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Add a 9V guitar tuner to at tube amp .

Discussion in 'Electronic Basics' started by fffcj, Oct 23, 2004.

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  1. fffcj

    fffcj Guest

    I have a nice tube amp .
    I'm modifying the case to lighten it etc ...
    While I'm at it , I'd like to incorporate my guitar tuner into the amp .
    Regarding the audio circuit , I'll probably just place the tuner into the
    FX loop .

    What I'm unclear on is how to find some 9V dc within the tube amp to
    power the guitar tuner .
    I guess I'll need maybe some more components to rectify / smooth the power
    from the amps own power transformer .
    I've not tested any ac voltages yet .

    What circuit should I be using ?


  2. tempus fugit

    tempus fugit Guest

    What kind of tube amp is it?
    Most people feel that a tuner in line degrades the signal, so if I were you
    I'd take a parallel connection from the FX loop.
    I doubt you'll find 9v in there, but if there are any opamps or other ICs,
    you might be able to get 15v. Often the EQ stages are opamps or transistors.
    I'd (very carefully) poke around there 1st. If you can get 15v, you could
    just use a 7809 regulator to get the voltage down to 9v.

    Always bear in mind that tube amps contain high voltages. Even when
    unplugged, the caps can hold a considerable charge (enough to KILL you).
    Make sure you know what you're doing before you go sticking probes around in
  3. fffcj

    fffcj Guest

    Thanks .
    I'm going to read a book about tube amps first .
    Especially the safety stuff .

  4. Spajky

    Spajky Guest

    lamps heating voltage ( ff ) is probably 6,3V AC coming from
    power transformer, followed by a graetz type rectifier & a more than
    1000uF/16V el.capacitor connected to rectifiers -/+ out properly;
    you will get approx. 9V DC ...

    But check first, that power ff source is not grounded; if is & the
    other wire is 6,3V AC to ground, IMHO would be enough instead of
    graetz rectifier to use just a diode ...

    If is grounded in the middle (2x 3,2VAC), than you will have to find
    another solution ...
  5. CFoley1064

    CFoley1064 Guest

    Subject: Add a 9V guitar tuner to at tube amp .
    Hi, Chris. If you don't need a lot of current, find a couple of small 6.3 VAC
    secondary transformers and do something like this (view in fixed font or M$

    120VAC 120VAC 6.3VAC | | +
    o---. ,-o-----------o-. ,------o--o~ +o-----o-----o
    6.3VAC )|( )|( | | |
    )|( )|( | | +|
    o---' '-o-----------o-' '------o--o~ -o-. --- 8VDC
    | | | ---
    '------' | |
    | | -
    created by Andy´s ASCII-Circuit v1.24.140803 Beta

    Pick off your 6.3VAC to the tube heaters (which will almost always be grounded
    -- you can't use that) and use it to backfeed the first transformer, and then
    use that output to feed the second transformer. You can take the output of the
    second and rectify/filter that to give you a little over 8VDC. Use schottky
    diodes instead of a standard bridge rectifier for a slightly higher voltage.

    First, you have to oversize the first transformer if you're backfeeding -- It
    should be rated at least 4X the VA you are using at the DC side. So, if you've
    got two 3VA transformers ([email protected]), you shouldn't expect to get more than
    80mA or so out of your DC without getting the first transformer really hot.
    And you do have to be careful about the heater power -- don't take too much, or
    you'll smoke the main xfmr. I like the concept of borrowing a couple of VA --
    that's about 1 extra tube's worth of current -- it's usually safe.

    Second, you'll have to use a brute force cap to reduce ripple on the output.
    The 120 Hz ripple might not be too good for your application, so you might want
    to use a 12VAC secondary transformer for the second one, and then feed that to
    a standard rectifier/filter/linear regulator like the LM317.

    .------. ____
    120VAC 12 VAC | | | |
    --o-. ,------o--o~ +o-----o---| 317|--o----o
    )|( | | | |____| | |
    )|( | | +| | .-. |
    --o-' '------o--o~ -o-. --- |240 | | |
    | | | --- | | | |
    2nd Transformer '------' | | | '-' +|
    | | | | ---
    | | o-----' ---
    | | | |
    | | .-. |
    | | | |1800 |
    | | | | |
    | | '-' |
    | | | |
    '---o-----o ---------o
    created by Andy´s ASCII-Circuit v1.24.140803 Beta

    Be sure to have a second transformer of less than or equal VA rating to the

    Also, be extremely careful here to avoid a situation where you connect a higher
    potential in to this circuit. It'll kill the LM317, and potentially cause a
    lot of other damage inside the amp.

    Good luck
  6. Way to complicated and not necessary. Why wont people realise that DC
    offsets are of no consequence for audio. Capacitors exist. The Tuner can
    float at any voltage, so just use the bridge directly off the heater AC
    to power the Tuner.

    Kevin Aylward
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
  7. Not really. The tuner can float at any voltage. It just needs capacitor
    coupling. Just slap a bridge etc across the 6.3 heater supply.

    Kevin Aylward
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
  8. CFoley1064

    CFoley1064 Guest

    Subject: Re: Add a 9V guitar tuner to at tube amp .
    First principle -- "Do No Harm". You're right, it is too complicated. For
    someone who knows what they're doing, like you (or possibly even me -- that's
    your call, I guess). But, the isolation makes it relatively difficult for a
    newbie to connect different potential commons by accident and let out the

    As the Guru Don once said,

    "Sometimes the technical solution and the people solution are wildly

    I'm hoping the OP has a couple of small transformers in his junkbox and saves
    himself grief if he makes a newbie mistake.

    Thanks for the spot -- you're right, of course.
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