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Tube Amp

Discussion in 'Electronic Basics' started by Dave.H, Apr 21, 2008.

  1. Dave.H

    Dave.H Guest

    Hi, I'm looking for plans for a 1 or 2 tube low voltage amplifier to
    go with a regenerative radio. Does not have to be the best, but needs
    to drive at least 250 mW speaker, preferably 1 watt or more.

    Thanks, Dave
    Australia
     
  2. neon

    neon

    1,325
    0
    Oct 21, 2006
    you are totaly confused low voltage amp using tubes for 250 mw. get to 99 cents store and by this thing for 99 cents.
     
  3. Mike Silva

    Mike Silva Guest

    How low is low? Anything much under 100V gets tough to make any
    power.

    You'll get the most power at a low voltage by using a tube intended
    for such use (kind of a DUH statement!). Look for a tube that has a
    higher heater current, since low-voltage tubes need more cathode
    emission to make decent power. That might be something like a 6CU5,
    6W6, 6Y6, 6DB5, etc. Some of those are miniature tubes, others are
    octals.

    You'll probably only need one triode stage of AF amplification between
    the detector and the output tube. Something like a 6C4, 6J5, etc. If
    one stage isn't quite enough you could try a tube with higher
    amplification factor like 6AV6 or 6SR7.

    BTW, you can talk tube radio homebrew to your hearts content on the
    glowbugs email list. Google should turn it up for you.

    Mike
     
  4. Mike Silva

    Mike Silva Guest

    Hmm, I thought I hit SEND but who knows where it went. Anyway, I had
    another idea so I'll reply again.

    First, what kind of low voltage are you talking about? Anything much
    under 100V is going to be very hard to make any power.

    You'll want an output tube designed for low voltage operation
    (typically around 110V). Look for a tube with high heater current,
    typically 1.2A at 6.3V. That's because low voltage tubes need big
    cathodes to produce enough current at low voltages to make decent
    power. Examples would be 6CU5, 6W6, 6Y6, 6DB5, etc.

    You'll probably only need one stage of AF amplification between
    detector and output tube. Look at triodes such as 6C4 or 6J5. If you
    need more gain look at a higher gain triode like 6AV6 or 6SR7.

    I don't know what voltage source you have or are looking at, but I
    like the Triad N68X isolation transformer for low (tube) voltage use.
    You can get 150-160V out of a bridge rectifier and filter. You'll
    need a separate heater transformer, but the combination usually still
    ends up being cheaper and easier to obtain than a combined tube
    transformer with both HV and heater windings.

    BTW, if you want to learn about tube radio homebrew you'll want to
    look into the glowbugs email list - best resource on the net that I
    know of. Google should turn it up for you.

    Mike
     
  5. Dave.H

    Dave.H Guest

    Found one at http://www.oldradios.co.nz/hikers/52octalhikersamp.jpg
    Would P-T156 at www.tubesandmore.com work for the transformer
     
  6. Dave.H

    Dave.H Guest

    Going off topic a bit here, concerning the regen radio previously
    mentioned, that I've yet to build, there are some connections I'm not
    sure of, first is the A connection, this doesn't appear to be the
    antenna connection, second is the B+ 1 1/5 v connection. Do they mean
    A+ 1 1/5, even though it's not connected to the + filament pin?

    Link to the schematic http://www.oldradios.co.nz/hikers/51hikers11.jpg

    Thanks, Dave
    Australia
     
  7. Mike Silva

    Mike Silva Guest

    "A" is the filament voltage, "B" is the plate voltage and "C" is the
    grid bias voltage (which is not present in the schematic you show).
    When you see things like B+1.5V and B+9V that's referring to taps on
    the B+ battery (many of them were made with such taps). Of course for
    B+1.5V you could also use a separate 1.5V battery. Here's a schematic
    that shows such a separate battery.
    http://www.schmarder.com/radios/tube/hikers1.htm

    BTW, the amplifier you're looking at won't give anywhere near 250mW,
    much less 1W. The specs for the tubes show that they just get to
    250mW output at 90V B+. At the 18V given in the schematic you'll only
    be in the tens of mW. And also, you'll need an output transformer to
    connect the amplifier to a speaker.

    Mike
     
  8. Dave.H

    Dave.H Guest

    I'll use the 2 watt IC amp I have sitting around, easiest option.

    If I have this right, and usually I don't, I connect the plus end of
    the filament battery to the terminal simply marked "A", and a separate
    "C" battery to the B+ 1 1/5v connection. Why is there only one
    connection for the bias battery?

    Will I get good lifetime out of a 9 volt transistor battery? I intend
    on using a "D" cell for the filament, as I do with my other regen
    radio (which BTW, you will find on Dave Schmarder's web page, very
    first radio on his tube radio page).
     
  9. Mike Silva

    Mike Silva Guest

    Wire up the batteries exactly as shown on the schematic at the bottom
    of the page I linked (http://www.schmarder.com/radios/tube/
    hikers1.htm). Yes, you should get good life out of your 9V battery.

    Mike
     
  10. Mike Silva

    Mike Silva Guest

    Actually the circuit is employing a trick that was sometimes used in
    low voltage applications. The tube control grid is connected to a
    small positive voltage to help accelerate the electrons towards the
    plate to obtain reasonable plate current, and the tube screen grid is
    then used as the circuit control grid. Unless you've run across that
    trick you'd be justified in thinking "what the heck?!"

    Mike
     
  11. Dave.H

    Dave.H Guest

    I'm planning on using a 1C5GT tube.
     
  12. Mike Silva

    Mike Silva Guest

    Here's what I think. I don't claim it's the only way, or even the
    best way, it's just what I would be inclined to do. Take the money
    you would spend on the 1:3 interstage transformer and spend it instead
    on a power transformer and build yourself a battery eliminator that
    will give you 90-110V to run a power output stage. Tap it down to run
    your other stage(s). If you want, add a low voltage regulated DC
    output for filaments/heaters. Then just use RC coupling between the
    detector and the AF stage.

    That's unless you were planning to use the radio away from AC power,
    in which case ignore everything I said.

    Mike
     
  13. neon

    neon

    1,325
    0
    Oct 21, 2006
    well if you like toasters that is the one for you, never mind that chinese sell these much better radio AM/FM for a $1.00. who would carry a shoe size box radio while while doing anythin and the power is one "a" battery. I guess to each it"s own.
     
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