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 Why use Vacuum Tubes?

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PostSubject: Why use Vacuum Tubes?   Fri Feb 27, 2009 1:51 pm

Vacuum tubes (or thermionic valves to our friends outside of North America) often get a bad rap. Although touted for their 'warmth' (enhanced 2nd order harmonic content) and their ability to soften the digital edge (non-linearities inherent in the tube circuit), they are often seen as finicky and unpredictable, sometimes noisy, and lacking in high frequencies. And unlike their solid-state transistor brethren, they sometimes need to be replaced.

The first actual vacuum tube triodes were developed by Irving Langmuir at General Electric in 1915. Like many inventions, these were spin-offs of earlier inventions and improved on theories and inventions of earlier scientists (including Tesla, Hittord, Goldstein, Fleming, De Forest and Edison). At it's most basic, a tube consists of electrodes in a vacuum surrounded by glass. The first electrode called a filament ( or 'cathode') releases electrons into the vacuum (thermionic emission) resulting in a negatively charged cloud which is drawn to the other electrode, the positively charged plate (or 'anode'). The result is current flowing from filament to plate in one direction (also called a diode). A triode tube adds a the third electrode, called a 'grid', between the filament and plate. By varying the polarity of the voltage applied to it, the grid controls the number of electrons flowing to the plate from the filament. Thus, the grid is used to control the plate voltage, turning it into a voltage amplifier.

Summit Audio has over 21 of years working with vacuum tubes. We've used vacuum tubes in many different circuit designs and have developed circuits that exhibit highly reliable and long lasting tube life with excellent high frequency response. We often see 10,000 + hours of tube life with very low noise floors and incredible consistency between units even 20 years apart in age. How do we do it?

Some of the areas where we focus include:

Plate Voltage. We're committed to tube designs with full, class A plate voltage to the plate. We use 185 volts there, and never use starved plate, low voltage designs. All amplifiers operate on a gain curve with a certain 'sweet spot' where the vacuum tube manufacturer's specifications are ideal. Starved plates are often characterized by unexpected gain curves and poor consistency from device to device and over time, with exaggerated distortion characteristics. By using full plate voltage, Summit Audio maintains a sweet sounding tube circuit without unwanted distortion and a very predictable, long life.

Regulated Voltage. By using fully regulated voltage on the vacuum tubes, Summit Audio gets much longer tube life, more predictable results from the tubes, with less intermodulation distortion. Regulating the power to the tube, a much more modern design than some old concepts, also means that it takes lower voltage to stay in the previously mentioned 'sweet spot'. For example, if we used unregulated power, it could take as much as 230V just to stay near the ideal voltage for the tubes, and any sag in the incoming voltage would increase the distortion and lower the gain of the tube itself. Regulated power means the tubes have predictable, clean power to run at their ideal gains, all the time.

Negative Feedback Designs. Different feedback and non feedback designs are concepts for controlling distortion and gain. Negative feedback design works by feeding back a portion of the output signal to counteract the input signal. This reduces distortion and keeps the operation of the tube closer to the intended gain. For example, if a tube's gain drops by 5%, so does the negative feedback pushing back on the input, so there is no net loss. In a no feedback design, there is no feedback to control the gain of the tube, allowing more of the tube characteristics to shine through. Summit Audio uses non feedback, full feedback and various levels between design concepts, choosing what is right for the product and the desired affect. For example, the 2BA-221 uses negative feedback, the TPA-200B uses no feedback, some other A-Line uses a different type of feedback called local feedback (uses feedback from the cathode), and some, like the DCL-200, use a proprietary, hybrid approach.

Vacuum tubes have been around for a long time. Although transistors have replaced tubes in the majority of circuit designs, correctly used vacuum tubes have many advantages in high end audio. By choosing the right design for the product, running high voltage to the tubes, and regulating that power, Summit Audio has been able to design highly predictable, long lasting vacuum tube gear that takes advantage of vacuum tube circuitry without the disadvantages of older designs.
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