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    new octal vta

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    kost

    Posts : 33
    Join date : 2009-05-12
    Age : 53
    Location : athens

    new octal vta

    Post by kost on Thu Jan 30, 2014 7:39 am

    Hi Bob

    I just changed my vta 12at7 board with the new octal version and i am very happy with the sound!
    Thank you Roy!
    I am very sceptical in changing the 0.33 audin coupling caps with better ones.
    In my 12at7 pcb i had the 0,22 pio k40 and i think they had fuller sound!
    If i go with even better ones(mundorf silver oil) what is the better capacitance to choose?
    0.33 or 0,22?
    What is the difference???
    The MUNDORFS are expensive ,i want to be sure about the capacitance choice. 

    regards

    kost

    arledgsc

    Posts : 340
    Join date : 2012-11-30
    Location : SF Bay CA

    Re: new octal vta

    Post by arledgsc on Thu Jan 30, 2014 12:10 pm

    Whatever the optimal values are recommended only using enough capacitance for desired low frequency response and nothing more.  Larger capacitance causes longer time constants for DC offsets.   These DC offsets throw off bias until they settle.  In the extreme DC excursions cause tube cutoff and nasty distortion.  In guitar amp world the phenomena is know as "blocking distortion".  

    The schematic values for the low-gain nine pin sockets are 0.1uF and 0.15uF for the preamp and PI stages respectively.  I did the calculations and the values are optimal for that circuit meaning any less capacitance and early low frequency rolloff.    I'm sure there is an acceptable range though.  Plus bigger caps cost more and are physically larger!   Bob and Roy have the answers so go by their recommendations.

    Congrats on your new board.  I'm using an 6SN7 type tube (with 9pin-to-8pin socket adapter) in the preamp position and really like it!

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