Announcement

Collapse
No announcement yet.

Help please! 1973 Marshall Super Lead - EL34 to 6550s bias question

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

  • #16
    Originally posted by Enzo View Post
    You alter the amp to accommodate your tubes, or you alter your tubes to accommodate your amp. Those are your choices. Altering the amp requires soldering parts.
    I've soldered parts. I've soldered the proper resistors all over this thing. I'd just rather not change that pot. That requires ordering parts and waiting. And frankly, these tubes are so far off I don't even know what pot I'd need...and it might not even work!

    I'm gonna try a different measurement method. I'll be installing the 1 ohm cathode resistors this evening.

    Comment


    • #17
      Originally posted by gtrplayr1976 View Post
      I've had to increase pots ,or resistors more than once to get the bias where I need it.
      Which resistor would I need to change? The 47k? Would I need to decrease it? Can I jumper in a parallel resistor just to see what happens?

      Comment


      • #18
        Originally posted by Greg_L View Post
        I'm using the figures off of an old Unicord schematic with figures for USA 6550s.

        15K before the diode
        15K with the 10uf bias caps
        47k in series with the bias pot
        150k grid leaks
        All checked and verified with an ohmmeter.
        Were these parts all there when it was running EL34's? If not, which did you change?
        "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

        Comment


        • #19
          About biasing tube guitar amplifiers

          https://music-electronics-forum.com/showthread.php?t=8291

          Bias adjusts depending on the + HV and working classes of service (A, AB, B) to the specified anode current value.

          http://web.archive.org/web/20120623045707/http://www.diyguitarist.com/GuitarAmps/PowerTubeBias.htm
          Power Tube Bias Charts

          http://web.archive.org/web/20110621211343/http://diyguitarist.com/Images/BiasChart-EL34.jpg
          EL34 Bias Chart

          http://web.archive.org/web/20110621211356/http://diyguitarist.com/Images/BiasChart-KT88.jpg
          KT88/6550 Bias Chart
          Last edited by vintagekiki; 10-14-2019, 11:24 PM. Reason: Upload Bias Charts
          Who does not know and knows that he does not know - teach him Confucius)
          Who knows and does not know that he knows - wake him Confucius)

          Comment


          • #20
            Originally posted by g1 View Post
            Were these parts all there when it was running EL34's? If not, which did you change?
            The grid leaks were 220k for EL34s. The bias power resistor was 27k. I piggyback paralleled resistors to get them down to 6550 specs. All readings have been verified.

            When I got the amp the conversion was halfass complete. I put it to proper EL34 bias specs and it worked perfectly. Now I'm putting it back to it's original 6550 specs and it isn't.

            Comment


            • #21
              Update -

              I soldered in a 1 ohm precision resistor from pins 1&8 to ground on one power tube socket. Took a reading across the resistor. I get the same reading as with the bias probe, so the probe is telling the truth.

              Just for kicks, I jumpered a 150k across the 47k that lives in series with the bias pot. That dropped the 47k to around 35k. My bias reading went up at the tube. Not much, but it did go up more in the right direction. My light bulb limiter got brighter though. It went from a very dull dim glow to noticeably brighter, so something is drawing more amps. Maybe these tubes are just bad.

              Comment


              • #22
                Yes, the hotter you bias the tube, the more current it draws. That current comes ultimately from the wall outlet mains supply, hence the bulb brightens.
                Education is what you're left with after you have forgotten what you have learned.

                Comment


                • #23
                  If you bias hotter, you are going to draw more mains current. The additional current comes from somewhere.

                  Edit: Enzo beat me!
                  "Yeah, well, you know, that's just, like, your opinion, man."

                  Comment


                  • #24
                    Originally posted by Enzo View Post
                    Yes, the hotter you bias the tube, the more current it draws. That current comes ultimately from the wall outlet mains supply, hence the bulb brightens.
                    Originally posted by The Dude View Post
                    If you bias hotter, you are going to draw more mains current. The additional current comes from somewhere.

                    Edit: Enzo beat me!
                    Ha, yeah, it just seemed to get brighter than I thought it should. Maybe? I don't know. It's a 300w bulb.

                    I had a rectifier wired backwards one time. That thing got really bright!

                    I'm gonna put it back to EL34 configuration and get readings with the EL34s.

                    Comment


                    • #25
                      I may be old, but I can still be quick.
                      Education is what you're left with after you have forgotten what you have learned.

                      Comment


                      • #26
                        I'm not sure the Mrs. would appreciate that statement.
                        "Yeah, well, you know, that's just, like, your opinion, man."

                        Comment


                        • #27
                          Taking bias readings with the light bulb or not?

                          Comment


                          • #28
                            Originally posted by mozz View Post
                            Taking bias readings with the light bulb or not?
                            Both

                            Comment


                            • #29
                              Originally posted by mozz View Post
                              Taking bias readings with the light bulb or not?
                              Good point! Bias should be set without the limiter at full/normal line voltage.
                              "Yeah, well, you know, that's just, like, your opinion, man."

                              Comment


                              • #30
                                Originally posted by Greg_L View Post
                                Both
                                Don't use the limiter to bias. You need full power.

                                Comment

                                Working...
                                X