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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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    My first Dynaco ST-70 with VTA driver board rebuild

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    docswanson


    Posts : 6
    Join date : 2019-11-14

    My first Dynaco ST-70 with VTA driver board rebuild Empty My first Dynaco ST-70 with VTA driver board rebuild

    Post by docswanson Fri Aug 06, 2021 2:24 pm

    Hello all.  This is my first post, and first tube project completed.
    First, I want to thank all the members of this group who provided me so much information while I browsed all the topics.  Your discussions were invaluable to my success in this rebuild.

    So, this is an ST70 that I acquired a couple decades ago for cheap.  It was not in too bad of shape, but definitely not ready for prime time.  I dug it out of my storage about 18 months ago and resolved to make it work (and look nice).

    My first Dynaco ST-70 with VTA driver board rebuild PmSNlbikj

    The only things I re-used were the three transformers and the chassis.  I ordered the VTA board, the stepped attenuator, and triode/ultralinear mode switch kit from Bob and Roy's site.  Also got new tube sockets hardware and misc. parts as needed.

    The chassis had too much pitting to leave polished without sending it out for refinishing, so I just cleaned and painted it.  I cleaned the transformers and gave them a coat of hammered black.  I wanted to make it look a little more modern, and to get better spouse by-in :-).  So I built a very simple drop in wooden enclosure out of African padouk I had lying around and made mount pounts for the inputs, attenuator, and a power indicator light.  Then life got busy, and it sat for months.

    This past week, I decided to finish the project, and started soldering.  It took a while, since I am slow and methodical.  

    I did a couple unique things I have not seen elsewhere.  Instead of the 4 ohm speaker connection, I connected the 16 ohm to post three to provide a connection point for my powered subwoofer's high level inputs.
    And since I would not be using the front power takeoff sockets, I needed a different bias contact points.  I added a test lead socket next to each tube. mounted in the existing vent slot (see the small red things in the picture), and a ground lead socket in between the triode/ultralinear swithces.  They work great.

    I got it done, and no smoke on first power up!!!  Just a bad solder on one of the triode/ultralinear switches (easily fixed).

    I do have a bias issue I will explain below.

    Here is the final result.

    My first Dynaco ST-70 with VTA driver board rebuild Pm4GrjB8j

    https://imageshack.com/i/pm3R4Yqoj" alt=""/>

    My first Dynaco ST-70 with VTA driver board rebuild PoClE1Q1j

    My first Dynaco ST-70 with VTA driver board rebuild PnjNq5cjj


    Now to my problem.
    I cannot get the tubes to properly bias.  With the bias pot turned all the way down, I cannot get below 0.45 V with brand new EL34's  (one would not go below .65 V).  When I tried some 6L6GC's, I could not get below 1.2 V.  House line voltage during the tests was at 121 VAC.

    The B+ to the driver board is 424V with a brand new 5AR4. The bias voltage between D1 and R39 is -70 VDC.

    It plays music with zero hum, and sounds great.  But I do want to be able to have proper bias.  Any suggestions would be welcome.

    Steve

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    docswanson


    Posts : 6
    Join date : 2019-11-14

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    Post by docswanson Fri Aug 06, 2021 6:09 pm

    I think I may have found the problem.
    While taking voltage reading and such, suddenly the output of the 5AR4 dropped to zero. Just no high voltage. (Darn soviet tube)
    So I dropped in a 5U4 to continue testing (all out of 5AR4's).
    Now I am getting normal adjustment range on the bias. I don't know how, but it must have been a faulty rectifier tube causing issues, even though the now dead tube was putting out the appropriate voltage.

    BTW, I sure like the look of the 5U4. I think the 5U4 helps to offset the modern high line voltage. Now I run at the end of the specified voltage range.
    solderblob
    solderblob


    Posts : 103
    Join date : 2018-05-20
    Age : 75
    Location : Placerville, California

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    Post by solderblob Sat Aug 07, 2021 5:03 pm

    That's a beautiful job you did!  I bet it sounds great too.

    Dave

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    docswanson


    Posts : 6
    Join date : 2019-11-14

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    Post by docswanson Sat Aug 07, 2021 7:29 pm

    Thanks solderblob. Even my wife thinks it looks nice. (which is high praise from her :-)

    I'm very happy with the sound. I noticed two things right off. First, how quiet it was with no input. Second, the instrument separation was so much better than with my solid state album. Vocal are very clear too.

    I first had a GE 12AU7 in the center spot on the driver board. And it sounded a bit too bright. I go right from my phono preamp to the ST70. So no tone controls to play with. So why not try my first real tube rolling? Found an ICE Mullard 12BH7A in my tube drawer to drop into the center. To me, that really tamed the high end, and gave it a much more balanced sound. Now to log a 100 or so hours of listening to get the tubes burned in ;-).

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    knotscott
    knotscott


    Posts : 46
    Join date : 2019-12-28
    Location : Rochester, NY

    My first Dynaco ST-70 with VTA driver board rebuild Empty Re: My first Dynaco ST-70 with VTA driver board rebuild

    Post by knotscott Sun Aug 15, 2021 2:34 pm

    Beautiful looking case.  I think I saw it and commented on FB too.

    I'm also running 12BH7A in my center spots.  Love it immediately and have never gone back.

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