Tech's Files: More Vacuum Tube Tales

Sep 1, 2009 12:00 PM, By Eddie Ciletti

MYSTERIES OF TWO VOX AC30S

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At top, two NOS (new old stock) American-made switches—the silver-plated solder lugs tarnished by time, except on the left—are shown recently polished. Between them are the two rockers from inside the Chinese Vox switches, looking older than their five years. The bottom-left pair are miniature Chinese switches that appear comparable to the higher-quality, silver-plated Japanese and American switches to the lower-right. The differences you can't see are the contact and housing materials. Chinese switch housings tend to melt when overheated during soldering.

At top, two NOS (new old stock) American-made switches—the silver-plated solder lugs tarnished by time, except on the left—are shown recently polished. Between them are the two rockers from inside the Chinese Vox switches, looking older than their five years. The bottom-left pair are miniature Chinese switches that appear comparable to the higher-quality, silver-plated Japanese and American switches to the lower-right. The differences you can't see are the contact and housing materials. Chinese switch housings tend to melt when overheated during soldering.

A simple excursion into geek territory can turn into a revelation or three; you might experience profound light-bulb moments or merely the simple reminder of a component not to be taken for granted. The Vox AC30 is a great amp. The preamp section has versatile EQ, reverb and an effects loop. The 30-watt power amp section has interesting features, like a tube rectifier (earlier versions were solid-state), and modern niceties like bias and filter capacitor switches on the rear panel for vintage/modern options.

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A few years ago, a famous band came through town with a troubled Chinese-made AC30 guitar amp. In the four hours allowed for a diagnosis, I replaced a broken bias resistor and a suspicious standby switch. The EL84 output tubes were also suspect because they were behaving as though microphonic — a strange, harmonically crunchy resonance when the amp was played loud enough to vibrate them. I had no EL84s in stock but would later learn that they were not the problem, so when another ailing AC30 came in, my curiosity was piqued.

The second AC30 was DOA. I replaced a blown fuse, and in such cases a gentle wake-up call from Mr. Variac is the safest way to bring troubled power amplifiers back from the dead. (Variac is the trade name for a variable-voltage auto-transformer, easily recognizable by its giant knob.) Rather than a traditional “hard-boot,” the Variac's soft-start lets me look for the warning signs — potentially excessive voltage or current — before damage can occur. For example, visible glass-fuse wires will “flex” during slow power-up; stop in time and they won't blow. The Variac was barely beyond 75 percent when the 5AR4/GZ34 rectifier tube began arcing. Clearly, something was wrong.

A Web search harvested plenty of info about short-lived 5AR4s. The general consensus is that modern tubes aren't up to vintage standards, but it's too easy to blame all of them. Even during the heyday of the Thermionic era, there were loser tubes. I confirmed this with my Uncle Vince and his pals from the early TV era. Sometimes only one particular manufacturer's version (or production run) would work in certain critical applications, yet all had the same designation. Guitar amp designs — and users — are often abusive, and I profess sympathy for the tubes.

Tube Amp ABCs

A vacuum tube's gradual warm-up time may give the impression of slow and gentle, but the power transformer and the power switch take the initial hit because a cold tube filament looks more like a short-circuit. (Its resistance goes up when warm.) Tubes take time to “warm up” because their filaments glow reddish-orange instead of the yellow-white of an incandescent light bulb. The orange heat is high enough to burn electrons off the cathode but low enough to ensure longevity. In an all-tube amplifier — one with a rectifier tube and no standby switch — the high voltage (HV) comes up slowly as the rectifier tube warms up. During this temperature transition, the preamp tubes may start out with a rush of noise that settles down eventually — reason enough to leave gear powered up during a session or at live gigs.

Once the filament is hot enough, negatively charged electrons need somewhere to go and the positively charged plate (the large metal structure under the glass) is absolutely irresistible, so the current flows easily. The bias circuit stops the stampede and optimizes current flow. Perhaps due to random noises, the standby switch was invented. This ultimate mute switch interrupts the HV power supply — aka, the B+ — leaving the aroused electrons with nowhere to go.






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