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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


    ST-70 Rebuild

    chad1376
    chad1376

    Posts : 23
    Join date : 2019-08-23

    ST-70 Rebuild Empty ST-70 Rebuild

    Post by chad1376 on Fri Aug 23, 2019 9:07 am

    Hello all - my first post here.

    I'm diving into the second re-wire of my ST-70, and thought I'd do a rebuild thread.  I originally bought and refurbished this amp about 20-years ago.    It was my first piece of tube equipment, and in hindsight, I had done several things that have been bugging me.  Most critically, it recently has been suffering from rectifier arc-over.  Searching for answers, I kept getting linked to this forum, and I believe I know the several things I had done wrong, and will correct them this time around.  In tearing down the amp, I also found several wires that had lost a good solder joint.  I'm not looking to hot-rod the amp too much.  I just want to have a solid and reliable amp again.

    A bunch of parts are on order - every cap and resistor is being replaced.

    I was going to link a few photo's, but it looks like I need to wait a week before I have rights.  I'll post them up as soon as I'm able.

    << I did get a pre-teardown picture in my avatar though Wink

    Chad
    chad1376
    chad1376

    Posts : 23
    Join date : 2019-08-23

    ST-70 Rebuild Empty Re: ST-70 Rebuild

    Post by chad1376 on Tue Sep 03, 2019 3:48 pm

    OK - by the time I could link pictures, the amp was darn near done. Here's a quick rundown of the resto:

    Stripped down chassis.
    ST-70 Rebuild I-d2HfwRQ-L

    Removed components and bits of clipped wire from the hardware to be re-used.  That's an older version of the Triode driver board.
    ST-70 Rebuild I-PJcf7Z7-L

    DIY chassis powdercoat
    ST-70 Rebuild I-33W8J46-L

    ST-70 Rebuild I-2wkVxKc-L

    ST-70 Rebuild I-Dw2WPtr-L

    ST-70 Rebuild I-nwQQNH6-L

    ST-70 Rebuild I-HC3Wrpn-L

    Rewire - essentially a stock circuit.  The only changes are:
    - Diode in lieu of the selenium rectifier
    - Diodes added to the rectifier
    - Used a 10 ohm resistors for the bias check, instead of the 14.5 ohm.  Bias set to 1v.
    The old cloth wires on the transformer were badly rotted and disintegrating.  I spliced new wires as close to the windings as I could.  I used silicone at the splices to keep the wires from moving around too much while tugged on them during wiring.
    ST-70 Rebuild I-3RzzfG2-L

    The amp fired up with no issues.  I set the bias (actually at 0.9v) and everything works great!
    ST-70 Rebuild I-6XFXNq8-L

    ST-70 Rebuild I-FwQ9xtj-L
    chad1376
    chad1376

    Posts : 23
    Join date : 2019-08-23

    ST-70 Rebuild Empty Re: ST-70 Rebuild

    Post by chad1376 on Mon Sep 09, 2019 11:24 am

    Update:  After firing up the amp, I found the bass to be quite weak.  I had hoped that it was new tubes that needed some run-in time.  I think it was the Triode board.  My board was an old version, and the latest documentation is for a board with a slightly different layout.  It's likely that I mixed up some resistor locations.  Rather than track all that down, I ordered a VTA board.  It was shipped very quickly, and I was able to solder it up this weekend.  There was quite a bit of re-wiring required, and I had to clip out a lot of freshly installed components.   It was 100% worth it.  The amp sounds the way it should now.  I'm confident I'll have a reliable amp for many years to come.

    ST-70 Rebuild I-rKthzMT-L
    PeterCapo
    PeterCapo

    Posts : 927
    Join date : 2008-12-05

    ST-70 Rebuild Empty Re: ST-70 Rebuild

    Post by PeterCapo on Sun Dec 01, 2019 1:45 pm

    Great color.
    MechEngVic
    MechEngVic

    Posts : 81
    Join date : 2019-01-16

    ST-70 Rebuild Empty Re: ST-70 Rebuild

    Post by MechEngVic on Sun Dec 01, 2019 5:58 pm

    At first I was "RED?!?!" but then you put on the burgundy, it's unique and gorgeous.
    chad1376
    chad1376

    Posts : 23
    Join date : 2019-08-23

    ST-70 Rebuild Empty Re: ST-70 Rebuild

    Post by chad1376 on Thu Dec 05, 2019 12:28 pm

    Thank you - I'm very happy with the look.  Just for reference - I used this powder coat:

    https://www.powderbuythepound.com/aussies-anodized-red-transparent-flat.html

    It's slightly translucent and needs a reflective undercoating.  The nickel plating worked well for this, except some of the darker pitting and scratches still show through (most of the deep scratches were on the bottom.)  I considered coating the transformer covers too, but I got lazy.  Those would have needed to be stripped, then an underlying coat of silver before applying the red.  I also wasn't sure I would get them to exactly match, since the undercoating would have been different.


    Last edited by chad1376 on Thu Dec 05, 2019 12:29 pm; edited 1 time in total (Reason for editing : added content)
    PeterCapo
    PeterCapo

    Posts : 927
    Join date : 2008-12-05

    ST-70 Rebuild Empty Re: ST-70 Rebuild

    Post by PeterCapo on Thu Dec 05, 2019 12:49 pm

    Transformer covers... I'm facing a difficult decision about mine. I'm poised to rebuild a Stereo 70, but I am not sure I will be able to manage refinishing the end bells. They are borderline, cosmetically.
    chad1376
    chad1376

    Posts : 23
    Join date : 2019-08-23

    ST-70 Rebuild Empty Re: ST-70 Rebuild

    Post by chad1376 on Thu Dec 05, 2019 12:58 pm

    PeterCapo wrote:Transformer covers... I'm facing a difficult decision about mine.  I'm poised to rebuild a Stereo 70, but I am not sure I will be able to manage refinishing the end bells.  They are borderline, cosmetically.

    FWIW

    Hammertone/Hammerite paint hides a lot of sins, and I think it looks appropriate for vintage equipment.  Depending how bad they are, I'd say straighten out the worst of the dings or whatever,  sand/roughen the stock paint,  maybe even apply a bit of body filler if needed, and hit-em with the hammer paint. Wink

    If you look very closely at my pictures (the one with the covers on the table), you can see the hammer finish.
    PeterCapo
    PeterCapo

    Posts : 927
    Join date : 2008-12-05

    ST-70 Rebuild Empty Re: ST-70 Rebuild

    Post by PeterCapo on Thu Dec 05, 2019 7:11 pm

    Thank you.

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