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The Millett Hybrid MOSFET-MOSFET-MAXed


Tubes, otherwise known as vacuum tubes or electron tubes, form the basis of the Millett gain stage circuit. The tubes give the audio "flavor" of the Millett headphone amplifier, and the MOSFET-MAX preserves this portion faithfully. 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
Rather than focus on complicated tube theory, however, some basic info on the specific Millett tubes may be more helpful. Besides giving the Millett MOSFET-MAX its unique sound "flavor" due to the tubes, the other unique aspect of the Millett tubes is that they are low voltage. They were originally designed for automobile radios, where voltages tended to be about 12V (ala car battery). If you refer to one of the actual tube specification sheets below, you'll see that
the typcial Millett tube is generally rated for 12V on the heater, and from 12V to 30V on the plate. As it turns out, 24VDC is a common power supply voltage for many popular DIY headphone amp designs, and this is perfect for the Millett. Most of us tweak that a little further and turn it up to 27VDC, for that little extra in performance. As you will see in the Heater Resistor section, the MOSFET-MAX can actually be setup for as much as 30VDC. The Millett tubes are known as "Duplex-Diode Triodes." The diodes are not used in the MOSFET-MAX and are simply tied into ground. "Triode" means the Millett tubes are single channel, so two tubes are needed for the stereo music circuit. The actual gain, or amplification factor, of the MOSFET-MAX is controlled by the tube. The higher gains are used for high-impedance phones, while lower gains are used for low impedance headphones.
There are 3 tubes commonly referenced for the Millett and Millett MOSFET-MAX:
  • 12AE6 (later version: 12AE6A) .... basic amplification factor of 14
  • 12FM6 ............................................... basic amplification factor of 10
  • 12FK6 ............................................... basic amplification factor of 7

  • The 12AJ6 is also sometimes mentioned, and while it will work in the circuit, the basic amplification factor is 55(!), making it pretty much inappropriate for headphone use. There is some negative gain applied in the basic Millett circuit, so these numbers are generally reduced by 2 or 3, but are a good indication of which tubes to choose for your particular needs.
    Reference data sheets for some commonly available Millett MOSFET-MAX tubes (pdf files):
    12AE6A 12AJ6 12FK6
    12FK6 Sylvania 12FM6 12FM6-GE
    Generally speaking, Millett MOSFET-MAX tubes and their sound is distinguished by brand and by construction. Since the Millett circuit requires one tube per channel, it is beneficial that you try to select two tubes that are similar in these two features. By construction, we specifically mean the getter placement. 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 Millett MOSFET-MAX tubes, these getters can be circular or square, and located on the top of the tube or the side of the tube. The photos at top are two tubes with circular side getters. Here are a couple of other photos that show these differences:

    The arrows point out the position of the getters, which in these tubes, are also round getters. The one with the circular getter at top is often referred to has a "halo" getter, for obvious reasons. Give your New Old Stock (NOS) tubes plenty of burn-in. Usually, you can tell when the tube has sufficient burn-in by how quickly the bias settles. The getters need time to burn off the gas molecules that have infiltrated the tubes while sitting unused for the last several decades.

    Generally speaking the brand differences are slight. I find the GE's best all around, with good detail and some bass thump. Sylvania's seem to have the most extended highs, while RCA's have the best midrange. Obviously, this is only anectodal and your mileage may vary. You will find that despite a plethora of different brand Millett MOSFET-MAX tubes, only 3 or 4 mfrs actually made the tubes - GE, RCA, and Sylvania, for sure. Further, each company may have only made two or three, but bought others from their competitors and re-branded them. I suspect that this also went on during stock shortages, as well. The RCA tubes generally have a flattened octagon label with the tube designation, while GE etched small dots on the tube glass. I haven't found specific differences in sound with different getter construction, but it seems reasonable to match this as well.
    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 Millett MOSFET-MAX tubes. A tube pin straightener:

    These can be picked up for a few dollars on e-bay.

    file last changed:Sunday, September 7, 2008 7:00:00 AM
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