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Newb needs help with tubes.

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  • Newb needs help with tubes.

    So i have a used Randall RD45 that i bought which i need to buy tubes for. I'm a complete newb to tube amps and any information about replacing tubes regarding that fact would be greatly appreciated. I need to know what type of tubes to buy and where they go in the slots on the amp, and if i need to do any biasing.
    Last edited by Joshua M; 11-26-2019, 10:30 PM.

  • #2
    If I am not mistaken, your amp uses dual 6L6 power tubes and 12AX7 preamp tubes. Are you sure you need to replace the tubes?

    A copy of the schematic can be found here -


    • #3
      I might not actually have to replace those two power tubes now that you mention it, but from what i've read replacing those definitely improves the tone according to people who have used this amp and swapped out the tubes. As for the preamp tubes that you mentioned they are, there aren't any so those i have to buy for sure. Do i have to do a bias adjustment if only replacing preamp tubes? or is it just for power tubes or changing any tube in general? Thanks for the help.


      • #4
        You do not set bias for preamp tubes.

        As for the Power Tubes, do not assume the bias was set correctly. You might want to check that. The schematic has a recommended setting. You should see the bias pots near the tubes (on the chassis) - VR4B25K and VR5B25K. Do you know how to SAFELY use a voltmeter to measure voltages in a tube amp? If not, let someone else do it. The amp has HIGH VOLTAGE points and you could injure yourself if you are not familiar with this process.

        Check a datasheet for the 6L6 -

        You might also read this thread for additional info -


        • #5
          Your amp uses two 6L6's and four 12AX7's. Generally, you only need to bias when changing output tubes, but it wouldn't hurt to check it even if you leave the existing tubes to verify they are working correctly.

          Edit: Another good link for calculating bias:

          "Yeah, well, you know, that's just, like, your opinion, man."


          • #6
            All good questions, and welcome to the forum!
            Preamp tubes generally are used in a circuit that makes them self-biasing, so that they do not need to be biased by the user.

            edit: I see other, more detailed, posts were made while I was composing. Carry on!
            If it still won't get loud enough, it's probably broken. - Steve Conner
            If the thing works, stop fixing it. - Enzo
            We need more chaos in music, in art... I'm here to make it. - Justin Thomas
            MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey


            • #7
              Thanks for the replies everyone! I got all the answers i needed AND more, i appreciate it. I just have a question or two more. I was reading there is enough voltage in an amp to kill you? so IF i was to learn how to bias myself would i be encountering that risk?


              • #8
                ANy time you expose the insides while it is powered up, that voltage is also exposed. You need to respect it. I mean the burners on your stove can incinerate your hand, so you need to respect that too, and not touch teh burners. It is a matter of paying attention to what you re touching. Nothing looks different just from having 500 volts on it.

                There are amps with EXTERNAL test points for bias, and EXTERNAL controls for that bias. In those cases, you generally are not exposed to danger.
                Education is what you're left with after you have forgotten what you have learned.


                • #9
                  That makes sense Enzo. So as long as you're knowledgeable in what you're doing and practice safety you'll be okay. I have no idea if my amp has external test points or controls. My other question was about the tubes. Do the tubes have to be the same model as are specified for the amp, or can you use other model tubes?


                  • #10
                    You need only be concerned about tube type (6L6 and 12AX7). Any brand will work.
                    "Yeah, well, you know, that's just, like, your opinion, man."


                    • #11
                      Also know that 12AX7, 7025, & ECC83 are all the same tube type and will work the same in your amp.

                      I would also stick with 6L6GC to start. There are other 6L6 types that are not the same spec as the GC version.

                      "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
                      "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
                      "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -


                      • #12

                        But any new tube sold these days with 6L6 on it or 12AX7 on it will work. You can learn for later about alternatives like 5881 for 6L6, or the 12AX7 types Justin mentioned.

                        Tubes are like guitar strings. You may prefer Fenders or D'Addarios, but ANY brand will work in your guitar.
                        Education is what you're left with after you have forgotten what you have learned.


                        • #13
                          Awesome! Thanks a lot guys. I'll just stick with the 6L6GC then. Are the power tubes the only ones that change the sound of the amp? or do preamp tubes affect it also?


                          • #14
                            Any time you change anything in the signal path or even in the power supply, it "could" (and probably does) affect the sound of the amp. But will you hear it? That is another question and highly debated. And the same can be said for speakers. I had a friend try to demo this for me with his amp and power tubes. He swore he could hear a difference. All I could hear was a guitar that was out of tune and horrific playing skills!!


                            • #15
                              I think the person playing the guitar has the upper hand hearing the difference or rather feeling it. 😋
                              When the going gets weird... The weird turn pro!


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