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    St70 rehab questions

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    BudK

    Posts : 2
    Join date : 2017-05-23
    Location : South Jersey

    St70 rehab questions

    Post by BudK on Thu Sep 21, 2017 11:37 am

    Hi Folks,

    my wonderful son has "gifted" me a ST70 Dynakit amp that was being tossed from the recording studio at the local university that he works at in addition to classes. I have changed the multicap, added diodes to the rectifier socket and replaced the bias and 6 large caps on the driver board. Sounds nifty but I have concerns about its reliability. Many of the foil traces lifted and self destructed while de-soldering and required jumpers.I have researched the vta driver board and after checking my tube stash have found a fistful of 12 at7's,12ax7's, 12bh7a's and more than a few 12au7's .

    At the moment I am using a TT run into the sound card of my pc to the st70 to a pair of old Bose 401's. ( my son absconded with my infinity rs3000's)

    My question is : what version of the vta driver board would be best for what I have now ? Does it make a difference? or can I get swappy with the tubes I have available. Thanks,

    Bud
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    Bob Latino
    Admin

    Posts : 2567
    Join date : 2008-11-26
    Location : Massachusetts

    Re: St70 rehab questions

    Post by Bob Latino on Thu Sep 21, 2017 11:44 am

    Hi Bud,

    Since you already have a supply of 12AU7 and 12BH7 driver tubes, you should upgrade your amp with the noval (12AU7) VTA ST-70 driver board. The octal version uses 6SN7 driver tubes. 6SN7 tubes draw double the current of a 12AU7 and will put a little extra stress on your already undersized original Dynaco PA-060 power transformer.

    Bob

    BudK

    Posts : 2
    Join date : 2017-05-23
    Location : South Jersey

    Re: St70 rehab questions

    Post by BudK on Thu Sep 21, 2017 12:39 pm

    . Thanks. So now I have only transformer envy to deal with. Sweet.
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    Peter W.

    Posts : 502
    Join date : 2016-08-07
    Location : Melrose Park, PA

    Re: St70 rehab questions

    Post by Peter W. on Thu Sep 21, 2017 3:05 pm

    BudK wrote:.  Thanks.  So now I have only transformer envy to deal with. Sweet.

    Only if you are leaning towards Octal upgrades. Keep in mind that there are stock 70s with tens of thousands of hours on them (I have one such) that - at least to-date - are still running cool and strong. Dynaco made 350,000 of them, +/- a few hundred. This is not to suggest that the stock 70 is by any means bullet-proof. And bad traces, aged caps and much more are par for the course.

    This gives you three choices:

    Upgrade and repair the OEM phenolic board - OK if you have the skills, patience and time to do so. Often more trouble than it is worth.
    Upgrade to a new glass board that is otherwise to the OEM circuit - better as the all the various parts are not costly, and reliability will be greatly increased. However, and note: They are no longer making 7199s as far as I know. There are (for now) plenty around, but it is a consideration.
    Upgrade to a new board with an 'improved' circuit - best in terms of reliability and ongoing maintenance. Higher cost, but it could be much worse. The VTA options (in my considered opinion) are the best options along those lines.

    After which, replacing the power-transformer and going all-in is next. Before I did that, I would purchase a new VTA amp outright with all the bells and whistles laid on, and keep the vintage 70 near-stock. I have always maintained that a Stock 70 is one of the best tube-amp learning tools out there as there are so many after-marked mods, changes, upgrades, tweaks and similar available such that you can try many different options from the same baseline.

    Enjoy it! (and get better speakers).

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