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    ST-70 driver board ID?

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    Rickenbackerman

    Posts : 5
    Join date : 2017-06-06

    ST-70 driver board ID?

    Post by Rickenbackerman on 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.

    34740611340_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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    PeterCapo

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    Join date : 2008-12-05

    Re: ST-70 driver board ID?

    Post by PeterCapo on Tue Jun 13, 2017 4:00 pm

    It's probably an earlier version of the Triode USA board designed by SDS Labs (Sheldon Stokes).

    Is it this one? http://www.quadesl.com/pdf/st70board_ef86.pdf

    or maybe this? http://www.quadesl.com/pdf/st70board_6au6.pdf

    I'd want to find out what caused the 1.5MΩ to fail.  You can try swapping the tubes from one channel to the other to see if the problem follows the tubes.  Or, I might replace the capacitor (0.05μF 400V) on the board nearest the 1.5MΩ that failed and also get a new pair of tubes, either EF86 or 6AU6, depending on which version of the board you have.  I should mention that there were some variations to this board over the years to make it more compatible with a wider range of tubes.  This plus changes in manufacturing probably account for why it does not look exactly the same as the one Triode now sells.  But, the basic design should be very similar.  If you can map your board to either of the two documents above, you have a basis for troubleshooting.

    Or, you could drop in the new board, as you suggested.


    Last edited by PeterCapo on Tue Jun 13, 2017 6:29 pm; edited 2 times in total (Reason for editing : Add information.)
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    corndog71

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    Re: ST-70 driver board ID?

    Post by corndog71 on 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.
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    tubes4hifi
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    Re: ST-70 driver board ID?

    Post by tubes4hifi on 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

    Rickenbackerman

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    Join date : 2017-06-06

    Re: ST-70 driver board ID?

    Post by Rickenbackerman on 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.

    Rickenbackerman

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    Join date : 2017-06-06

    Re: ST-70 driver board ID?

    Post by Rickenbackerman on 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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    PeterCapo

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    Re: ST-70 driver board ID?

    Post by PeterCapo on Wed Jun 28, 2017 3:16 pm

    Curious as to whether you made all these changes at about the same time or did you do them piecemeal over a period of time?

    Rickenbackerman

    Posts : 5
    Join date : 2017-06-06

    Re: ST-70 driver board ID?

    Post by Rickenbackerman on 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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    PeterCapo

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    Re: ST-70 driver board ID?

    Post by PeterCapo on Wed Jun 28, 2017 5:12 pm

    I see, so there are some changes to both the amp and the preamp at the same time.

    By the way, bypassing the tone controls in a PAS does not necessarily have to result in DC offset at the output of the PAS.  To bypass PAS tone controls and eliminate DC offset from its output would be to emulate the original Dynaco PAS-2X or PAS-3X with tone pots centered.  Dynaco originally included a capacitor in the PAS-2X and PAS-3X that kept DC offset from the outputs.  Best procedure is here: http://www.audioregenesis.com/documents/PAS%20Tone%20Control%20Bypass.pdf

    Rickenbackerman

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    Join date : 2017-06-06

    Re: ST-70 driver board ID?

    Post by Rickenbackerman on 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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