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differential hybrid parafeed headphone amplifier

The 12A*7 Tube family (NOTE: the Torpedo III is optimized for the 12AY7 tube)

Tubes, otherwise known as vacuum tubes or electron tubes, form the signal stage of the TORPEDO III! Other than the output transformers, no other component has as much effect on the sound quality of the amplifier. 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, we'll try to give some basic info on the specific TORPEDO III! tubes that can be used. Most of the entire 12A*7 tube family may be used in the amplifier. This includes the 12AU7, 12AT7, 12AY7, and 12AX7. However, some may perform better than others. As referenced above in the section title, the TORPEDO III! is optimized for the 12AY7 tube.
With few exceptions, every tube in the 12A*7 family is a dual-triode tube rated for 300V on the plates. For the TORPEDO III! differential hybrid parafeed headphone amplifier, the triodes in each tube are paralleled, with the power transformer voltage providing the individual plate voltage for each tube (2 plates each) at 125V. The result is a true high-voltage tube circuit that is more or less self-balancing for the tube pairs. Combined with the differential solid-state stage after the tubes and the output transformers, the result is a very high-performance amplifier. As stated elsewhere on this site, when combined with the Cinemag output transformers, the result is world-class performance.

The 12AY7 has the following basic attributes when run at 250V (125V) on the plates:
  • Basic amplification factor of 44,
  • Transconductance of 1750 micromhos, and
  • Plate resistance of 25000 ohms.
  • EH 12AY7 Tube Data     EH 12AY7 Tube Curves
    (The 12AY7EH is the basis of the Torpedo III differential hybrid parafeed amplifier)
    The 12AY7 tube is part of perhaps the most prevalent miniature small signal dual-triode family in history. Both the 12AU7 and 12AX7 are very familiar to most headphone and guitar tube amplifier users. Both tubes may be used in the Torpedo III, but each represent opposite ends of the gain spectrum in this tube family. The Torpedo III is specifically optimized for the 12AY7, but 12AT7 and 12AV7 tubes are acceptable. Here are data sheets from three different mfrs of the 12AY7 (pdf files):
    6072 (GE) 12AY7 (TungSol) 6072 (RCA)

    Other Tubes for the TORPEDO III!

    There are many other tubes that can be used with the TORPEDO III! TDSL (Tube Data Sheet Locator) indicates tubes equivalents. Among the full equivalents for the 12AY7 tube are the 6072, 6072-A, CV2650, and CV3650. Near-equivalents for the other tubes in the 12A*7 are almost too numerous to mention. The 12AT7 and 12AV7 tubes are closely related in amplification factor and could be a good fit for usable gain with most headphones. Those tubes have equivalents as follows: 12AT7: ECC81 (European equivalent), 12AT7WA, 6060, 6201, 6679, 7492, 7728, etc.; 12AV7: 5965, CV10175.

    The equivalents for the 12AU7 are almost limitless: ECC82 (European equivalent), 12AU7A, 12AU7WA, 5814, 5963, 6067, 6189, 6680, etc. The equivalents for the 12AX7 are similar: ECC83 (European equivalent), 5751, 6057, 6681, 7025, 7382, 7729, etc., etc. If one drills down deeper at TDSL by starting with the full equivalent variants such as the 12AU7A, 12AX7A, etc., you can find even more.

    This table and paragraph are also on the Tech Highlights page, but they are worth repeating here:
    The circuit is designed to work well with most of the 12A*7 family of tubes, but is optimized for the 12AY7. Different tubes will provide different levels of gain, but some types may not perform as well as others, with high-frequency performance possibly suffering in some instances. The 12AY7 provides optimal performance and approximately 6X (15dB) gain into 300 ohm loads and 2X (6dB) into 32 ohm loads. Other tubes and their approximate gains are shown below:

    Tube Typegain - 32 ohmsgain - 300 ohms

    Notes on some specific tubes (from Dsavitsk):
    12AU7 - probably has too little gain to be very useful, but if you have a particularly hot source and/or tend to listen quietly, it might be worth playing with.
    12AX7 - has a higher mu (gain) than the other tubes listed, but it also has a higher plate impedance. This will result in lower gain and some high frequency loss. It might still be worth playing with, but it is probably not the ideal tube here.
    12AT7 - has similar looking gain to the 12AY7. However, the circuit runs it at lower voltage and current than are idea which will result in slightly higher distortion. Subjectively, we found it to sound more tubey which some people may like.
    12AZ7 - looks like another decent option, but I have not tried it.

    6DJ8, 6922, 6H6, 6H31, 6N23, 6N1p, etc. are not compatible with the circuit. At the very least, if you try them you'll probably damage the tube, but there is a good chance you'll damage the amp, too. If you are in doubt about a particular tube, please ask before you try it.

    Additional Tube Data Sheets for the TORPEDO III:
    12AT7 (GE)
    12AT7 (RCA)
    12AT7 (Tung-Sol)
    6201 GE
    6201 RCA
    12AU7 (GE)
    12AU7 (TungSol)
    12AU7A (RCA)
    12AV7 (GE)
    12AX7A (GE)
    12AX7A (RCA)
    12AX7 (EH)
    12AX7 (GE)
    12AX7 (TungSol)
    12AY7 (GE)
    12AY7 (RCA)
    12AY7 (TungSol)
    6072 (GE)
    6072 (RCA)
    12AZ7 (RCA)
    12AZ7 (TungSol)

    Generally speaking, TORPEDO III! tubes and their sound is distinguished primarily by tube type, but sometimes 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 TORPEDO! 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 of this web page are tubes with different getters - the Electro-Harmonix 12AY7's have top "UFO" or disc getters, the other - GE 5963's have top halo getters. Here are a couple of photos that show another getter type:

    The tube above has a top-square getter. The getters need a bit of time to burn off the gas molecules that have infiltrated the tubes while sitting unused for the last several decades. That said, at 200V it really doesn't take too long. Unlike some of the low-voltage tube hybrids, whatever's in the tubes in the TORPEDO! gets burned off pretty fast!

    Selecting and Buying TORPEDO III! Tubes -
    In selecting/buying TORPEDO III! tubes, one can simply choose to purchase new manufacture tubes, period. Active warranties and many retailers are available. Purchasing NOS tubes can result in those magnificently "magical" finds, but it can be a crapshoot. The thing to remember is that a lot of 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 when new - they were nowhere near as consistent in their performance a sthe solid-state devices we're familiar with today. The sound of your TORPEDO III! can depend greatly on the tubes. While the transformer choice between Edcor and Cinemag may have an even greater effect, they can't be rolled in and out of the amp.

    What does this mean to the TORPEDO III! builder and owner? Quite simply, buy more tubes and try more tubes! You may be delighted to find how different they can be. You will find with the 12A*7 tube family, the options run the gamut from inexpensive to some of the most expensive tubes found. The differences range from slight to great, but can be inconsistent. Ordinarily, one might recommend a certain brand for a type of sound, but you may find a particular tube construction stamped/marked by any of the manufacturers.

    Tube Accidents

    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:Thursday, November 12, 2015 7:37:05 AM
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