Tubes, otherwise known as vacuum tubes or electron tubes, form the basis of the Millett Hybrid
gain stage circuit. The tubes give the audio "flavor" of the Millett Hybrid headphone amplifier, and the Mini-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 VacuumTubes.net.
Rather than focus on complicated tube theory, however, some basic info on the specific Millett tubes (we'll take the liberty of calling them "Millett tubes") may be
more helpful. Besides giving the MiniMAX 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
photo by DIYForums user "vixr"
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 MiniMAX 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 MiniMAX
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 MiniMAX 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 Hybrid MiniMAX:
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 also the 12FT6 tube, with an amplification of 15, but I can find no one who can substantiate whether it actually existed. Anyway, there is some negative gain applied in the basic
Millett circuit, so the tube's rated amplification factors are generally reduced by 2 or 3. Nevertheless, they are a good indication of which tubes to choose
for your particular needs.
Reference data sheets for some commonly available Millett MiniMAX tubes (pdf files):
Generally speaking, MiniMAX 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. 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 between different constructions 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 Millett MiniMAX tubes, these getters can be circular, horse-shoe, or
square, and can be located on the top of the tube, the side of the tube, or even inbetween. 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.
Selecting and Buying Millett Tubes -
In selecting/buying tubes, one thing to remember is the great variation that 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 meaning of the acronym "NOS" also means that great differences are likely to exist. The sound of your MiniMAX can depend almost exclusively on the tubes. While the buffer and different transistors have a great effect, they can only provide current to the signal that the tube has already provided.
What does this mean to the MiniMAX builder and owner? Quite simply, buy more tubes and try more tubes. You may be shocked to find how different they can be.
The brand differences range from slight to great, but can be wildly inconsistent. I used to offer opinions on brands (found on other MAX websites), but as mentioned below, you will almost find a particular tube construction stamped/marked by any of the manufacturers.
So Who Made What?
Despite a plethora of different brand
Millett MiniMAX tubes, only 4 mfrs actually made the tubes (from most reports) - GE, RCA, Sylvania, and Tung-Sol, for sure. Further, each
company may have only made two or three types, but bought others from their competitors and re-branded them. I suspect
that this also went on during stock shortages, as well. The re-branding became ubiquitous. Moreover, many of the mfrs shared common plates.
Tracing the true manufacturer becomes very difficult. The RCA tubes generally have a flattened octagon label
with the tube designation, while GE etched small dots on the tube glass. Sylvania's are usually distintinguished with the box at the top of the plates and above the top mica spacer, while RCA seems to be the only ones to use the horseshoe side-getter (but they used other types, too). GE's seem to use the ring getter more often, but this is not consistent. While they all have shared some of the plate designs at one point or other, GE seems to be the only one with stamped "J" plates for the lower gain tubes. Others trimmed their plates, and they were not stamped in one piece. The 12AE6's are consistent in having mostly whole plates. One imgagines that the tube design was often the same, and a manufacturer simply trimmed part of the plate to cut down on gain for a different model tube. Obviously,
this is only anectodal and your mileage may vary. There are exceptions and you may find RCA octagons and GE-etched dots on the same tube! So, it seems prudent to match plate construction and perhaps getter construction, as well - the getter does seem to determine a particular series, period of manufacture, or both.
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, December 29, 2008 6:00:00 AM
Please contact the MiniMAX webmaster for questions about these web pages.