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    ST-70 Rebuild


    Posts : 3
    Join date : 2011-06-15
    Age : 53
    Location : Los Angeles

    ST-70 Rebuild

    Post by ss900f on Fri Jul 01, 2011 1:52 am

    First off I would like to say.... great board! I have been reading and learning and developing a great respect and appreciation for all of you out there! and now the time has come for me to ask for HELP/Advice. I recently decided to re-build my trusty rusty old ST-70 after the quad cap finally went. In my search for parts and information I discovered your nice friendly place here (had I found it a day sooner... I would have gotten the VTA driver board).
    I used the stock transformers, the Dynakitparts 80,40,30,20 Quad cap, as well as their Bias update kit, and I used the Triode electronics ef86 driver board.
    After rebuilding I first shorted the inputs, and hooked up some old POS speakers and powered it up with no tubes at all... nothing exciting happened so following directions from Triode I plugged in my new 5AR4 (only the 5AR4) and when none of the magic smoke was released...... I proceeded to check voltages on the driver tube sockets (I don't know if anyone knows much about this board here or not but I will present my findings so you all will understand my concern)

    2 EF86 Sockets:
    pin 1 Screen, should be 55-80V, is 373V
    pin 3 Cathode, should be 1.2 - 1.6V, is .96
    pin 6 Plate, should be 80 - 110V, is 418V

    1 12AU7 Socket
    pin 1 Plate 1, should be 235 - 265V, is 478V
    pin 2 Grid 1, should be 80 - 110V, is 416V
    pin 3 Cathode 1, should be 90 - 120V, is 0
    the second half of the tube reads the same numbers

    Hmmmm I say, those look a tad high! I got my handy dandy color chart out and checked placement and value of each component on the board, All is as it should be, I also re-confirmed all hook-ups on the board, all is as it should be. (I will write and ask Triode's Tech support after I finish here).

    So, I fired it up again and checked all the Dynaco test points (heres where I think you all can help) I don't know if these numbers are supposed to be checked with or with out tubes in but here is what I find with only the 5AR4 in:

    EL34 Sockets (consistant on all 4 sockets):
    pin 2 6.3
    pin 3 500V
    pin 4 500V
    pin 5 -47Vac
    pin 6 -47Vac
    pin 7 6.3

    pin 2 504V
    pin 4 367Vac
    pin 6 365Vac
    pin 8 501

    Quad Cap (as per Dynakit parts , this cap is installed 1/4 turn counter clock wise from the original with the 80 lug facing the back of the amp but I used the original Dynaco lug/location numbers for this check, not the symbols) :
    lug 3 422V
    lug 4 476V
    lug 1 492V
    lug 2 494V

    Selenium Rectifier (now a diode)
    + 52Vac
    - -70Vdc

    Circuit board eyelets 3 and 18

    Now with no load on it and the old PTX on modern line voltage hmmm I can see 20% hotter numbers, but the driver board... WoW!!

    any thoughts, suggestions? (other than "Yo! don't put any tubes in that thing!!" lol) but seriously any help would be greatly appreciated!

    Thanks one and all!
    The New guy

    Bob Latino

    Posts : 2648
    Join date : 2008-11-26
    Location : Massachusetts

    Re: ST-70 Rebuild

    Post by Bob Latino on Fri Jul 01, 2011 6:42 am

    Hi Will,

    You are going to have to deal with Triode on the voltage specs for their board. I have never used their circuit and don't know the voltage readings. If all your voltages are high then my question is > Do you have all the tubes plugged in ? If you have just the rectifier tube plugged in and no other tubes then your voltages WILL be high because the other tubes are not in and there is no "load" on the high voltage system. My advice is ...

    1. Check your line voltage and see if it is higher than 120 volts.
    2. Plug in ALL the tubes and redo the voltage check.


    Posts : 3
    Join date : 2011-06-15
    Age : 53
    Location : Los Angeles

    Re: ST-70 Rebuild

    Post by ss900f on Fri Jul 01, 2011 1:43 pm

    Hi Bob,
    Thank you! I needed that confirmation, I suspected the voltages under the hood were not dramatically out of line (about +20% with no load/Tubes) I am still waiting to hear back from Triode before I plug anything into the driver board tho... those voltages are about +200 to 300% too high! Yicks!! and they are supposed to be checked with no tubes installed, I am half surprised components on the board didn't start going off like popcorn lol
    I am headed out of town after work this weekend so I won't be able to do anything with it until next week *sigh*
    Thanks again

    Posts : 3
    Join date : 2011-06-15
    Age : 53
    Location : Los Angeles

    Re: ST-70 Rebuild

    Post by ss900f on Tue Jul 05, 2011 11:01 pm

    I heard back from Chad at Triode Electronics Monday, He says that these voltages are within spec with no Tubes/Load on the system, the instructions are in error and are being rewritten. I am listening to my my freshly rebuilt Dynakit ST-70 as I type and MmmMmm I do love these Amps! thanks again for your help Bob. Next project... I just won an AR ST-70-C3 NIB kit on FleaBay! this should be fun!

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    Re: ST-70 Rebuild

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