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  • LTPI little help, please.

    Hello,
    I did a ccs to force in balance my 12au7 PI. I used what I have on hand...another 12au7. The circuit is not finally tuned but it seems it is nothing to tune...The PI works as well imbalanced with ccs as a fixed 22k resistor. Can You take a look to see what I did it wrong ,please? There are not power mosfets here lying around so I ask for a suggestion how to get the best ltpi balance with this 12au7 tube, Thanks.

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    "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

  • #2
    Have you missed decoupling off the 'other' grid?
    My band:- http://www.youtube.com/user/RedwingBand

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    • #3
      Originally posted by pdf64 View Post
      Have you missed decoupling off the 'other' grid?
      I did..sorry, fixed.

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      It was just a drawing issue, the cap is there together with 1M dc reference
      "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

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      • #4
        By my estimation, the plate resistance of your 12AU7 CCS is only about 15K to 20K. Merlin talks about using the same tube to get 36K, but I don't see it. http://valvewizard.co.uk/ccs.html

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        • #5
          Originally posted by Tony Bones View Post
          By my estimation, the plate resistance of your 12AU7 CCS is only about 15K to 20K. Merlin talks about using the same tube to get 36K, but I don't see it. http://valvewizard.co.uk/ccs.html
          Excellent point!

          I get a current source impedance of 15.5k (@Ip= 7ma), which means that the 12AU7 makes a rather poor CCS.
          - Own Opinions Only -

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          • #6
            Originally posted by Tony Bones View Post
            By my estimation, the plate resistance of your 12AU7 CCS is only about 15K to 20K. Merlin talks about using the same tube to get 36K, but I don't see it. http://valvewizard.co.uk/ccs.html
            Thanks
            I did not any estimations, but it realy measured like 15-20k. 22k fixed tail did not help to much, nor 27k either. Over that I cannot pass. Want to keep hk potential under 180v
            Last edited by catalin gramada; 08-11-2019, 05:13 PM.
            "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

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            • #7
              I don't think you can get more resistance at 7mA with 1/2 of a 12AU7. Maybe both halves in cascode...
              Last edited by Tony Bones; 08-11-2019, 05:46 PM.

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              • #8
                Instead of trying to increase non-inverting gain you may simply decrease inverting gain by tapping/splitting the plate resistor at the inverting plate for better signal symmetry.
                - Own Opinions Only -

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                • #9
                  Originally posted by Helmholtz View Post
                  Instead of trying to increase non-inverting gain you may simply decrease inverting gain by tapping/splitting the plate resistor at the inverting plate for better signal symmetry.
                  Thanks.
                  Sorry, but don't get it.... It is not the same result in effect, please?
                  "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

                  Comment


                  • #10
                    Originally posted by catalin gramada View Post
                    Hello,
                    I did a ccs to force in balance my 12au7 PI. I used what I have on hand...another 12au7. The circuit is not finally tuned but it seems it is nothing to tune...The PI works as well imbalanced with ccs as a fixed 22k resistor. Can You take a look to see what I did it wrong ,please? There are not power mosfets here lying around so I ask for a suggestion how to get the best ltpi balance with this 12au7 tube, Thanks.

                    [ATTACH=CONFIG]54668[/ATTACH]
                    It might be rubbing you your inner need for vacuum tubes the wrong way but you'll much better balance at a fraction of the cost by using a couple of 10 cent transistors. The ratio of signal current thru the cathodes to that through current the source is about 54dB (500:1) with this arrangement. Near perfect balance.

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                    Experience is something you get, just after you really needed it.

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                    • #11
                      Thank You Nick.
                      "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

                      Comment


                      • #12
                        Originally posted by catalin gramada View Post
                        Thanks.
                        Sorry, but don't get it.... It is not the same result in effect, please?
                        My proposal means matching the signal level of the left PI output to the lower level right output.

                        How much is your signal asymmetry?
                        - Own Opinions Only -

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                        • #13
                          Originally posted by Helmholtz View Post
                          My proposal means matching the signal level of the left PI output to the lower level right output.

                          How much is your signal asymmetry?
                          Thanks
                          Clearly understood. What wasn't is: what made a difference if I match inverted output in respect with non inverted one , or I match non inverted output in respect with inverted one (as already have a trim pot soldered there to adjust signal level) ?. Does it matter, cause for me seems the same thing
                          The difference between outputs is exactly 10 percent. And can be compensated replaced 47k non inverted plate resistor with 53 k one. The voltage gain of stage is 5,8 on 100k loads
                          No global feedback will be applied over the stage
                          "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

                          Comment


                          • #14
                            Originally posted by catalin gramada View Post
                            Thanks
                            Clearly understood. What wasn't is: what made a difference if I match inverted output in respect with non inverted one , or I match non inverted output in respect with inverted one (as already have a trim pot soldered there to adjust signal level) ?. Does it matter, cause for me seems the same thing
                            The difference between outputs is exactly 10 percent. And can be compensated replaced 47k non inverted plate resistor with 53 k one. The voltage gain of stage is 5,8 on 100k loads
                            No global feedback will be applied over the stage
                            But that means that you could achieve signal symmetry with your circuit. Now I don't understand your problem.

                            Also consider that the 2 triode systems may easily differ by 10% or more.


                            BTW, how did you measure current source impedance?
                            Last edited by Helmholtz; 08-11-2019, 08:26 PM.
                            - Own Opinions Only -

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                            • #15
                              What is surprising for me,with 12au7 clip relative hard . Why ? The outputs are quad of el34 triode strapped into 4 ohm load resistor.
                              Put some pics with output and PI output scooped together. No nfb was used.
                              Is something you don't want to use for guitar, I guess...

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                              Last edited by catalin gramada; 08-12-2019, 03:48 AM.
                              "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

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