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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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    ST-70 driver board ID?

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    Rickenbackerman


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    Join date : 2017-06-06

    ST-70 driver board ID? Empty ST-70 driver board ID?

    Post by Rickenbackerman Tue Jun 13, 2017 3:50 pm

    Right channel died on my amp about a week ago - I spent a bunch of time debugging and it turns out there was a 1.5M resistor that went open on the driver board.  In troubleshooting the thing, however, I realized that I couldn't find an exact match for this board online.  It looks similar in design to the triode board but at the same time there are differences.  Anybody recognize it?  It was in there when I bought the amp 8 or 10 years ago.

    ST-70 driver board ID? 35143239825_9e82d981ea_z34740611340_1f231e35a1_k by rickenbacker_man, on Flickr

    I got the right channel working again but it still doesn't sound quite as loud or as good as the left channel.  Is it worth upgrading to the VTA board?
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    Guest
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    ST-70 driver board ID? Empty Re: ST-70 driver board ID?

    Post by Guest Tue Jun 13, 2017 4:00 pm

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    Last edited by PeterCapo on Thu Dec 03, 2020 11:34 pm; edited 3 times in total (Reason for editing : Add information.)
    corndog71
    corndog71


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    Post by corndog71 Tue Jun 13, 2017 5:34 pm

    Yes, I recommend the VTA board. It's probably a better circuit and there's way better support here.
    Roy Mottram
    Roy Mottram
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    Post by Roy Mottram Thu Jun 15, 2017 6:42 pm

    Is it worth upgrading to the VTA board?

    you won't be asking that questions after you try one . . . your amp will sound 10x better

    there's about 1000 people on this forum that will tell you the same thing
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    Rickenbackerman


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    Post by Rickenbackerman Mon Jun 19, 2017 2:58 pm

    Shortly after I started this thread I ordered the VTA board - it arrived and I've already stuffed and installed it.  Hope to finish up the wiring tonight.
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    Rickenbackerman


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    ST-70 driver board ID? Empty Re: ST-70 driver board ID?

    Post by Rickenbackerman Wed Jun 28, 2017 11:50 am

    My rig is now complete. PAS3 w/ SDS labs cap board, orange drop coupling caps, LED pilot light and tone controls bypassed into a recapped ST70 w/VTA board (and old Svetlana's) into recapped A-25's. I got everything hooked up last night and gave it a quick test drive.

    All I can say is daaaaaaaaaaamn.
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    ST-70 driver board ID? Empty Re: ST-70 driver board ID?

    Post by Guest Wed Jun 28, 2017 3:16 pm

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    Last edited by PeterCapo on Thu Dec 03, 2020 11:34 pm; edited 1 time in total
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    Rickenbackerman


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    ST-70 driver board ID? Empty Re: ST-70 driver board ID?

    Post by Rickenbackerman Wed Jun 28, 2017 4:07 pm

    This round of mods consisted of the VTA board, PAS3 coupling caps and LED pilot. Everything else was done in earlier rounds.

    I should also mention that I put some input coupling caps on the ST-70 since I read that bypassing the tone controls in the PAS introduces DC at the outputs.
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    Post by Guest Wed Jun 28, 2017 5:12 pm

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    Last edited by PeterCapo on Thu Dec 03, 2020 11:35 pm; edited 1 time in total
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    Rickenbackerman


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    ST-70 driver board ID? Empty Re: ST-70 driver board ID?

    Post by Rickenbackerman Wed Jun 28, 2017 5:49 pm

    Thanks Peter,
    I used an old VanAlstine article when I first did the bypass - it was in an 80's article of audio basics I think. That pdf you linked to suggest putting coupling caps on the PAS output which I don't recall doing when I did the mod. Either way, not being sure whether it had DC at the outputs or not, I stuck some caps in the 70 as I was buttoning it up just to be safe. These two will be with me forever anyway so I'm not too worried about the PAS being paired with anything else.

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