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Two styles of bias, two very different results. Opinions, please.

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  • #1

    Two styles of bias, two very different results. Opinions, please.

    Hello Folks.

    I biased up a 1959 earlier today and read very different results with two different methods of reading bias voltages. So I am trying to determine which I should be following.

    Over the last few years, I have been checking bias using the "Plate/cathode current method" as described by Randall Aiken at his webpage here or Rob Robinette's site here

    Holding the opinions of both Mr Robinette and Mr Aiken in high regard, this is the method I have been using.

    Of course everyone knows the adding a 1Ω, 1% resistor to ground from pin 8/1 to ground. This is the second method in reference to the topic.

    Today, I just quickly set bias by the resistor to ground method and later wanted to check against that by doing the other method to cross check. I had a feeling the amp was running too hot, tubes seem to be hotter than usual.

    Going the 1Ω resistor to ground method... my B+ at the tubes is 505VDC, according to the usual formula, my bias should be 34mA. And I set it as such... ranging from 30 to 36 mV on my meter.

    Went back later and did the other method...

    Output Transformer CT to Pin 3 on v4 and v5 = 19.2Ω
    Output Transformer CT to Pin 3 on v6 and v7 = 17.7Ω

    Plate Voltage: 505VDC

    Voltage Drop on CT to v4 and v5 = -.98VDC
    Voltage Drop on CT to v6 and v7 = -.93VDC

    plate current = volts/resistance = .051 and .052 A !!! Which would mean I am dissipating a full 25 watts per tube!52mA!??

    So, what gives? Any recommendations and/or opinions?

    Thank you!

    I'm sure the topic has been beat to death, please indulge me in the opinions!
    Last edited by big_teee; 08-02-2019, 03:08 PM. Reason: add the t in styles
    "'He who first proclaims to have golden ears is the only one in the argument who can truly have golden ears.' The opponent, therefore, must, by the rules, have tin ears, since there can only be one golden-eared person per argument." - Randall Aiken
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  • #2
    I hope I understood well, but I suppose that these measures (51 and 52mA) are for each pair, so you should divide it by two.
    26mA across the plate matches approximately with ≈32mA in the cathode, since this last reflects the sum of the plate and screen grid intensities.
    This, in the case that each pair is composed of identical tubes. They may not be symmetric and you simply read the sum of the two.

    Comment

    • #3
      Originally posted by Pedro Vecino View Post
      I hope I understood well, but I suppose that these measures (51 and 52mA) are for each pair, so you should divide it by two.
      26mA across the plate matches approximately with ≈32mA in the cathode, since this last reflects the sum of the plate and screen grid intensities.
      This, in the case that each pair is composed of identical tubes. They may not be symmetric and you simply read the sum of the two.
      Naaa... unfortunately that is at each tube. I should have been more clear there.

      I lowered it to get .041A using that method, but I can not go any lower... rather any more counter-clockwise on the bias pot. Pin 5 is reading -50Volts which I would think is cold..?? With the bias adjusted, the plates are now 512VDC.
      "'He who first proclaims to have golden ears is the only one in the argument who can truly have golden ears.' The opponent, therefore, must, by the rules, have tin ears, since there can only be one golden-eared person per argument." - Randall Aiken

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      • #4
        just leave off two tubes and measure voltage again,what do you see?

        Comment

        • #5
          Output Transformer CT to Pin 3 on v4 and v5 = 19.2Ω
          Output Transformer CT to Pin 3 on v6 and v7 = 17.7Ω

          Plate Voltage: 505VDC

          Voltage Drop on CT to v4 and v5 = -.98VDC
          Voltage Drop on CT to v6 and v7 = -.93VDC
          As described here you measured the voltage drops produced by 2 tubes per side. (Or did you remove 2 tubes for this test?)

          The explanation by Pedro is logical and correct.
          - Own Opinions Only -

          Comment

          • #6
            Holy &%#$.

            I am extremely sad to report that I did not know/misunderstood this.

            I spend hours and hours researching and reading and find I didn't understand this correctly! Sad.

            So do I pull the outer two tubes? And do i need to make an impedance adjustment?

            Thank you so very much!
            "'He who first proclaims to have golden ears is the only one in the argument who can truly have golden ears.' The opponent, therefore, must, by the rules, have tin ears, since there can only be one golden-eared person per argument." - Randall Aiken

            Comment

            • #7
              You can't measure the bias current of a single tube without removing the other one because they are in parallel (their plates are connected together).

              There is no need to make an impedance adjustment. Bias current is set at zero signal. Turn all volume pots to zero.

              Comment

              • #8
                You don´t need to pull tubes, adjust impedance, nothing of that sort.

                Just-divide-52mA-by-two. Period.

                Measuring across transformer half primaries gives you the current of all tubes connectd to it, what else?
                Then divide by amount of tubes.
                Juan Manuel Fahey

                Comment

                • #9
                  Agree with the others here, just divide the results by 2 and set bias to desired current.
                  If you want to check the matching of the tubes, I would just do one tube at a time on one socket. For testing purposes having only one power tube installed won't cause any problems.
                  Setting bias with less than all 4 tubes installed will not be accurate because the B+ will fluctuate depending on how many tubes are loading it.
                  Originally posted by Enzo
                  I have a sign in my shop that says, "Never think up reasons not to check something."


                  ​

                  Comment

                  • #10
                    And there it is on Randall's site:
                    Again, if there is more than one tube on each side, you must divide the total current by the number of tubes.
                    I apologize that I missed that and thank you for pointing this out. I have luckily only previously tried this on 50 watt amps, so that (sort of) explains it to myself.

                    Helps to read things over after time has passed so nothing so important is missed!!!!

                    Thank you all!!
                    "'He who first proclaims to have golden ears is the only one in the argument who can truly have golden ears.' The opponent, therefore, must, by the rules, have tin ears, since there can only be one golden-eared person per argument." - Randall Aiken

                    Comment

                    • #11
                      What I want to know is how did you get the omega symbol for ohms to show? THATS what I want to know. Show us that, and all else is forgiven!
                      It's weird, because it WAS working fine.....

                      Comment

                      • #12
                        awwww, man! You got it...

                        hold alt key and type "234" - alt+234 - release the alt key and the Ω should appear - this is on Windows 10.

                        on Linux I always had to use on-screen keyboard and on Mac I think it's alt+z
                        "'He who first proclaims to have golden ears is the only one in the argument who can truly have golden ears.' The opponent, therefore, must, by the rules, have tin ears, since there can only be one golden-eared person per argument." - Randall Aiken

                        Comment

                        • #13
                          Creating the Omega symbol

                          Randall....

                          When using MS Word, I use the font called "Symbol". I start in Ariel, I type the letter "W" as a cap, I select that "W" and change the font to Symbol. I then get the Omega character. Unfortunately, you cannot copy and paste from Word into this box and get the symbol to show up.

                          Go to this site for reference - https://www.keynotesupport.com/websi...-symbols.shtml

                          With the number lock on, hold the ALT key and enter 234 (using the number keys). This should give you the Omega symbol. Ω. You can then increase the character size with the text tools in this box.
                          It's not just an amp, it's an adventure!

                          Comment

                          • #14
                            Originally posted by TomCarlos View Post
                            Randall....

                            When using MS Word, I use the font called "Symbol". I start in Ariel, I type the letter "W" as a cap, I select that "W" and change the font to Symbol. I then get the Omega character. Unfortunately, you cannot copy and paste from Word into this box and get the symbol to show up.

                            Go to this site for reference - https://www.keynotesupport.com/websi...-symbols.shtml

                            With the number lock on, hold the ALT key and enter 234 (using the number keys). This should give you the Omega symbol. Ω. You can then increase the character size with the text tools in this box.
                            Yes, must use the number pad... otherwise it will not work.
                            "'He who first proclaims to have golden ears is the only one in the argument who can truly have golden ears.' The opponent, therefore, must, by the rules, have tin ears, since there can only be one golden-eared person per argument." - Randall Aiken

                            Comment

                            • #15
                              Ω Yay!!
                              It's weird, because it WAS working fine.....

                              Comment

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