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    Bias setting on ST 70

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    jtee1340

    Posts : 3
    Join date : 2014-12-13

    Bias setting on ST 70

    Post by jtee1340 on Sat Dec 13, 2014 2:09 pm

    I am putting some JJ 6L6 tubes in a GCI modded ST 70. Any idea as to the correct Bias setting?
    I went to the website and all the Numbers are not really making since. scratch
    Thanks,
    J.T.

    Bob Latino
    Admin

    Posts : 2411
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Bias setting on ST 70

    Post by Bob Latino on Sun Dec 14, 2014 5:21 pm

    Hi JT,

    I do not know how the bias system is set up with the GCI board and what is the value of the bias resistor? You did mention in a personal Email to me that this amp has individual bias controls for each output tube. What you can do (with the amp OFF) is measure the resistance to chassis ground at each of the four bias measuring points. If you get 10 ohms, then you bias each tube at .400 volts DC like a VTA driver board. If you get something other than 10 ohms, then the bias setting will be different than .400 volts DC.

    Bob

    jtee1340

    Posts : 3
    Join date : 2014-12-13

    Re: Bias setting on ST 70

    Post by jtee1340 on Sun Dec 14, 2014 8:44 pm

    Thanks Bob,
    Well I checked and each bias tests (4) point is 10 ohms to ground.
    I checked the bias on each separate tube that is currently installed and it reads .843 or so volts DC.
    Is this because I am reading each tube individually, or are these tubes biased at .800 volts DC, double what you suggested? Shocked

    Guest
    Guest

    Re: Bias setting on ST 70

    Post by Guest on Sun Dec 14, 2014 10:25 pm

    jtee1340 wrote:Thanks Bob,
    Well I checked and each bias tests (4) point is 10 ohms to ground.
    I checked the bias on each separate tube that is currently installed  and it reads .843 or so volts DC.
    Is this because I am reading each tube individually, or are these tubes biased at .800 volts DC, double what you suggested? Shocked

    that is WAY TOO high!...it is indeed double to the recommended BIAS setting. For INDIVIDUAL tubes each with its own 10 OHMS BIAS resistor, it should be .400VDC
    So, get it down there pretty quickly, that high BIAS setting will really shorten the life of your power tubes!

    Bob Latino
    Admin

    Posts : 2411
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Bias setting on ST 70

    Post by Bob Latino on Mon Dec 15, 2014 11:41 am

    Just to be sure .. See if you can post on our Forum here a photo of the inside wiring. (See sticky above on how to post a photo on this forum) One of us should be able to figure this out for you .. As MontanaWay has mentioned, .800+ VDC with a 10 ohm bias resistor per each output tube is way too high for a SINGLE output tube. This would equate to 80+ milliamps of bias current for each output tube.

    Bob

    jtee1340

    Posts : 3
    Join date : 2014-12-13

    Re: Bias setting on ST 70

    Post by jtee1340 on Mon Dec 15, 2014 7:49 pm

    Thanks for the input guys. I will try to post a picture, but I think I have to be a member for a week. I thought that is what I read.
    I installed the 6L6 tubes tonight and tried to bias down to 40ma but the lowest I could go was 52 or so.
    Do I need to modify/swap a resistor value to allow me to use 6L6 tubes?
    It sounds great at this bias, but I don't want to burn up the tubes.
    Again, Thanks for the help.
    J.T.

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