DYNA ST-70 Harmonic Distortion Testing using all 1960's Heathkit Tube Technology

Discussion in 'Tube Audio' started by JRUBIN, May 14, 2018.

  1. JRUBIN

    JRUBIN Member

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    In this video I will demonstrate harmonic distortion testing of my Dynaco ST 70 amplifier using all Heathkit tube gear from the 50's and 60's. This demonstration will show the THD produced from this amplifier at 2KHZ using three different power inputs.

    The equipment used includes:

    Heathkit IG-72 Audio Generator
    Dynakit ST70 Power Amplifier
    Heathkit IM-12 Harmonic Distortion Meter
    Heathkit O-12 Oscilloscope

    The entire process is shown in the video over several iterations, some have been shortened

     
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  2. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Jordan; Do you know where I can get a copy of the IM-12 manual? I've got an IM-12 but no manual, and don't want to use it unless I have the manual.
    Thanks
    Larry
     
  3. primosounds

    primosounds Parallel single ended EL84 ,EDCOR OPT

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    Hey thanks for posting the info. Always nice to see the vintage tools being used to good advantage.
     
  4. ConradH

    ConradH Addicted Member

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    Nice! People underestimate Heathkit test equipment. It may not hit the numbers of the HP and similar of the time, but it gets the job done. You can easily fix it if it breaks and/or mod it for improved performance.
     
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  5. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Nice job! That distortion reading correlates well with my own tests at 1 kHz on the ST-70. It would be interesting to know what the distortion is of the signal generator alone without the ST-70 in circuit. For any expected distortion reading, its always best if the source has an internal distortion level of no greater than 10% of the distortion reading produced by the device under test. In this instance then, the Heath generator would hopefully have an internal distortion no greater than .014% at the output level required to drive the ST-70 to 32 watts output, or about 1.25 vac rms.

    The old Heath test equipment was great gear at a very affordable price point. Thanks for sharing!

    Dave
     
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  6. s-petersen

    s-petersen Scott Subscriber

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  7. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Thanks Scott. Just what I was looking for.
     
  8. Audiovet

    Audiovet Well-Known Member

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    As Dave G said.

    I read that the Heathkit IG-72 specifies a typical distortion of 0,1%. This will only be when optimally adjusted, and not frequency-independant.

    Not to 'rain on the parade' (we need that here in South-Africa, not on the thread!), but that will need to be taken into account. Otherwise an informative thread!
     
  9. s-petersen

    s-petersen Scott Subscriber

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    I have upgraded from the IM-48 and IM-58 to the solid state versions IM-5248 IM-5258 and IG-5218(with the Morray? mod)
     
  10. drtool

    drtool It might get loud In Houston Subscriber

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    Thanks for the post.
     
  11. JRUBIN

    JRUBIN Member

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    The THD from the test set into the alayzer was .12% at this frequency... I should have specified this in the video to reduce confusion.
     
    Last edited: May 21, 2018 at 7:32 AM

     

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  12. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Thanks JR. Just goes to show how cascaded signals that each contain their own distortion elements can often produce a cancellation effect to produce less distortion in the output, than the simple addition of the two distortion products would suggest.

    Dave
     

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