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The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Tubes4hifi VTA tube amp and preamp kits and all Dynakitparts.com products


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    VTA PAS 3 preamp

    tubenutr
    tubenutr


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    VTA PAS 3 preamp Empty VTA PAS 3 preamp

    Post by tubenutr 19th December 2015, 3:53 pm

    I have a VTA PAS that I would like to temporarily use with a SS power amp (ya I know, evil).
    If the PAS has a 1k output impedance and the power amp has a 22k input (in parallel with 62 pF cap), why does it sound
    thin, bright and just plain terrible.

    Any help would be greatly appreciated.

    Mike
    peterh
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    Post by peterh 19th December 2015, 4:18 pm

    The PAS don't have 1k output impedance, rather it needs > 100k to be even reasonable good.
    You need a buffer / cathode follower, and as PAS don't have enough power for an extra tube
    it will have to be an external unit.

    A very primitive workaround might be a series resistance of 100k on the outputs,
    that would reduce the signal to 1/10 but it might be enough to your setup.
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    Post by Guest 19th December 2015, 5:52 pm

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    Last edited by PeterCapo on 2nd December 2020, 4:24 pm; edited 1 time in total
    peterh
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    Post by peterh 19th December 2015, 7:19 pm


    Good reading. And we are discussing a PAS ( not -X ) and "eskimo" experiences
    "thin, bright" sound. Just to be expected on 22k loading.
    Bob Latino
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    Post by Bob Latino 19th December 2015, 8:16 pm

    The PAS-3 was made to work with the 470K loading of the Dynaco tube amps of the day. The PAS-3 does not work well with amps that have input impedances of less than 250K or so. You can see from the chart that the low end is really depressed even at a 100K loading. With a 22K load from your SS amp, the low end has undoubtedly dropped out of sight - probably 3 to 5 dB down for any frequency below 100 Hz.

    Bob


    VTA PAS 3 preamp PAS3load_zpsz0hpr32n
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    Post by Guest 19th December 2015, 8:38 pm

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    Last edited by PeterCapo on 2nd December 2020, 4:25 pm; edited 1 time in total
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    GP49


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    Post by GP49 19th December 2015, 11:38 pm

    My "work:" I would put the owner's PAS preamp on the bench along with his power amplifier, and remove all the load resistors.  I substituted a ten-turn variable resistor in place of the load resistors, while sweeping the line input through the low-to-midrange, and watching the synchronized output on a scope as I adjusted the resistor.  I'd do this at several different output voltages but generally that didn't make much difference as long as I kept to "reasonable" levels.  When satisfied that the response was as flat as I could get it (generally I weighted my judgment more heavily to 40Hz and above, while making sure the 20-40 Hz range didn't go too far off), I'd measure the potentiometer and use 1% resistors to make up that value load resistor and install it.  Same was done on both channels.  The specific tubes used in each channel were noted and the owner advised that if he rolled tubes, he might change the response.  Of course the owner would also have to use only that particular power amp or another identical one.

    I generally found that in a stock PAS, the lowest I could go was around a 25K load.  Below that, irregularities appeared, not all of them in the bass range.
    peterh
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    Post by peterh 20th December 2015, 2:25 am

    We seem to agree that 22k is to low for a PAS3
    ( eskimo named it a "VTA PAS" but i assume he was referring to a "Dynaco PAS3" )

    The fact that nu suitable stand-alone buffer / cathode follower seems available on the market
    opens up a niche for such a device. ( yes there is lots of "bear-like" chinese things on ebay, but
    none of them is a cathode follower as far as i know)

    Any takers for designing such a thing complete with 110/230V AC power supply ?
    tubenutr
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    VTA PAS 3 preamp Empty VTA PAS 3 aka tubes4hifi PAS zmod

    Post by tubenutr 20th December 2015, 2:54 pm

    Just to clarify what preamp I referring to.

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