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    st-70 question about bias problem

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    keaster

    Posts : 2
    Join date : 2012-04-16

    st-70 question about bias problem

    Post by keaster on Mon Apr 16, 2012 3:46 pm

    Hi all, New to the forum with some questions about a amp I'm working on. Its a push pull EL-34 based on a st-70 that I built from a different driver board I found

    kta-hifi.net/projects/amp_page/st70amp/st70.html

    and Stock power supply but I used a different power transformer, a Edcor XPWR002-120 it has the same high voltage @ 720 but 5 more volts on the bias

    So between the stock and modified circuit I used it has some minor differences the resistor were the negative bias voltage inputs is about 100k more and the cathode resistor to ground is one 15R where the other has two 15R's

    so the problem I'm having is that I can't get the bias voltage to go much higher then 400mV and not the 1.56 that it should be.

    I read on here that that changing the to 10k's on the bias power supply to 5k might help and it did but not enough should it be lower or do I need to look somewhere else for a problem?

    I hope I've given enough info please let me know if more is needed

    thanks

    danf

    Posts : 58
    Join date : 2009-01-19

    Bias fix

    Post by danf on Mon Apr 16, 2012 10:20 pm

    If I understand your message, you have a 15 ohm cathode resistor on each EL34. For a tube current of 50 ma, you would want to read 0.75 v across each resistor. I might try for a bit less, or about 0.68 V. The bias voltage from your bias supply is a bit too high. The bias supplies are set up as voltage dividers. To get a lower bias voltage increase the resistor between the bias cap and the pot or decrease the resistor between the pot and ground, or both. Read up on voltage dividers and you can calculate the correction from where you are now.

    keaster

    Posts : 2
    Join date : 2012-04-16

    Re: st-70 question about bias problem

    Post by keaster on Tue Apr 17, 2012 9:09 pm

    Thanks for the reply, Im still new to all this so what your saying is if the negative voltage in is lower the bias voltage will be higher??
    Also what about the 270k resister before the grid, would lowering that to 120K change anything? (because the driver I used was setup for 4 bias controls, one for each tube but the person who's it is said I could set it up like the stock driver)

    danf

    Posts : 58
    Join date : 2009-01-19

    Grid voltage more negative less tube current

    Post by danf on Wed Apr 18, 2012 6:09 am

    If the bias voltage applied to the grid is more negative, the tube current decreases and the voltage across the cathode resistor decreases. The reason for this is that the grid controls the flow of electrons from the cathode to the anode. A more positive grid increases the flow of negatively charged electrons, and a more negative grid decreases the flow of electrons. In this case, let's say that you are now supplying -38 volts of bias to the grid. In order to increase the tube current, you need to supply -36 volts of bias voltage. Since the bias supply is always negative, my previous email stated how to decrease the amplitude of the bias voltage, making it less negative. Some people incorrectly refer to the voltage measured across the cathode resistor as the bias voltage, so this can be confusing for newcomers.

    (I think that this agrees with your summary, except you are calling the voltage measured across the cathode resistor the bias voltage.)

    The KTA circuit calls for 120K grid resistors. I would use this value unless you have a good reason to change it. This resistor doesn't affect the bias voltage, but does affect the load seen by the previous stage. Your bias voltage is most likely too negative because your transformer has a higher voltage bias winding.

    Do you have just one 15 ohm cathode resistor for each pair of output tubes? If so, 90 ma for both tubes should drop about 1.35 volts across this resistor. I think that I now understand that you are trying to use a completely stock bias set-up with the KTA board.

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