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    Octal driver board when used for VTA ST-120

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    StevieRay


    Posts : 67
    Join date : 2009-01-09
    Age : 61
    Location : Central VA

    Octal driver board when used for VTA ST-120 Empty Octal driver board when used for VTA ST-120

    Post by StevieRay Sun Jun 25, 2023 3:01 pm

    Many months ago I purchased a octal driver board to swap for my stock noval board in my ST-120 amp (purchased ~2018 I think).

    I first tried a 6SN7 with a tube adaptor in the center tube, liked the results, and ordered the octal driver board.  Now I finally have time to install it!!!

    Anyway, I see that there are some different values in the resistors supplied with the octal board.  Of course the instructions say for ST-70 all over them.
    Is this due to the different B+ voltage in the ST-70 vs. ST-120, or due to the 6SN7 tubes, or what?

    I know I need to move the Russian PIO caps from old to new.

    Here's what I found that is notable in the resistor differences:
    Resistor:    Noval driver board value:      NEW Octal driver board value:                              Notes:
    R3 & R4                         270k                                           220k                                     Just sets input impedance? negligible
    R7 & R8                         7.5k                                            6.8k                                      Part of the negative feedback filter?
    R15 & R16                      4.7k                                           6.8k
    R19,20,21,22                  30k                                            27k
    R27 & R28                      10                                              8.2                                       part of the LM334 CCS circuit (!)
    R39                                jumper                                       1k                                          jumper I think in ST-120 for more bias; jumper again here

    Should I move any of the resistors from old to new?

    Thanks in advance for any insight or advice anyone can offer.  I suppose I could email Roy, but I'd be interested in any opinions from the group.
    Roy Mottram
    Roy Mottram
    Admin


    Posts : 1839
    Join date : 2008-11-30

    Octal driver board when used for VTA ST-120 Empty Re: Octal driver board when used for VTA ST-120

    Post by Roy Mottram Tue Jun 27, 2023 6:51 pm

    all of the values you show for the octal board are correct, most are minor changes
    Roy

      Current date/time is Sat Apr 27, 2024 3:37 pm