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    I need help with fundamental tube bias information

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    mrconclusion

    Posts : 15
    Join date : 2012-10-18

    I need help with fundamental tube bias information

    Post by mrconclusion on Thu Oct 18, 2012 1:55 am

    Hello! This is my first post but I've been reading the forum for a while. I have one of Bob Latino's VTA ST70's and am about to build another. The problem is, I don't really understand the jump from calculating desired bias in amps, to measuring it in volts. I want to play with various tubes without always asking "where should I set the bias for this one?" so it would be nice to know what I'm doing!

    I understand measuring plate voltage, dividing it by 70% of the tube's rated dissipation, and getting the answer in amps... but how is this translated into volts measured at the test points? I've found info online (mostly re: guitar amps) that talks about transformer resistance or cathode resistor value... am I barking up the wrong tree?

    I bet this is a very stupid newbie question and I apologize if it's been covered before. If so, point me at a useful old thread and I'll be happy! (The existing threads I keep finding concern bias problems or bias for one particular tube.) Fortunately I have no amplifier problems - just a lack of knowledge. Thanks you in advance for your assistance!

    Bob Latino
    Admin

    Posts : 2415
    Join date : 2008-11-26
    Location : Massachusetts

    Re: I need help with fundamental tube bias information

    Post by Bob Latino on Thu Oct 18, 2012 11:17 am

    Volts = amps X resistance therefore amps = volts divided by resistance. In all the VTA amps we use a 10 ohm bias resistor therefore the amps that the tube will be using will always be 1/10 that of the measured voltage. If you measure the bias voltage across that 10 ohm resistor and you measure .400 volts or 400 millivolts then that tube is using 1/10 of that value in amps - so - the tube is using .040 amps or 40 milliamps.

    The reason you use VOLTS to measure bias is that it is easier to do with a meter. To measure amps with a meter the meter has to be in series with the circuit and the meter itself with it's own resistance becomes a part of the circuit. It is hard to get an accurate reading this way.

    Bob


    Last edited by Bob Latino on Thu Oct 18, 2012 2:49 pm; edited 1 time in total

    mrconclusion

    Posts : 15
    Join date : 2012-10-18

    Re: I need help with fundamental tube bias information

    Post by mrconclusion on Thu Oct 18, 2012 12:10 pm

    Thanks you very much Bob, that's an excellent explanation. This explains why I couldn't get the math to make sense when I was looking for a cathode resistor. I feel silly because the term 'bias resistor' should have clued me in. Using a 10 ohm bias resistor sure does make the math easy!

    Tom

    Posts : 166
    Join date : 2011-04-04

    Re: I need help with fundamental tube bias information

    Post by Tom on Fri Oct 19, 2012 8:17 am

    Dang, that is a great explaination! Thanks!
    This is one of the reasons I keep coming back here.
    Smile

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