tubes, impedance matching, and holy grail equivalents

Discussion in 'Amp Tech Center' started by SoPro, Oct 22, 2019.

  1. SoPro

    SoPro TDPRI Member

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    I've posted a variety of related questions here and on other sites, so this may seem vaguely familiar, but I've arrived at the following:

    Background - I've had a '65 U.S. Vox Berkeley V8 tube amp for decades. I got it with a Buckingham cabinet (2x oxford alnico 12s.) As part of a restoration project, I finally got a beautiful Berkeley cabinet and trolley, but it came from a solid state version which used different speakers. The "new" cabinet has a pair of 8 ohm Celestion 7424s, instead of the 16ohm 7442s the V8 would have come with. I will keep looking for Vox/Celestion 7442s, but it will probably be a long & difficult search, since the 7442's have achieved mythic status.

    Question 1 - Since my 7424s are from same era as the holy grail 7442s and have N1B4 cone stamps, are they identical except for the impedance? (Would they sound equally grailish?)

    Question 2 - I have a half dozen 70V multi-tap transformers lying around that were pulled from very hi-fi N.E.A.R. outdoor speakers.
    The primary wires are: Com, 128w, 64w, 32w, 8w, 4w. Secondary: Com, 8ohm, 4ohm.
    My minimal web research (a dangerous thing) makes me think the secondary of these is simply an autoformer. Can I use the secondary (only) as an impedance matcher? In other words, if I were to completely ignore the primary taps, could I safely try out my 4ohm cab with my 8ohm amp by running the amp into the 8ohm tap of the secondary, with speakers fed by the 4 ohm tap?

    Thanks very much. Please forgive if I post the same question elsewhere.
    Randy
     
  2. kbold

    kbold Tele-Holic Silver Supporter

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    Best to match speaker impedence with transformer. The easiest road would be to select correct impedence speakers.
    You may have to settle for something less mythical.
    Can't help with Q 2.
     
  3. elpico

    elpico Tele-Holic

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    You don't need to bother, you can safely try the 4 ohm cab directly connected to the amp. There will be a slight difference in harmonic content and volume versus using an 8ohm cab, but that's about it.
     
  4. SoPro

    SoPro TDPRI Member

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    Thank you in kbold.
    Unfortunately "best" isn't an option here. I want to keep my 55 year old all orig amp (tubes and few caps notwithstanding) in its current state. Hence my specific questions.

    Also confused. Was that an answer to question 1? If so, I don't know whether it was yes or no.

    Randy
     
  5. SoPro

    SoPro TDPRI Member

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    Elpico,
    I've read that it's ok to double or halve impedance and that it's done all the time. But I've also read stern warnings NOT to do that with vox.
    Majority of people I've tended to trust say not to risk it. (Ok at medium volumes but diming being risky. And if I can't dime it, where's the joy?)
    How are you sure they are wrong about vox vulnerability?
     
  6. elpico

    elpico Tele-Holic

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    Math? Not sure what kind of an answer you really expected to question loaded that hard :lol:

    There's a lot of legend and mythology surrounding tube amps and I'm not crazy enough to think anybody can change that at this point. It's your amp and you should do as you want with it.

    I can only explain that power pentodes are essentially current sources. Imperfect ones, but current sources all the same, so if you're thinking cutting the speaker load in half would double current like a solid state amp I can assure you that's not the case. That only happens with voltage sources. Since we're dealing with a current source cutting the load from 8ohms to 4ohms increases current by less than 5%. More importantly, running a lower load decreases voltage stress on the output transformer because there's less impedance for that current to develop a voltage across. Going from 8 to 4 decreases voltage stress on that old output transformer by almost half. It's actually quite a bit safer than running the specified 8ohm load, which was never chosen because it was the "safest" load in the first place. It was chosen to create maximum power output.

    Using a higher load than the specified one is a different kettle of fish. That increases voltage stress on the output transformer and tubes. Most amps should tolerate a small mismatch in that direction, but that's not the situation you asked about.

    There's a reason tube amps use shorting jacks for the speaker jack. Lower load = safer, higher load (or god forbid open) = sad times.
     
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