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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


    Restoring a Stereo 70 Series II

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    Andy B

    Posts : 1
    Join date : 2021-04-25

    Restoring a Stereo 70 Series II Empty Restoring a Stereo 70 Series II

    Post by Andy B Sat May 01, 2021 11:18 pm

    I'm poring over the posts on your site, and finding lots of ideas and opinions regarding the vintage Dynaco gear - LOVE IT !!
    I recently acquired a Stereo 70 Series II which was headed to the dumpster due to a blown Left channel. From a visual inspection, it's obvious that cathode resistor R9 is blown, and the tubes on the Left channel are marked Dynaco - so they're probably the originals from 1990.
    The antique tube tester I borrowed (Seco Model 88) shows the same reading for the stock tubes on the Left channel and the newer Sovtek EL34G's on the Right side - finding it hard to trust that tube tester...
    My plan is to replace both R9 and R8 with 2watt, 15 ohm metal film resistors - not too comfortable with the idea of using lower wattage resistors as fast-acting fuses. Also planning to replace all four finals (but keep the Sovteks as spares.) I'm curious about the idea of the screen grid resistor modification, but not ready to try that without more detailed information - what resistor value? - which side of C17/C27? - is it really worth the effort to make this mod?? Are there other land mines I should know about with this amplifier, or any other simple enhancements? Unfortunately, I got rid of my 1970's Dynaco A25 speakers a long time ago - not sure that my JBL 500 bookshelf speakers are going to do this amp justice - but it will be a good excuse to dust off some of those old LP's and take a break from Sirius XM radio.
    Thanks in advance for the assistance, advice, and opinions...
    Andy B

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    peterh
    peterh

    Posts : 1565
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Restoring a Stereo 70 Series II Empty Re: Restoring a Stereo 70 Series II

    Post by peterh Sun May 02, 2021 12:46 am

    Andy B wrote:I'm poring over the posts on your site, and finding lots of ideas and opinions regarding the vintage Dynaco gear - LOVE IT !!
    I recently acquired a Stereo 70 Series II which was headed to the dumpster due to a blown Left channel. From a visual inspection, it's obvious that cathode resistor R9 is blown, and the tubes on the Left channel are marked Dynaco - so they're probably the originals from 1990.
    The antique tube tester I borrowed (Seco Model 88) shows the same reading for the stock tubes on the Left channel and the newer Sovtek EL34G's on the Right side - finding it hard to trust that tube tester...
    My plan is to replace both R9 and R8 with 2watt, 15 ohm metal film resistors - not too comfortable with the idea of using lower wattage resistors as fast-acting fuses. Also planning to replace all four finals (but keep the Sovteks as spares.) I'm curious about the idea of the screen grid resistor modification, but not ready to try that without more detailed information - what resistor value? - which side of C17/C27? - is it really worth the effort to make this mod?? Are there other land mines I should know about with this amplifier, or any other simple enhancements? Unfortunately, I got rid of my 1970's Dynaco A25 speakers a long time ago - not sure that my JBL 500 bookshelf speakers are going to do this amp justice - but it will be a good excuse to dust off some of those old LP's and take a break from Sirius XM radio.
    Thanks in advance for the assistance, advice, and opinions...
    Andy B
    The cathode resistors R8 and R9 are low wattage film resistors of a reason,
    when a flashover happens they will blow saving the output transformer from damage. Replacing them with anything sturdier will make the transformer at risk.
    Using fuses is also worse as they will allow more current for a while before blowing.
    As for screen "stoppers" ny value between 100 and 500 ohm is ok. Preferably non-spirol resistors , carbon comp is perfect here.
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    Solder Slinger

    Posts : 20
    Join date : 2009-05-24

    Restoring a Stereo 70 Series II Empty Re: Restoring a Stereo 70 Series II

    Post by Solder Slinger Sun May 16, 2021 12:08 pm

    I'd agree with peterh, use original wattage value resistors on the cathodes, they act as a fuse.

    Regarding the screen resistors, I use 3 to 5 watt metal films or non-inductive wirewounds for this application. Dale CPF3 are a good choice, can get them from Mouser. Regarding the value, you might want to experiment, the traditional value is 100 ohms, but others prefer higher values. Dennis Grimwood @ http://oestex.com/tubes/ suggests up to 1/4 the transformer primary winding resistance, 4300/4 or up to about 1100 ohms. I personally use about 700 ohms on my amps which run in triode mode. Note that running these higher values will cut your power very slightly, a watt or 2 depending on the value chosen. Main thing is to use screen resistors; any value. They make things easier on your output tubes.

    Don't know how efficient those JBL's are, if reasonably efficient, you could try your amp in triode mode by moving that resistor from the Ultralinear transformer lead connected to pin 4 to instead connect pins 3 to 4 on each output tube, disconnecting and insulating the end of the transformer UL lead going to the Pin 4 screen. Easy enough to do and totally reversible.

    Enjoy your new amp

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    Restoring a Stereo 70 Series II Empty Re: Restoring a Stereo 70 Series II

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