Announcement

Collapse
No announcement yet.

JCM900 5881 to EL34

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

  • JCM900 5881 to EL34

    Anything special about converting the JCM900's that came with 5881's for EL34 use?
    Aside from bias, is the output transformer different impedance or the power trans can't handle the load difference if there is one?
    I forget.
    Thanks.

  • #2
    I'm sure the OT impedance will be forgiving enough, heck you might find you get a bump up in power. The problem I found in the Marshall series of amps equipped with sovtek 5881 is they usually had a "cage" over the output tubes that wasn't tall enough to allow EL34's. Of course you could can the cage or cut holes in it.
    This isn't the future I signed up for.

    Comment


    • #3
      Change the screen resistors from 470 to 1k. You also might have to mess with the bias circuit a bit to get the bias in range.
      "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

      Comment


      • #4
        The transformers are the same. There are differences in the bias circuit, screen resistors, and possibly other differences I forget (feedback?).
        Schematics for both versions of power amp are available, best to go through them and make all the changes.
        Originally posted by Enzo
        I have a sign in my shop that says, "Never think up reasons not to check something."


        Comment


        • #5
          I figured the tranny's were the same, The bias and screen grids I know about.
          All these amps have a funny mod with a resistor and cap attatched to the output transformer for the negative feedback.
          And I never understood the way the output impedance could be factory wired for 4 (16) or 8 Ohms.
          How are you supposed to know?
          If it's a head 16, combo 4?
          Anyway, thanks.

          Comment


          • #6
            Originally posted by drewl View Post
            And I never understood the way the output impedance could be factory wired for 4 (16) or 8 Ohms.
            How are you supposed to know?
            If it's a head 16, combo 4?
            There was no way of knowing properly, without testing the amp with a scope and load resistors.

            When Marshall "went cheap" and put in a slide switch instead of 3-way for output impedance, they caused confusion & consternation for their owners. Typically you'd see something like 8/16 on the slide switch and (4) in parenthesis. The OT actually has all 3 taps, but a tech had to unsolder the unwanted tap and substitute the wanted one. How many 900's I've seen with wrecked output tubes because a musician saw (4)16/8 and thought he could run 4 ohms with the slide pushed that way, actually engaging the 16 ohm tap.

            What I did was ask the player what impedances he intended to run, wire accordingly, and CROSS OUT the non-selectable impedance with a thick black marker. The heartbreak and $$$ lost by "going cheap." Big misteak Marshall!
            This isn't the future I signed up for.

            Comment


            • #7
              Originally posted by drewl View Post
              I
              And I never understood the way the output impedance could be factory wired for 4 (16) or 8 Ohms.
              How are you supposed to know?
              If it's a head 16, combo 4?
              Yes that is correct. Head was 8 or 16, combo was 8 or 4. Parentheses were on the number it could NOT be (unless rewired).
              Real dumb as Leo mentioned. We also did the black marker on the parentheses number.
              It was covered in the handbook, but how many users look at those?
              Originally posted by Enzo
              I have a sign in my shop that says, "Never think up reasons not to check something."


              Comment


              • #8
                Bias has enough range, but any idea why it's drifting up to like -70v in standby?

                I did replace the .047uf/250v AC bias feed cap with a standard .047 rated at 630v.

                Is it supposed to be an AC cap? I'll swap it and see.

                Comment


                • #9
                  The bias settles down when the standby is switched on?
                  If so, the rise to -70 when standby is off is not a concern.
                  I've seen it before.

                  Comment


                  • #10
                    Bias is fine when playing.
                    Why do some amps do it and others don't?
                    Something to do with the cathode protection circuit?

                    And where the heck is Wernersville? I'm in the Philly burbs.

                    Comment


                    • #11
                      (Assuming you mean the voltage goes more negative) As long as it settles in when you fire it up, I would think this would be actually a good thing- tube current slowly rising. I don't see it as a problem. If it went more positive on standby, that might be hard on tubes if substantial enough.
                      "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

                      Comment


                      • #12
                        The change in bias voltage is due to the type of bias circuit (capacitor coupled from HV winding). You will also find the AC voltage on that winding rises when on standby (unloaded), probably by the same proportion.
                        Not sure what you meant by AC cap? But real good idea to change to 630V type. If you want to know why, measure the AC volts across that cap when on standby .
                        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 drewl View Post
                          Bias is fine when playing.
                          Why do some amps do it and others don't?
                          Something to do with the cathode protection circuit?

                          And where the heck is Wernersville? I'm in the Philly burbs.
                          Wernersville is due West of you.
                          State Route 422.
                          About half way between you & Hershey.

                          Comment


                          • #14
                            Looks like a good place to drop this although probably a bit late now.. Think JPB may have posted this originally
                            Attached Files

                            Comment


                            • #15
                              Originally posted by drewl View Post
                              I did replace the .047uf/250v AC bias feed cap with a standard .047 rated at 630v.
                              Good move. I've had to replace .047uF/250VAC gone bad in a couple of 900's.
                              This isn't the future I signed up for.

                              Comment

                              Working...
                              X