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a single output transformerstereo tube amp ??

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

  1. lj_robins

    lj_robins Guest


    I was able to download it, I'll email it to you.

  2. lj_robins

    lj_robins Guest

    Ok, good point I don't see a phase inverter either.

    But... What this comes down to is how the speakers are wired together,
    they are wired in MONO not stereo, even the schematic says "External
    Speaker" (singular not plural) where it shows the external speaker jack.

    Well, I've said all I'm going to say, I'll show the schematic to my my
    father who has been fixing tube equipment since the 1950's I'll see what
    he has to say about it.

  3. But I bet he was incredibly proud of his design. "Only a mother could love
  4. I bet he got a lot of funny looks at the old stereo factory!
  5. propman

    propman Guest

    wrote in

    FWIW, I use that site pretty much on a day to day basis and it's really
    not that weird. I just tried logging on using the web address cited in
    this message without any problems at all. You do need Javascript turned
    on to download files (and the site will automatically tell you if you
    don't have it turned on). From the sounds of it, probably you are
    clicking on an "upload" hyperlink rather than a download one.

    Once the page comes up, mouse click on the "Free"
    This will bring up another screen which will include a "validation" box
    where you type in the numbers that are required (numbers are in a little
    Press the "Return" or "Enter" key on your keyboard, or click on the
    "Download from" button........depending on how you have your web browser
    set up, the file will either start downloading automatically or a pop-up
    will ask if you want to "Save to disk" or "View the file".

    Hope this helps....... :)
  6. Guest

    ah... my browser isnt popup vulnerable.

  7. Guest

    thank you, I'll retrieve it from the spam dump later on, and see if I
    can get any clues from it.

  8. Jim Land

    Jim Land Guest


    You got one speaker set (woofer, two tweeters) in the case, and one set

    They are not wired for mono. Take a look at the schematic. That common
    connection the two speaker sets share? It also goes to ground. What
    else is connected to ground? The second transformer in the output
    circuit. Hence, stereo.
  9. Jim Land

    Jim Land Guest

    Can anyone dig up the name of the engineer who came up with this circuit?
    We ought to have it available for throwing around, when we come across
    weird circuits.
  10. Andy Cuffe

    Andy Cuffe Guest

    Here's a higher resolution scan of that schematic.

    The way I see it, the transformer connected to the plates of the 6L6s
    (T1) is the difference channel and the one connected to the center tap
    (T2) is the sum channel. The 6L6s are operating in single ended class
    A. There's no phase splitter, so it can't be a PP amp.

    If both plates have the same signal on them (a mono signal), they will
    cancel out in the primary of T1. Since T2 is in the B+ path, it will
    have the current from both 6L6s passing through it. This will induce
    a voltage in the secondary of T2 and pass through the secondary of T1
    to both speaker outputs in phase.

    Now, if the plates of the 6L6s have a 180 degree out of phase signals,
    the current through T2 will be constant. DC through T2's primary
    means no signal on its secondary. Since T1's secondary is grounded at
    the center tap, you'll get equal, but out of phase signals on the two
    speaker outputs.

    Finally, if only the top 6L6 has a signal, it will result in the same
    signal on the secondaries of both transformers. T2's secondary will
    be in phase with the signal on the top half of T1's secondary, but out
    of phase with the signal on the bottom half. This results in a signal
    only on the black wire from T1.

    It looks ok in theory, but it won't save anything unless it's designed
    assuming there's limited channel difference.
    Andy Cuffe

  11. propman

    propman Guest

    wrote in
    I'm not talking about a javascript pop-up.......

    Lord it's no wonder you were having so much trouble with that site when
    your response to someone trying to help you is a snotty "ah... my
    browser isnt popup vulnerable.".
  12. lj_robins

    lj_robins Guest


    I was under the assumption that the "External Speaker" was an option not
    a permanent part of the circuit, going on that, YES this is stereo.
    Guess that is what I get for assuming something.

    Not a design I particularly like, had it been me I would have used four
    6L6's and two output transformers.

  13. I bet he designs software these days - probably for Microsloth.

    + Required crap appended to avoid restrictions imposed by brain +
    + damaged idiots.
    + Server Response: '441 Posting Failed (Rejected by POST filter)', +
    + Port: 119, Secure(SSL): No, Server Error: 441,
    + Error Number: 0x800CCCA9
  14. robb

    robb Guest

    Here are some comments from amplifier group....

    mykeymykey wrote ..
    sounds like a cool amp, can you post us a link to the schematic?
    this type of output was class A, about 12 watts per channel and
    popular among budget hi fi buffs in the 50s-60s. the output
    tranny was built like this just to save money and space.
    its not very unusual but its uncommon to find one in good
    condition that works. But yes tube amps and questions about
    them are always welcome here.

    po excuse 4 me wrote ............
    Yeah, it's an old trick to get low $ 'stereo' when 'stereo'
    only ment 2 speakers, 1 L, 1 r.. I love the way the '3rd'
    tranz works the OT..Hate to see what it would cost today
    against $$$ design..hehehehe.

    It's almost like old McIntosh tube amps with weird NFB based OT.
    I got from Ned years ago, a donut OT with a 'screen' tap. NOT
    a tube screen, but an xtra winding BETWEEN the pri/sec, that
    can be...controled. Also multi-UL taps, just a cool hunk of

    Ned wrote .......
    It's a cheap (but clever) piece of shit, like the rest
    of the stuff Columbia (apparently) gave away as premiums with
    record-club memberships.

    OK, think about this. Having a single output transformer
    is cheaper and means no core gap. If one 6L6 is amplifying
    something the other isn't, the signal appears out of phase
    on the opposite channel speaker. Feed that back to the input
    tube of the opposite channel, it cancels itself out, mostly.
    Not perfect stereo separation, but it's not intended to be,
    it's intended to give "stereo effect" at the lowest
    possible cost.

    flipper wrote .........
    I'm not real familiar with the topology but it looks like an old
    matrix stereo amp, a technique that was experimented with back in the
    1950's with the idea being 'cheap', er 'economical'. You get a
    'stereo' amp from not much more than a PP 'mono'. Saves iron in the
    OPT even though still SE.

    I say 'looks like' because the matrix circuits I've seen inverts one
    channel, and that one doesn't, so maybe it's either a variation or an
    even cheaper attempt at a similar concept.

    In the ones I've seen it's analogous to analog stereo radio
    multiplexing the L and R channels for transmission and then demuxing
    in the receiver. In the 'amplifier stereo' case you have R and -L (the
    inversion on the ones I've seen) amplified through the amp and then
    summed (subtracted, actually, since on opposite sides of the CT) at
    the OPT primary for R+L (this, btw, is 'normal' P-P polarity). Since
    the OPT center tap is a common mode null it is R-L, ah hah!, so if you
    then feed that R-L back (the purpose of the second transformer on the
    CTs) into the main OPT you subtract it out (or add it in, take your
    pick), opposite polarity on each side of the CT. So on one side you
    have R+L-(R-L) for 2L and, conversely, R+L+(R-L) for 2R on the other
    (if all your components were balanced, flawless, and without parasitic
    capacitance, leakage inductance, etc)

    It's likely that the missing 'inversion' on the front end, compared to
    what I've seen, is taken care of by proper phasing of the OPT and CT
    feedback but I'm not familiar enough with the topology to be
    completely sure if it's 'done right' or a dodge.

    But, what the heck and winging it here, that amp looks like both are
    in phase, at the input, so it'll be R-L in the OPT primaries. Now,
    that sounds bad, like there's 'nothing' with a common mode (mono)
    signal, but the second (matrix) transformer is on the primary side CT
    and will have the signal (and idle) current from both sides going
    through it, in phase, like a double tube SE, so it's R+L. Ah hah!
    Taking again the simple case of a common mode (mono) signal, the OPT
    secondaries 'on the ends' would, in a normal P-P amp, null each other
    (R-L) but the matrix transformer couples the primary CT R+L signal to
    the output CT 'adding/subtracting' (depending on which side of the CT
    you're on) it in. So, bingo, we have R-L+(R+L) for 2R, again, and
    R-L-(R+L) for 2L on the other. Dreadfully crude analysis but it
    illustrates the basic principle.

    From what I've read the matrix amps didn't work terribly well, though
    (components aren't flawless), with poor separation but did have the
    interesting characteristic of sounding louder than normal for the same
    power rating.
  15. Guest

    The cct diag of this bizarre amp is on this horrid server:

    Unfortunately the image quality is so poor that some relevant details
    are not legible, eg switch & input markings etc. A first look at the
    speaker wiring suggests this is one of those units where one speaker
    was in the amp case, and the other was in its own separate box. So at
    least one can rule out the speakers themselves being wired in mono -
    unless I've misread something.

    I'm now wondering why 2 tweeters per cabinet.

    OK, I've found a phase splitter. Where the input is, it goes to a
    double valve, and below that valve on the diag is a switch. In its
    upper position, ie not as shown on diag, the switch decouples the
    cathodes, making the double valve a conventional pair of amplifiers.
    But in the position the switch is shown, there is no cathode
    decoupling, and the double valve acts as a long tailed pair. Note also
    in this posn the grid feed to one is shorted out. So in this position
    we get traditional push pull operation on the output, ie a mono high
    power class B amp using the bigger transformer. I havent looked at the
    smaller output tf yet.

    6L6 = KT66, a fairly high power high quality high price valve. So this
    doesnt look much like a cut price unit. And it still has 2 output
    transformers, which is no cheaper than having 2 as usual.

    OK I see the smaller output tf sees the total B+ current of both output
    valves, and unbalances the speakers by an amount proportional to this.
    What I dont know is what the tf ratios are, so dont know if theyre the
    same or out by a ratio of 10:1.

    Lets guess theyre the same ratio, as its the most likely and it makes
    analysis relatively easy.
    Where B1, B2 are the anode current in valves 1 and 2:

    top speaker output is then B1-B2 + B1+B2 = 2x B1
    btm speaker output is then B2-B1 - (B1+B2) = -2x B1

    but driving both speakers off one valve's anode current, thus having to
    run it class A, makes as much sense as stuffing mushrooms. As does
    running the speakers in antiphase!

    And the above equation holds regardless of what the early stages do. I
    must have made a mistake, as it doesnt make any sense to run an amp
    that way - but I'm not seeing where.

  16. Guest

    I'll admit to not knowing why it was produced. Understandable to design
    such things to try to get an economic advantage, but surely 4x EL84s
    would have been cheaper than 2x KT66 (6L6), and there would be no
    crosstalk issue. And it would be simple enough to switch the speakers
    in series or parallel for mono max power use.

  17. Jim Land

    Jim Land Guest

    The "amplifier group" sounds interesting. Where are they located?
  18. robb

    robb Guest

    hello there are two that i know of they are
    guitar amp news groups but they like any
    amplifier related stuff
    they can be searched/seen on google-groups
    if your ISP news server does not list them

  19. 2 @ 4 ohm = 1 @ 8 ohm.
  20. There's a demented logic in it but it would have made more sense if the mono
    was push pull and the stereo was single ended.

    Probably be in the next Borat movie?
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