, so maybe you're not going to test tubes, although testers are available from eBay, some with full-service documentation. So when purchasing tubes, request that the dual-triodes be selected for a best match. I happily paid a $1-per-tube additional charge last year (well worth the cost), compared to the extremely poor-quality tubes I was sent the first time by the same company!

TESTING: ONE, TWO, THREE


Whether in a guitar amp or a mic preamp, a high-gain circuit is the perfect rig for noise tests. There's no space here to provide enough examples: Simply tap on all of the tubes while the gain is up to determine “the sensitive one.” Then, short the input connector (if applicable) and listen to the “tube symphonia” as the preamp is powered up. A whole array of whooshes and clinks can be heard as metals expand. When things settle down, note the noise. Your memory should be good enough for comparisons, but for documentation, find a high-resolution digital meter. Cool Edit Pro's level meter is worth the price of the software alone at www.syntrillium.com.

The noise spectrum will vary considerably from loud to not-so-loud, rocks or hiss, plus another variable known as microphonics. High-gain tubes must be built to exacting tolerances for mechanical noise immunity. Unlike transistors, tubes can resonate to the point of oscillation if they're in the same enclosure as a soloing guitarist's loudspeaker. For the purpose of microphonics and noise testing, all of the 12A?7 family can be tested in the high-gain socket. Keep in mind that lower-gain tubes like the 12AU7 will have less noise.

BIAS


Of the many contributors to a guitar amp's efficiency and overload characteristics, output tube bias is one of those voodoo variables. Mesa Engineering solved this issue by “fixing” the bias to a value that matched its pretested tubes, choosing only those valves that make the grade both at the time of manufacture, as well as future replacement. Other amps have adjustable bias that can easily be misadjusted.

The purpose of bias is to set a comfortable operating environment for the output tubes that will optimize their life and minimize nasty crossover distortion in a push-pull (Class-A/B) circuit. Many amps run the plate voltage at maximum so that only higher-performance tubes can do the job. Electro-Harmonix (www.sovtek.com) claims its 6V6 EH can tolerate the juice that Fender Deluxe and Champ amps deliver. The easiest way to observe this is to watch the tubes in a dark room. Only the filament is supposed to glow red and not the outer shell, called the plate. This is the result of the grid being too positive (beyond Class-A).

A grid that's too negative (Class-B) will make the amp run much cooler, but it won't sound very good. There are many ways to optimize bias, including some after-market boxes from Groove Tubes (www.groovetubes.com) that make the process “more safe.” The highest voltages are at the output tube sockets, so if you don't want to invest $120 in the kit, then you may want to leave this tweak to a pro.


Eddie Ciletti is Mix's resident tech. Visit www.tangible-technology.com for more fun.






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