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Vacuum Tube Amp

Discussion in 'General Electronics Discussion' started by jmoschetti45, Oct 2, 2011.

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  1. jmoschetti45


    Oct 2, 2011
    To start off, I know almost nothing about tubes, but want to learn, so I figure this is a good starting project.

    I want to built a 5-10 watt amp using tubes. I've searched around, and found a few schematics, but most of them are only for a few watts of power.

    Main question I guess is, can I run tubes in parallell? I've noticed you can run one or both 'triode sections' of a 12AX7. Running them in parallel gives more output power. Not an issue there, I can just use 2, one per channel. I've read about the whole 'matched sections' thing, but for a first tube project, not too worried about it.

    Can I run power tubes like an EC88 in parallel as well?

    Also, do I really need to use a 12AX7 to run a EC88? I've seen some schematics where only a 12AX7 is used. Can't I just run the power tube? Input would be from a cd player/ipod, so it wouldn't need preamplification.

    Things would be simple if I could just run 2 or 3 or 4 12AX7's in parallel per channel, or could just use 2 or 3 power tubes in parallel. My goal is to keep the parts count as low as possble.

    I've also seen ignition coils used as output transformers, and besides the higher risk for shock, see nothing wrong with this?
  2. duke37


    Jan 9, 2011
    As a starting project, a 10W amplifier is a bit ambitious. All the tubes you mention are triodes designed for amplification and would give out less than a watt.
    To get 3 or 4W you could go for a single 6V6 or EL84 with a suitable ratio of transformer.
    You say that no amplification of the signal would be needed, not so, the tube would need a few volts swing on its grid.
    To get 10W out you could use three 6V6 in parallel and a very special transformer. If you used two 6V6 in push/pull, you could get more output and could probably find a mains transformer which would do an adequate job for a first try.
    If you do run tubes in parallel, they should be matched so they share the load properly, this is a pain.

    Why not find an old tube radio or TV, get the schematic and take the required parts out of that. Remember that you will need a high voltage power supply that could kill you, but only once.

    The design of audio output transformers is complex if you want high quality and most amateurs use commercial designs. An ignition coil is useless for this purpose. It is designed to step up voltage but you need to step up current. It is intended for pulse operation, what it would do to an audio signal is anyone's guess.

    Better to start with transistorised equipment until you understand the principles and can then take the addtional risk of high voltages.
  3. OLIVE2222


    Oct 2, 2011
    If you absolutely want to go for valve amplifer, maybe a OTL ( output transformer less) amplifier can be a good start.
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