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    New VTA70 mod - tubes don't bias independently

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    david24x7

    Posts : 11
    Join date : 2013-12-17
    Location : Oakland, CA

    New VTA70 mod - tubes don't bias independently

    Post by david24x7 on Wed Dec 18, 2013 2:08 pm

    Hi All,
    I just completed my VTA70 upgrade on my ST70 and it sounds great. As I am biasing my tubes, I noticed some unexpected behavior. Since there is a bias pot for each tube, I expect them to be adjusted independently but when I adjust either tube on the left, the bias for both tubes have the same value and go up and down in tandem as I adjust either of the bias pots. Is this expected behavior or did I forget to remove a jumper wire or something?

    Note that left and right channels bias independently but the tubes in either right or left seem to bias together. I am checking bias through the front octal sockets.

    What do you guys think?

    peterh

    Posts : 679
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: New VTA70 mod - tubes don't bias independently

    Post by peterh on Wed Dec 18, 2013 4:54 pm

    david24x7 wrote:Hi All,
    I just completed my VTA70 upgrade on my ST70 and it sounds great.  As I am biasing my tubes, I noticed some unexpected behavior.  Since there is a bias pot for each tube, I expect them to be adjusted independently but when I adjust either tube on the left, the bias for both tubes have the same value and go up and down in tandem as I adjust either of the bias pots.  Is this expected behavior or did I forget to remove a jumper wire or something?

    Note that left and right channels bias independently but the tubes in either right or left seem to bias together.  I am checking bias through the front octal sockets.

    What do you guys think?
    It is an effect of unregulated B+ and bias- sources ( provided that you followed the schematics, all
    of the "old bias" stuff is to be removed)
    When you add more bias to a tube ( make it more negative, this reduces current through that tube ) causes B+ to raise a bit and have the other tubes draw a bit more current.
    The bias process will be a stepwise refinement , course at the beginning.

    Make shure you choose the right pot ( front leftmost  tube uses front leftmost  bias pot  and so on)

    Bob Latino
    Admin

    Posts : 2413
    Join date : 2008-11-26
    Location : Massachusetts

    Re: New VTA70 mod - tubes don't bias independently

    Post by Bob Latino on Wed Dec 18, 2013 5:57 pm

    david24x7 wrote:Hi All,
    I just completed my VTA70 upgrade on my ST70 and it sounds great.  As I am biasing my tubes, I noticed some unexpected behavior.  Since there is a bias pot for each tube, I expect them to be adjusted independently but when I adjust either tube on the left, the bias for both tubes have the same value and go up and down in tandem as I adjust either of the bias pots.  Is this expected behavior or did I forget to remove a jumper wire or something?

    Note that left and right channels bias independently but the tubes in either right or left seem to bias together.  I am checking bias through the front octal sockets.

    What do you guys think?

    Did you remove the wire on the original Dynaco ST-70 that goes from pin 8 on one output tube to pin 8 of the other output tube? That wire must be removed so that the tube will bias independently. Another thing ... Since all four output tubes share the bias current, If you turn the bias UP on one tube, the other three should go DOWN very slightly ... This is normal ..

    Bob

    david24x7

    Posts : 11
    Join date : 2013-12-17
    Location : Oakland, CA

    Re: New VTA70 mod - tubes don't bias independently

    Post by david24x7 on Thu Dec 19, 2013 3:50 am

    Hi Bob,
    Just checked and you were right. I didn't remove the wire connecting pin 8 on the two output tubes. That makes perfect sense as to why there were reading the exact same bias. I'll remove the wires tomorrow and give everyone an update.

    Much appreciated!
    -David

    david24x7

    Posts : 11
    Join date : 2013-12-17
    Location : Oakland, CA

    Re: New VTA70 mod - tubes don't bias independently

    Post by david24x7 on Sat Dec 21, 2013 11:15 pm

    Quick update. Removed the wire that connected pin 8 on each output tube (right and left) and now they bias independently. Thanks Bob! I'm really enjoying listening to the ST70 with VTA70 upgrade. Thanks Roy!

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