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    ST-70....Help!! Please.

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    cc43fan

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    ST-70....Help!! Please.

    Post by cc43fan on Sun Mar 18, 2012 7:40 pm

    I just finshed the last wiring of my ST-70. I followed the instructions that covered first start up. I put the driver tubes in (I used 12AU7's because I didn't have any 12AT7's), and the tubes did not light up. I checked the AC voltage across the incoming heater wires and got 6.7 VAC. The B+ voltage to the board is 480VDC, "-50 VAC" voltage was -53.5, and pins 1 & 6 across all 3 tubes are 480VDC. I've attached pics so those kind enought to help can see my work. I used only the parts supplied with the board, and used the instructions found here to connect the VTA to the sds pwr supply board as well.

    I'm stumped. Any and all thoughts, suggestions, etc. welcomed. I thought I would be listening to sweet, sweet music this Sunday evening, but no joy in Mudsville tonight. I'm certain this will get solved, but I need the wisdom of the great minds here.
    Thanks for any and all help!
    Chuck




    Bob Latino
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    Re: ST-70....Help!! Please.

    Post by Bob Latino on Sun Mar 18, 2012 8:30 pm

    You have the two pairs of filament wires only connected to ONE of the "FIL" eyelets on the VTA driver board on each pair of filament wires. Unsolder each of the wires on each pair that is not connected to a "FIL" eyelet and connect that wire to the open "FIL" eyelet on both channels.

    Bob

    cc43fan

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    Re: ST-70....Help!! Please.

    Post by cc43fan on Sun Mar 18, 2012 9:24 pm

    Bob Latino wrote:You have the two pairs of filament wires only connected to ONE of the "FIL" eyelets on the VTA driver board on each pair of filament wires. Unsolder each of the wires on each pair that is not connected to a "FIL" eyelet and connect that wire to the open "FIL" eyelet on both channels.

    Bob

    Hi Bob,
    You made my day!! That was the exact problem. It's playing sweet music!!
    On a side note, I used the "VTA-Rebuild.pdf" instructions, and according to step 2 from "Connections to the VTA Driver board" section it explains "Connect the two twisted pair of wires coming from V3 on the LEFT side of the amp to the two open UNMARKED eyelets below the left tube socket on the bottom of the driver board. These UNMARKED eyelets are set at a about 45 degree angle to the sides of the board. Shorten the wires to proper length (s)."
    I was wondering, has the board been updated since this .pdf instuction file was created?

    I just want to say how much I appreciate you helping me solve the problem. This ST-70 is a great project, I'm thinking I want to build another one!!
    Regards,
    Chuck

    Bob Latino
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    Re: ST-70....Help!! Please.

    Post by Bob Latino on Sun Mar 18, 2012 10:10 pm

    Hi Chuck,

    Glad to hear that you are up and running ..

    Where did you get that pdf file ? We will have to update that file ..

    Yes - those two unmarked eyelets are now marked "FIL" to avoid any confusion about which eyelets you connect those two pairs of filament wires to ..

    Bob

    cc43fan

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    Re: ST-70....Help!! Please.

    Post by cc43fan on Mon Mar 19, 2012 6:54 am

    Bob Latino wrote:Hi Chuck,

    Glad to hear that you are up and running ..

    Where did you get that pdf file ? We will have to update that file ..

    Yes - those two unmarked eyelets are now marked "FIL" to avoid any confusion about which eyelets you connect those two pairs of filament wires to ..

    Bob

    Hi Bob,
    Below is a link to the .pdf file I mentioned.

    Thanks again,
    Chuck
    http://www.tubes4hifi.com/VTA-Rebuild.PDF

    tubes4hifi
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    Re: ST-70....Help!! Please.

    Post by tubes4hifi on Mon Mar 19, 2012 2:09 pm

    if you have 480vdc going to that board I'm surprised it hasn't blown up yet, it should have 380-400v to the board.
    On the quad cap you should have a 2.2K 2-3 watt resistor to drop that voltage down,
    www.tubes4hifi.com/VTA70wiringdiagram3A.gif


    Last edited by tubes4hifi on Mon Mar 19, 2012 3:13 pm; edited 1 time in total

    cc43fan

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    Re: ST-70....Help!! Please.

    Post by cc43fan on Mon Mar 19, 2012 2:57 pm

    I'm using the SDS Power supply board. Would I still use the resistor you mentioned?

    tubes4hifi
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    Re: ST-70....Help!! Please.

    Post by tubes4hifi on Mon Mar 19, 2012 3:12 pm

    yes, you need to change R9 on the SDS/Triode board to 2.2K 2-3 watt and make the B+ connection to the driver board from there
    see diagram below:

    cc43fan

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    Re: ST-70....Help!! Please.

    Post by cc43fan on Mon Mar 19, 2012 4:44 pm

    After I read your post it freaked me out....so to the workbench I go, I turned on amp (all tubes in place, speakers connected, input connected to preamp but not on), and my B+ voltage was approx 230VDC . I read the thread here and explained how to "marry" the SDS and VTA, and one of the pointers was to jumper either R9 or R10 if needed to get voltage to acceptable range. When I jumper R10 B+ is 370VDC, when I jumper R9 it's approx 300VDC, and when I jumper both it's approx 440VDC.

    I'm not in love with the idea of putting a solding iron to that board again, and I noticed your SDS and VTA notes used different connections than the post I read here. Do you think it's safe and acceptable to go with what I've done to this point and just jumper both R9 and R10? Below is the post I keep referring to.
    btw, I'm using the SDS bias supply.

    Thanks so much for the great help,
    Chuck

    http://dynacotubeaudio.forumotion.com/t36-using-the-sds-cap-board-with-the-vta-driver


    Last edited by cc43fan on Mon Mar 19, 2012 4:51 pm; edited 1 time in total (Reason for editing : Added info)

    Bob Latino
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    Re: ST-70....Help!! Please.

    Post by Bob Latino on Mon Mar 19, 2012 7:43 pm

    Hi Chuck,

    To use the SDS cap board with the VTA driver board, R9 on the SDS board should be (as Roy mentioned) a 2200 or 2.2K (2 or 3 watt) resistor. You then place a jumper in R10 if you have the VTA driver board.

    If you have a STOCK ST-70 driver board, then R9 on the SDS cap board should be a 22,000 ohm (22K) resistor and R10 should be a 6800 (6.8K) resistor. Both of these resistors should be 2 or 3 watt.

    Bob


    Last edited by Bob Latino on Mon Mar 19, 2012 8:26 pm; edited 1 time in total

    cc43fan

    Posts : 29
    Join date : 2012-01-30
    Age : 53
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    Re: ST-70....Help!! Please.

    Post by cc43fan on Mon Mar 19, 2012 8:16 pm

    Thanks Bob,
    I've ordered the 2.2k 3w. Sounds like that will get me closer to 400VDC.

    Thanks again,
    Chuck

    cc43fan

    Posts : 29
    Join date : 2012-01-30
    Age : 53
    Location : Upstate New York

    Re: ST-70....Help!! Please.

    Post by cc43fan on Mon Mar 19, 2012 8:39 pm

    Hi Bob,
    The VTA kit came with a 2.2K, 3w for the can cap if I'm not mistaken?

    Chuck

    Bob Latino
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    Re: ST-70....Help!! Please.

    Post by Bob Latino on Mon Mar 19, 2012 8:42 pm

    Yes - You use that 2.2K 3W on the SDS board now as R9 ...

    cc43fan

    Posts : 29
    Join date : 2012-01-30
    Age : 53
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    Re: ST-70....Help!! Please.

    Post by cc43fan on Mon Mar 19, 2012 8:47 pm

    I did, and now voltage is 397.5 VDC. You guys are awesome, and I'm glad I remembered the 2.2k. It sucks to order a .90 resistor and pay $6.00 in postage.

    Thanks very much to you and Roy for your help.

    Chuck

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