Amps have been prodded, poked and pimped up with countless technical innovations over the years. Manufacturing techniques have been optimized to a point approaching perfection. But despite all the improvements, there’s one component at a critical point in all tube amps’ signal chains that can be as temperamental as the most mercurial prima donna. As irony would have it, the very thing that makes an amp sound so rich can make a player sound so poor. Guitarists have had to come to terms with this fickle force that most of us revere and sometimes have cause to fear. This diva among the electrical components of a tube amp is – of all things – the tube itself.
Power tubes, especially, operate under heavy loads, and they can’t be simply swapped without further ado. Most amps’ power tubes come in sets of two or four, and it takes some tweaking to adjust the bias of each tube to arrive at the optimum operating point. This can be a laborious, time-consuming undertaking. However, with a bit of technical nous and the right touch, this is the point where a lot can be done to ensure a tube amp’s future reliability and sound quality.
A brief digression on matters of technical concern: In most tube amps, power tubes work in pairs. Even in the smallest possible combination – say a pair of power tubes such as those in a TubeMeister 18 or Switchblade 50 combo –two tubes have to work together in perfect symbiosis, each amplifying one half-wave of the audio signal. Together they are strong; divided they are puny. The better the two tubes are matched, the more symmetrical and sweeter the signal and the happier the guitarist will be.
To this day amplifier tubes are still made largely by hand. Looking through the glass envelope at the bowels of a tube, it doesn’t take an engineering degree to see that thin sheet metal and wires are delicate, sensitive components whose tolerances can vary wildly. If the schmo on the assembly line with the tweezers and soldering iron is having a bad day, chances are the tube will be having a bad life. It’s not uncommon for dust or dirt to be sealed into the glass envelope. Oddly enough, a little grunge doesn’t always adversely affect tone. In other words, squeaky-clean tubes don’t necessarily sound better.
In any event, tubes are not assembled under anything like clean-room conditions. Sneak a peek at a typical shop floor and you’ll agree that we’re talking about tolerance standards that may have made guys in lab coats happy in the 1950s, but are not readily reconciled with post-millennial technology and fall far short of today’s demands for quality.
Determined to assure a consistently high level of quality for output tubes within a narrow tolerance range, we developed a largely automated method of testing tubes in 2009. It measures and verifies the properties and performance parameters of a power tube under real-world conditions. With this tool, it’s easy to rate and select tubes to put together perfectly matched pairs and sets of four. And that’s a good thing because tubes’ ratings have to be similar if not identical to ensure smooth operation and stellar tone.
This tool we developed is called the Vacuum Tube Inspector, or VTI short. Here’s how it works: After tubes are given a little time to get toasty, it slams them with up to 4000 volts in an actual amplifier circuit, there by safely pinpointing in any weaknesses. A small rubber mallet applies mechanical stress to simulate the vibrations that jar an amp on top of a speaker cabinet or during transport. In other words, VTI hammers the tube to see how it will respond when the going gets tough. Dirt, dust and other foreign material inside the tube are burned in during the test, which makes for a more stable, durable tube.
Power tubes are both sophisticated and simple. They’re delicate by design, yet they have to be tough enough to stand up to heavy mechanical and thermal loads. In the era of smartphones, DSPs and nano technology, they are relics from a bygone era. The VTI system is here to bring this technology up to current standards and our demands. It pinpoints and minimizes tubes’ weaknesses to maximize their great strength – tone.
This test, which takes about half an hour including warm-up time, allows us to draw remarkably accurate conclusions about a tube’s reliability and sound quality. As it turns out, only around 70% of all the tubes we test are good enough for use in our amps. No power tube has left our facility untested since 2009. Technology of Tone – that sums up our philosophy, and VTI stands as yet another testament to it.
First published: April 24 2014. Most recent update: October 16 2015.