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The Starving Student Millett Hybrid

The 19J6 Tube
photo by Head-Fi user "kansei"
 Tubes, otherwise known as vacuum tubes or electron tubes, form the basis of the Starving Student Millett Hybrid gain stage circuit. The tubes give the audio "flavor" of all Millett Hybrid headphone amplifiers, and the Starving Student is no different. Many books have been written about tubes and their sound quality. An excellent article on "How Tubes Work", their parts and operation is online at Dale and Roy's VacuumTubes.net.
Rather than focus on complicated tube theory, however, some basic info on the specific Starving Student tubes (we'll take the liberty of calling them "Starving Student tubes") may be more helpful. Besides giving the Starving Student Millett Hybrid its unique sound "flavor" due to the tubes, the other unique aspect of the Starving Student tubes is that they use 19V heaters. This allowed Pete Millett to come up with the ingeneous idea of splitting the voltage among the heaters of two tubes, connected
photo by Head-Fi user "kansei"
to a simple single MOSFET as an output buffer and biased by the heater current. The entire hybrid is fed with a 48VDC, Cisco VoIP type switching power supply. The 19J6 tube is a dual-triode tube rated for 100V on the plates. For the Starving Student Millett Hybrid, the triodes in each tube are paralleled, with the Cisco's power supply voltage providing the plate voltage at 48V, about half of the normal operating recommendation. The result is a medium-reduced voltage tube hybrid that is more or less self-balancing for the tube pairs.
The 19J6 has the following basic attributes when run at 100V on the plates:
  • Basic amplification factor of 38,
  • Transconductance of 5300 micromhos
  • Plate resistance of 50 ohms.
    • One thing of note, particularly important to the Starving Student, is that the 19J6 has a quoted heater warmup time of 11 seconds. However, the Cisco power supply combined with the Starving Student circuit and tubes will experience an even longer delay in startup. Please be aware of this when you are preparing to plug in your headphones - allow the tubes plenty of time to warm up.
    It's been reported that perhaps RCA was the only true manufacturer of 19J6 tubes. Regardless, here are data sheets from three different mfrs of the 19J6 tubes (pdf files):
    19J6 (GE) 19J6 (TungSol) 19J6 (RCA)
    Generally speaking, SSMH tubes and their sound is distinguished by construction. By construction, we specifically mean the getter and its placement. While the getter style probably has nothing whatsoever to do with the sound quality (most likely the plates do that), it is still a pretty good visual indicator of different overall construction of the tubes, period.
    The tube getter, accompanied by the silver "splotch," is a loop of wire. The loop creates a localized charge field that when catalized by the chemical in the silver splotch, burns up excess gas molecules that may have leaked inside of the tube. In the SSMH tubes, these getters can be circular, square, or horseshoe shaped (sometimes called a "D" getter) and can be located on the top of the tube or over to the side of the tube top. The photos at top are two tubes with different getters - one has a top horseshoe getter, the other - a top square getter. Here are a couple of photos that show other getter types:
    photo by Head-Fi user "bmwpowere36m3"
    photo by Head-Fi user "bmwpowere36m3"
    		    


    The tube at left has a side horseshoe getter. The tubes on the right all have top-square getters. Give your New Old Stock (NOS) tubes plenty of burn-in. The getters need time to burn off the gas molecules that have infiltrated the tubes while sitting unused for the last several decades.

    Selecting and Buying SSMH Tubes -
    In selecting/buying SSMH tubes, one thing to remember is that variation exists in NOS tubes. They are not like opamps, transistors or other semiconductors. With few exceptions, silicon-based active devices are consistent to a certain level of performance. While tubes were similarly rated, even new - they were nowhere near as consistent in their performance. The sound of your SSMH can depend almost exclusively on the tubes. While the MOSFETs have a great effect, they can only provide current to the signal that the tube has already provided.

    What does this mean to the SSMH builder and owner? Quite simply, buy more tubes and try more tubes. You may be delighted to find how different they can be. The differences range from slight to great, but can be wildly inconsistent. Ordinarily, one might recommend a certain brand for a type of sound, but you will probably find a particular tube construction stamped/marked by any of the manufacturers.


    photo by Head-Fi user "abcheng"
    		 This tube was unfortunately destroyed when the tip was accidentally broken off. However, it is an educational photo from the perspective of the getter "flash." Instead of a chrome silver, the getter has completely oxidized and turned white, indicating a total lack of vacuum in the tube. As you might guess, the tube is irreparably damaged. It's nice to know what to look for if your tubes go bad, though - without an accident.

    Finally, there's an interesting accessory you might be interested in if you start buying/collecting many tubes. A tube pin straightener:

    These can be picked up for a few dollars on e-bay.    		    
    file last changed:Monday, May 27, 2024 12:21:44 PM
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