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Where's the Tube? (Randall RG75-G3)

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  • Where's the Tube? (Randall RG75-G3)

    I have posted a picture of the guts, and I have read a multitude of posts that says this has a tube (even specifying it as 12AT7), and others the opposite. My problem is a no sound issue (have power, guitar and cord works on other amps). Checked for cold solder joints and am at a loss. Worked fine before I moved.

    If somebody can identify the tube, can you point it out in the image, because I surely don't see it.




    https://drive.google.com/file/d/1NXn...ew?usp=sharing

  • #2
    I see no reason to believe there's a tube inside:

    Randall RG75-G3: https://www.musiciansfriend.com/ampl...itar-combo-amp

    has the "character of power tubes" but no actual tubes.
    --
    I build and repair guitar amps
    http://amps.monkeymatic.com

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    • #3
      No offense, but are you sure that is a G3 and not a G2? I can't find any pictures of a G3 with front graphics that look like that. All the G3s I see have the G3 right under the switch. Then again Randall seems like they are sometimes kinda loose with model numbers and the actual components.
      Last edited by glebert; 05-25-2020, 05:32 AM.

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      • #4
        Originally posted by glebert View Post
        No offense, but are you sure that is a G3 and not a G2? I can't find any pictures of a G3 with front graphics that look like that. All the G3s I see have the G3 right under the switch. Then again Randall seems like they are sometimes kinda loose with model numbers and the actual components.
        No offense taken, mainly because you are right. Not sure why I had G3 in my head all these years. This was a special camo series, but it is a G2.

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        • #5
          You replied back while I was adding something to my post, so putting it here instead.

          Assuming you have an effects loop try plugging your guitar directly into the effects return jack. See if you get some sound, and if it is clean or distorted (it may be a bit low level, that is OK). If you get sound doing this trying using a cable to jump from the send to the return to rule out a dirty switchjack. Also, see if you have anything at the headphone jack (dirty jack here can also cause no sound). I'm assuming you checked that the speakers were connected.

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          • #6
            Originally posted by glebert View Post
            You replied back while I was adding something to my post, so putting it here instead.

            Assuming you have an effects loop try plugging your guitar directly into the effects return jack. See if you get some sound, and if it is clean or distorted (it may be a bit low level, that is OK). If you get sound doing this trying using a cable to jump from the send to the return to rule out a dirty switchjack. Also, see if you have anything at the headphone jack (dirty jack here can also cause no sound). I'm assuming you checked that the speakers were connected.
            Yep, tried the return on the FX loop (nothing), and tried going line out to another working cab (nothing). Haven't been able to locate my headphones, but tried to go from the headphone jack to the other cab with nothing.

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            • #7
              Local copy:

              Click image for larger version

Name:	20200522_194753.jpg
Views:	1
Size:	3.62 MB
ID:	857829

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              • #8
                Originally posted by jperrizo View Post
                Yep, tried the return on the FX loop (nothing), and tried going line out to another working cab (nothing). Haven't been able to locate my headphones, but tried to go from the headphone jack to the other cab with nothing.
                Next I would check voltages to see if you have the right (or at least reasonable) voltage rails for the power amp and preamp.
                Last edited by glebert; 05-25-2020, 06:11 AM.

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                • #9
                  Does this power amp schem. look right?
                  I'm dealing with a similar one where the muting circuit that enables Q8 is causing intermittent 'no sound on power-up'.
                  In my case some of the silk screening of component names does not match schematic so be careful. (ex.Q4 & Q5 names reversed).
                  Attached Files
                  "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

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                  • #10
                    Originally posted by g1 View Post
                    Does this power amp schem. look right?
                    I'm dealing with a similar one where the muting circuit that enables Q8 is causing intermittent 'no sound on power-up'.
                    In my case some of the silk screening of component names does not match schematic so be careful. (ex.Q4 & Q5 names reversed).
                    Wonderful Don't you love it when things don't match up. I have been working on a lawn tractor where the manual shows the incorrect routing of the main drive belt. Anyway, I was afraid I was going to get in too deep with this beyond my capabilities and either have to take it to a shop or just bag it and buy a new amp. I am sure many of you would say why bother fixing the POS. However, their is some nostalgia associated with, but probably not enough to warrant paying too much to fix it.

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                    • #11
                      Originally posted by jperrizo View Post
                      Wonderful Don't you love it when things don't match up.
                      This is a frustration I have with Randall amps. I am working on an RX120rh were the power amp is *almost* the same as the one posted above, but with a few more transistors and other components, and of course those were were the problem was. Finally found a schematic for a 200 watt power amp that showed a protection circuit that was really close to what I had and I was able to at least get it working, until I overheated it and blew the power transistors I liked the sound of it, though, while it worked.

                      Me personally I have a problem of trying to fix just about every POS amp project I can get. Luckily right now I have the luxury of time and enough money to buy parts. POS amps are a good way to practice and develop new skills or learn about different amp designs.

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                      • #12
                        Originally posted by glebert View Post
                        This is a frustration I have with Randall amps. I am working on an RX120rh were the power amp is *almost* the same as the one posted above, but with a few more transistors and other components, and of course those were were the problem was. Finally found a schematic for a 200 watt power amp that showed a protection circuit that was really close to what I had and I was able to at least get it working, until I overheated it and blew the power transistors I liked the sound of it, though, while it worked.

                        Me personally I have a problem of trying to fix just about every POS amp project I can get. Luckily right now I have the luxury of time and enough money to buy parts. POS amps are a good way to practice and develop new skills or learn about different amp designs.
                        Thanks for the help. Since you are (somewhat) local, if you are interested, I can give you the amp. Maybe you can figure it out easily, sell it and make a buck. Send me a message if you have interest.

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