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240vac xformer and tube amp

Discussion in 'Electronic Basics' started by Michael, Nov 28, 2005.

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  1. Michael

    Michael Guest

    I've been in electronics a long time but it's been years since I've messed
    with tube stuff but I thought I "may" build a small guitar (practice) amp.

    First question is...

    Has anyone explored the possibility of using a simple (and generally
    inexpensive) 120 to 240vac step up transformer with a solid state bridge
    rectifier to get high voltage for a tube?

    Is there a design/schematic someone can refer me to on a particular tube
    that could operate on the voltage from this arrangement....I would imagine
    you could get close to plus 280-290 volts or so under load?
     
  2. Doggone

    Doggone Guest

    You need Duncan's very fine tools.

    Go to:
    http://www.duncanamps.com/software.html

    And download PSU Designer. While you're there, you'll want to get the other
    tools too. TDSL is my fav. You'd be hard pressed finding better value on the
    net.
     
  3. John  Larkin

    John Larkin Guest

    If it's an autotransformer, and most are, it won't be isolated and
    will be a shock hazard.

    John
     
  4. Michael

    Michael Guest

    I see where Hammond makes a few 230vct transformers, one that will supply
    ..35a for about 20 bucks....not an autotransformer.

    Wonder if anyone has seen a design with these for a small practice amp with
    just a few watts?
     

  5. That's an 80 VA transformer. Use a bridge rectifier on it and get 325
    VDC out. You could get more than a few watts out of that with a class
    B output stage. Use the center tap as a 162 volt DC output for the
    earlier stages.
     
  6. Bob

    Bob Guest


    That's almost all I use! (Sorry, my schemo is a proprietary device!)

    The peak voltage will be around 330 but will drop under full load back down to
    250 or so - perfect for a 6BQ5 power tube. If you run a class A SE amp the
    voltage will be stable... don't go over 300...

    Preamp tubes like the 12AX7A will run on anything over 150 volts with no
    problem...

    Don't forget you need a 6 volt filament supply at a few amps...

    Oh, I hope you have experience with wiring a high Z high gain wide bandwidth
    device - or it won't work... it'll either hum or oscillate or both, probably...

    Then there's the matter of Tone...

    Bob
    Amp builder
     
  7. Bob

    Bob Guest

    Not at load... I get 275 volts after a 150 ohm filter... about 10 less than the
    bridge output, for a SE EL84 5 watt amp. Higher loading would reduce it more...
    You can't use the center tap if you use the full secondary with a bridge...

    Bob
    Amp builder
     
  8. Why not?
     
  9. Doggone

    Doggone Guest

    Hi Bob.
    You can use the center tap. It will be at exactly 1/2 the B+.
     
  10. Bob

    Bob Guest

    Sorry - my bad - you can... I must have been pre-occupied!
     
  11. Bob

    Bob Guest

    You're right - I must have been thinking of getting the bias from there, which
    is negative...
     
  12. Phil S

    Phil S Guest

    Hey Bob!
    You're the one who told me to tie two secondary (82v + 125v) windings of
    that old Heathkit TV power tranny in series and it would give more than
    enough B+ for a Champ like thing. I haven't seen you around in a while.
    Thought I'd mention that it worked out just like you said it would. As I
    recall, B+ was in the neigborhood of 250VDC. I built it with a diode
    bridge, 12AT7, and 6AQ5. Works great! Amp building is addictive. I built
    two more since then (5E3-almost, and JTM45).
    Phil
     
  13. Bob

    Bob Guest

    Hi Phil - I remember you!

    I don't come here too much because of the off topic political bullshit - some
    people can't understand this is a guitar amp group...

    As for addictive amp building - I'm now working on number twenty something...
    I'm trying toroids this time!
     
  14. Phil S

    Phil S Guest

    Well you dropped in at a decent time. Between the filters and whatnot,
    things are down below a dull roar at the moment. I don't think it will stay
    that way. Anyhow, it was nice to see you drop by. Toroids, huh? What's
    the deal with them?
    Phil
     
  15. Doggone

    Doggone Guest

    Amp junkies can be itentified by the solder burns on their hands.

    Do you know of a source for toroidal transformers with both a high voltage
    and filament windings or do you wind your own? I found Hammond have decent
    toroidals but don't have mixed windings. They also tend to be quite pricey. I
    find it had to justify the extra cost when recycled iron can be had so cheap.

    Is there such a thing as a toroidal output transformer?

    <thinking>......... R.K. style......................................

    Would the core end up being unrealistically large? What do you think?
     
  16. Bob

    Bob Guest

    -snip-
    A toroid has the advantage of a more compact magnetic field, and I thought that
    would give an advantage to players who like to sit on their amps, and get hum
    broadcast into their single coils. I'm putting the toroids on the bottom of the
    amp chassis, and there should be no radiated field outside the amp.

    I will test it with my Strat, a sort of $1000 magnetic field finder!

    Toroids aren't all peaches and cream though, their wide frequency response can
    bring more line noise into the amp... I may put a ferrite choke on the power
    wires...

    not to mention their price, about double a standard unit. I need 2, for filament
    and plate supply, cost me about $100.
     
  17. Bob

    Bob Guest

    -snip-
    I'm trying a pair of Hammond's right now! I will do some tests between identical
    amps with and without torus transformers. I've never seen a tube type power
    toroid... you'd have to order it made.

    I might also try a standard filament tranny with a toroid plate tranny... I
    bought a box of E-I filament trannys cheap - about $7 each, with slight damage
    cosmetically speaking... 12.6VCT @ 2A ... was a good deal so I want to use them.
    I haven't seen one - don't know if there is an advantage to one...
    I'm not a tranny expert by any means, although I have wound a lot of them! I
    couldn't tell you how a torus would do as an output... but for fun I may try a
    toroid filament tranny as an output... it would match an EL84 to 4 ohms! But it
    may saturate with the DC...
     
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