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a single output transformerstereo tube amp ??

Discussion in 'Electronic Repair' started by Boborann, Oct 28, 2006.

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  1. spake thus:
    Nope. No way could it work.

    What you say is correct, I'm guessing, for what goes on on the *primary*
    side of the xfmr. But on the secondary side, as someone else pointed out
    here, there's no way the split secondary windings are going to be able
    to selectively pick up only that part of the magnetic flux intended for
    that channel. In other words, the secondary can only act as one winding
    (albeit one split in half), so, as someone also pointed out, what you
    end up with is a stereo amp with crosstalk: 100% crosstalk.

    Just as McDonald's is where you go when you're hungry but don't really
    care about the quality of your food, Wikipedia is where you go when
    you're curious but don't really care about the quality of your knowledge.

    - Matthew White's WikiWatch (
  2. Boborann

    Boborann Guest

    The Way I see it is there is a stereop cartridge where each channel goes
    through two stages of 12aX7 amplification and the each to its own 6L6 The
    output transformer is CT with one speaked on one side of the CT in the box
    The second speaker ( remote ) on the second side of the CT
    There is even a stereo /Mono switch when when in Mono mode ties both sides
    of the cartridge together nd adds a cap across teh cathode biasing resistor
    of teh first amplifier stage
    I will put a scope on it tomorrow and see how teh outputs are biased as I
    assume teh6L6's are running class A
  3. Boborann

    Boborann Guest

    Schematic posted to alt.binaries.schematics.electronic

  4. No, I believe it would work OK and the proof is in the pudding.
  5. Jim Land

    Jim Land Guest

    Your analysis is correct, as far as it goes. It's the *second*
    transformer in the output circuit, connected to the output transformer's
    primary and secondary, that makes stereo come out of the speakers.
  6. Jim Land spake thus:
    Ah, there it is; I missed that the first time I looked at that (crappy)
    picture of the schematic.

    So now I'm wondering: what, if anything, would be the advantage of such
    a circuit? Again, it looks like they're trying to save a nickel or two
    on transformers, but there are two anyway: why not just use two standard
    output transformers? Must have been someone's brainstorm.

    Just as McDonald's is where you go when you're hungry but don't really
    care about the quality of your food, Wikipedia is where you go when
    you're curious but don't really care about the quality of your knowledge.

    - Matthew White's WikiWatch (
  7. The difference signal is small so the extra transformer is much smaller.
    Also, instead of two full tube push pull amps you have just one. Same design
    philosophy as Detroit.
  8. lj_robins

    lj_robins Guest

    Sorry, no, this is not stereo, this is not even a stereo circuit.

    This is a standard two 6L6's in push-pull mode circuit, one pushes the
    up-swing of the sine wave, the other pushes the down-swing of the sine
    wave, each half of the wave is fed into the opposite side of the

    Regardless of what output tubes they use, nearly every push-pull circuit
    I have seen has been designed this way.

    That little transformer that is hooked to the primary side's center tap
    does NOT make it stereo, if you follow the circuit backwards from the
    transformer it leads back to B+ voltage coming out of the rectifier
    tube. For whatever reason that they decided to do it this way, the
    center tap of the secondary of the output transformer self modulates the
    B+ voltage going into that transformer in phase which the signal that is
    going through the output transformer.

    My best guess on this is that it's a poor mans' volume booster to get
    more volume going to the speakers than the amplifier design would do on
    it's own.

    Secondly, the speaker's are not on separate circuits (this was mentioned
    in another posting), the two tweeters and woofer are arranged in a
    series-parallel circuit with a capacitor stuck in there to act as a poor
    man's cross over circuit to separate the high and low frequencies going
    to the speakers.

    The way that these are arranged is MONO but instead of using SIGNAL and
    GROUND to wire the speaker(s) to they used the PUSH-PULL of the up and
    down swings to drive the speakers with.

    Looking at the INPUT of this entire circuit there IS something on there
    that says STEREO, it is probably a phonograph input. The rotary switch
    that switches between RADIO, STEREO, and MONO, in Radio mode the lower
    half of the 12AXT tube is grounded out and it only runs in MONO mode,
    when switched to MONO mode the same thing happens.

    But in STEREO mode both halves of the 12AXT are turned on and it also
    acts as a PUSH-PULL circuit and feels both halves of the signal to the
    rest of the circuit, which from here on out is in push-pull mode.

    Hope this clears up the confusion.

  9. Does too.
    That is one of the two speakers.

    Go look again. This amp has STEREO input, STEREO volume control, STEREO tone
    controls and two sets of speakers, left and right.
  10. Guest

    Maybe its time the OP posted the cct somewhere we can all see, eg
    tinypic. Not everyone has binaries groups access.

  11. Didn't he do that? He also attached it here (naughty, naughty).

    You can analyse it by assuming the two inputs are tied together; and then
    using one only while tying the other one to ground. First one transformer
    works, then the other.
  12. Guest

    I can get binaries, but I don't see a link.

    I really would like to see the print for this contraption.

  13. spake thus:
    What's up with this? There's no link; look at the OP's 3rd post here. It
    has a JPG of the schematic attached.

    Just as McDonald's is where you go when you're hungry but don't really
    care about the quality of your food, Wikipedia is where you go when
    you're curious but don't really care about the quality of your knowledge.

    - Matthew White's WikiWatch (
  14. Many ISPs (including mine) strip attachments off text only newsgroup
    posts. Otherwise their servers would be jammed with HTML files...
  15. Guest

    not that I saw anywhere.
    Unfortunately analysing a circuit without knowing the circuit is one of
    those skills I never did learn.

  16. Boborann

    Boborann Guest

    Thanks for the heads up I did'nt realize that how it worked and will post to
    a binaries group ( non Porno) in the future
  17. Others won't let you post html or binaries to a non binary group - and won't
    let you post html to a binaries group even when it makes sense. Assholes.
  18. James Sweet

    James Sweet Guest

    Could someone email me the schematic? I'd like to have a look at it.
  19. Meat Plow

    Meat Plow Guest

    Ok check your mail, don't share my address.
  20. lj_robins

    lj_robins Guest


    Yes I will agree with you that it has two speakers, it could have four,
    eight, or sixteen speakers and still be a mono circuit depending on how
    the speakers are wired together.

    Yes it has stereo inputs, they stay stereo until they get to the 6L6's
    when it becomes a MONO push-pull circuit.

    Take another look at the schematic, the two 6L6's are creating a
    Push-Pull circuit, one produces the up-swing of the sine wave while the
    other produces the down swing of the sine wave, then they mix together
    in the output transfer to form an alternating current (complete) audio
    sine wave.

    Now, take a look at the output of the output transformer, follow the
    wires going to the speakers (notice that there are only two), one wire
    goes to the left side of one of the speakers, the other goes to the
    right hand side of the other speaker.

    Here is the important part, the two speakers are tied together right in
    the middle (series), then the woofer is wired to the left side of the
    left speaker and the right side of the right speaker.

    This is a series-parallel MONO circuit (using Ohms Law this probably
    comes to 4 or 8 ohms, maybe 16), to make this a true stereo circuit you
    would need to have at least three wires (two signal wires and a ground).

    For a final test, play something through it that you know is STEREO, The
    Doors and old Pink Floyd are well known for doing weird freaky things
    with stereo.

    If the guitar is on one speaker and the keyboard/organ is on the other
    on a song that you know plays this way on a stereo they yes this amp is
    stereo. If the same sound is blended and comes out the same on both
    speakers it is mono.

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