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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


    E80CF/7643

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    rodulfo23

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    Post by rodulfo23 on Thu Nov 12, 2020 11:37 am

    Hi! what was your experience trying out these tubes with adapter (in place of 7199) on a Dynaco ST-70? cheers!
    Bob Latino
    Bob Latino
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    Post by Bob Latino on Thu Nov 12, 2020 10:00 pm

    rodulfo23 wrote:Hi! what was your experience trying out these tubes with adapter (in place of 7199) on a Dynaco ST-70? cheers!

    To my knowledge there is no direct replacement for a 7199 tube that is plug compatible. I believe that a 6U8 and a 6GH8A may be used but only if you a use a proper adapter that alters the "pin out" of the tube. Dynakitparts sells such an adapter at the link below ..

    7199 to 6GH8A adapter at Dynakitparts

    Bob
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    boscode

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    Post by boscode on Tue Nov 24, 2020 2:58 am

    I tried a pair of Philips 7643 in my Dyna SCA 35 with adapters from Tubedepot. I don't recall the exact problem, this was a few yrs ago, but they didn't sound good. (The amp had been fixed-up with many new parts and sounded quite good with the old 7199.) I think the adapters came with a pair of Tungsol 6U8A tubes. Those didn't sound very good either. What has worked well, are a pair of Siemens 6U8A ECF82. Also a pair of 1987 Philips 6U8A tubes sounded good, too, surprisingly. The later two pairs just had more output that was clearer and richer, more full-bodied, less noise. Could be that the adapter has something to do with it. All these tubes tested very well on my Sencore tube tester. The 6BQ5 tubes are wired in triode mode. I haven't tried any 6BH8A tubes.
    peterh
    peterh

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    Post by peterh on Tue Nov 24, 2020 8:31 am

    boscode wrote:I tried a pair of Philips 7643 in my Dyna SCA 35 with adapters from Tubedepot. I don't recall the exact problem, this was a few yrs ago, but they didn't sound good. (The amp had been fixed-up with many new parts and sounded quite good with the old 7199.) I think the adapters came with a pair of Tungsol 6U8A tubes. Those didn't sound very good either. What has worked well, are a pair of Siemens 6U8A ECF82. Also a pair of 1987 Philips 6U8A tubes sounded good, too, surprisingly. The later two pairs just had more output that was clearer and richer, more full-bodied, less noise. Could be that the adapter has something to do with it. All these tubes tested very well on my Sencore tube tester. The 6BQ5 tubes are wired in triode mode. I haven't tried any 6BH8A tubes.
    Changed to triode will upset nfb.
    As wired originally 6U8 / and E80CF will work just fine.
    Reason is that sca35 has a more complex scheme with both nfb and positive feedback.
    Thus my recommendation is to restore to factory configuration.
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    boscode

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    Post by boscode on Wed Nov 25, 2020 8:58 pm

    Thanks, Peter. Good point about the complex feedback arrangement. Now I remember reading and trying to understand it a few years ago, giving up. I didn't change the 82k feedback resistor, or any of the other feedback components. (I know just enough to get myself into trouble.) I will connect the 6BQ5 back to ultralinear. Maybe my 7643 are excellent tubes.
    Also, my 7199 to 6U8 adapters are not from Tube Depot. I don't know their origin (except that they came from a friend).
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    boscode

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    Post by boscode on Thu Nov 26, 2020 12:52 am

    And sorry I talked too much about the SCA-35, when the original poster asked about the ST-70. I'd guess, that trying the E80CF/7643 in the ST-70 with the adapter might be an economical way to enjoy the amp, as opposed to building a new input board. These 6U8 types did work well in my SCA-35. I hope to see some responses that deal with the ST-70.
    All the best,
    b

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