Announcement

Collapse
No announcement yet.

Help in determing bias on Blue Jr. Mark 1

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • Help in determing bias on Blue Jr. Mark 1

    Hello,
    Have a greenboard 90's blues junior purchased new. Swapped speaker and fooled around with changing tubes. I have some knowledge of electrics, metering and soldering. Have done soldering on tube mounting board in the past as it solder joints decayed from heat. I never biased the amp and left it stock. I now want to check the plate voltage and current bias of my el84s. Must be doing something wrong. Measured output transformer connections on board to ground and got basically 334.5 -335 Vdc to ground on all taps. Measured the input coils at 97.2 and 97.5 Ohms on the transformer. My issue is when measuring voltage from each leg to the center tap I am only getting.634 and .671 Vdc. The amp is hot and is functioning. What dumb mistake or factor am I missing?
    Thanks for any assistance or advice.........

    Don't actually recall the date or the revision. Using Rev B print.

  • #2
    I have been servicing the BJ from day one and never needed to measure the anode current on the output valves. If the bias voltage is around -11 volts, all is good. You can, however, fit a 1R 1W resistor temporarilly in series with the output transformers centre tap, red wire and HT supply to measure the voltage drop across it. You can work out the quiescent current using Ohms Law quite easily.
    It suffices to say that if you fit a 33k resistor in position R31, check/replace the screen feed resistors R24/25 if you replace them use fp type MF or MO 1W 100R to avoid a fire if the EL84s fail, all will be fine.
    The EL84s will run slightly cooler and last a lot longer.
    Also, if you are Australia or UK, change the power supply primary tapping to 240volts. You won't be sorry!
    Support for Fender, Laney, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk
    If you can't fix it, I probably can.

    Comment


    • #3
      Please post schematic.
      - Own Opinions Only -

      Comment


      • #4
        Originally posted by Jon Snell View Post
        I have been servicing the BJ from day one and never needed to measure the anode current on the output valves. If the bias voltage is around -11 volts, all is good. You can, however, fit a 1R 1W resistor temporarilly in series with the output transformers centre tap, red wire and HT supply to measure the voltage drop across it. You can work out the quiescent current using Ohms Law quite easily.
        It suffices to say that if you fit a 33k resistor in position R31, check/replace the screen feed resistors R24/25 if you replace them use fp type MF or MO 1W 100R to avoid a fire if the EL84s fail, all will be fine.
        The EL84s will run slightly cooler and last a lot longer.
        Also, if you are Australia or UK, change the power supply primary tapping to 240volts. You won't be sorry!
        Hello Sir,
        Thank you for your reply. I was following a YT video and I didn't see them install a load resistor and just reading from the center and HT taps. Got lost in my thinking or lack of... I am initially thinking of adding a resistor of the appropriate value in parallel to R37 to lessen it's value. Can get to it without pulling the board. Are there any other fixes you've seen to enhance the longevity of the amp, if you don't mind sharing. I am happy with the sound. YT is not as reliable as it once was IMHO....Think when I pull the board I will take your advice on the screen resistors.

        Comment


        • #5
          Originally posted by Helmholtz View Post
          Please post schematic.
          A bit reticent on posting diagrams. Found it easily on Google.

          Comment


          • #6
            Not familiar with the amp.
            But if there's a way to insert 1R resistors in series with each power tube cathode, that would be a permanent solution allowing to monitor individual tube currents and balance.
            Might require cutting traces?
            - Own Opinions Only -

            Comment


            • #7
              Originally posted by ArturoQ View Post

              A bit reticent on posting diagrams. Found it easily on Google.
              Sorry, not my job to find schematics.
              - Own Opinions Only -

              Comment


              • #8
                Originally posted by ArturoQ View Post

                A bit reticent on posting diagrams. Found it easily on Google.
                Then please at least post a link to the one you are looking at. There is no reason to be hesitant about posting schematics that are already available online. It is considered a courtesy to aid the discussion.
                For schematics that are not available online, and you have signed an NDA to not share them, then it is understandable.

                I am attaching the Rev.B schematic that I hope is the one you mentioned.
                Attached Files
                Originally posted by Enzo
                I have a sign in my shop that says, "Never think up reasons not to check something."


                Comment


                • #9
                  Sorry for the inconvenience, I was unsure so here is a link. Thanks for the info about posting the online information. That is the version of the print I was using, thank you kindly for posting the print!! Not an expert at this.

                  https://el34world.com/charts/Schemat...ice_manual.pdf

                  Here is a the address. Looking at the REV B first schematic, it seems to match but nothing on the amp that I've found, indicated the correct version. It is a 90's greenboard.
                  Last edited by ArturoQ; 10-09-2023, 04:07 AM.

                  Comment


                  • #10
                    Can someone explain why I am getting such low voltage from HT to Center taps when the amp is on. CP3 to CP2 as well as CP1 to CP2. .634 and .671 Vdc

                    Comment


                    • #11
                      Originally posted by Jazz P Bass
                      Not sure what 'center tap' you are refereing to.
                      In the first post it seems that you measured the B+ at 334 Vdc with respect to ground.
                      Which should be the correct way.
                      Sorry, I was referring to the output transformer, primary windings. Don't know what I am doing wrong. Measured the CP1,2 & 3 connectors on the board for voltage to ground which seemed correct, but measuring from the transformer B+ CP2 to CP1 or to CP3 , I get exceptionally low voltage. Inferred in the above 1 volt Vdc range.
                      Thanx for any insight...
                      Last edited by ArturoQ; 10-09-2023, 04:59 AM. Reason: Wasn't clear in description and ter

                      Comment


                      • #12
                        Not exactly sure what you mean by 'transformers HT line' which I think is the same as center tap.
                        You should be measuring DC volts from CP2 to CP1, then CP2 to CP3. Just to be sure there is nothing funny going on, remove the PI tube V3 while doing those measurements.
                        Originally posted by Enzo
                        I have a sign in my shop that says, "Never think up reasons not to check something."


                        Comment


                        • #13
                          Originally posted by g1 View Post
                          Not exactly sure what you mean by 'transformers HT line' which I think is the same as center tap.
                          You should be measuring DC volts from CP2 to CP1, then CP2 to CP3. Just to be sure there is nothing funny going on, remove the PI tube V3 while doing those measurements.
                          Yes, thanks and I noticed my improper terminology and made corrections. Measured as CP2 to CP1 @ .634 Vdc and CP2 to CP3 @ .671 Vdc. Hope I am finally making myself understandable! I will remove the V3 and try again. Except for the version of the amp, I am doing the readings at the correct locations on the greenboard.
                          Will report back tomorrow. Thanx

                          Comment


                          • #14
                            If you still get same results, in case there is any discrepancy from the schematic to the CP designations on your actual unit, measure from B+ to V4 pin7, then B+ to V5 pin7.
                            Originally posted by Enzo
                            I have a sign in my shop that says, "Never think up reasons not to check something."


                            Comment


                            • #15
                              1R cathode resistors would make this easy peasy, removing opportunity for measurement errors / circuit disturbances etc to mess things up.
                              My band:- http://www.youtube.com/user/RedwingBand

                              Comment

                              Working...
                              X