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The Relic, or Why VTI?


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.

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Leave a comment

Joe Brindley on May 3, 2014 Reply

Are VTI tested tubes only installed in German made h&k amps or are they also in Chinese made h&k amps like the Grandmeister?

    Hughes & Kettner on May 3, 2014 Reply

    Hi Joe,

    we shipped similiar testing machines to the production line in China, shipping tubes around the globe to test them in Germany would add costs and sacrifice quality. The EL84 used in all Meister-amps are way less fragile than high power tubes, so the testing is not as “sophisticated” as with EL34, but still very effective.

    Best regards

Matt Wells on May 1, 2014 Reply

4000 volts???

I think that should be 400volts.
4kv would surely cause internal arcing?

    Hughes & Kettner on May 5, 2014 Reply

    Hi Matt,

    up to 4.000 volts (depending on the type of tubes) is correct, but it is a very short spike voltage.

    Best regards.

Frederick Glazier on April 25, 2014 Reply

I want an amp that has a channel to mimic classic fender clean, another for vox chime, another for edge of breakup fender bassman, and a final for marchall madness

Griff on April 25, 2014 Reply

Will you be rebranding and selling in the future like Mesa does? I think any time an amp manufacturer develops their own strict policy on tube screening it’s a good thing. I’ve always thought that Mesa’s screening and color code matching has done a good service to the musicians who own their amps. It creates a lot less hassle of going through the biasing process, and shows a very professional attitude toward the product name.
Anyway, I certainly hope that’s the direction you’re going with this article.

    Hughes & Kettner on April 25, 2014 Reply

    Hi Griff,

    you can order VTI-tested tubes as a spare part from the Hughes & Kettner service department, but there are no plans to sell them in stores as an own brand, the capacity of the VTI machine is limited and just enough for our own production needs … but maybe we will have to build some more VTI machines for the future? 🙂

    Most of our amps have a part of the VTI machine built-in, it is called TSC (Tube Safety control), an automatic tube testing an biasing module. TSC makes sure, that the amp will work fine if original VTI-tested tubes are not available. More about TSC: http://hughes-and-kettner.com/#rock-on-stage (scroll down to “peace of mind”)

    Best regards

Ector Soto on April 25, 2014 Reply

It is an unargueable fact that tones that are generated from power tubes are comparable and second to none. But the fact of the matter is technology is comin up fast. For exam

todd on April 25, 2014 Reply

smooth and simple genius,you have discovered the core of good sound and how to tame it,anyone who uses an amplifiershould know the name Hughes and Kettner.Bravo