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Musicman RD-50 112 mod

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  • Musicman RD-50 112 mod

    I'd like to share some of the mods I did to smooth the harsh top end of the Limiter channel typical for Musicman amps. In general, changed the V1a and b stages per the *umble OD stages.
    In Particular:
    1. disconnected the cathode grounds and ground leads from C10,R12 at V1-8.
    2. Added a 5uf and a 1.8k at V1-8 and connected the grounds to the C10,R12,ground node.
    3. replaced plate resistors R9 (470k) with a 180k, and R11 (4.7M) with a 150k.
    4. replaced R10 with a wire jumper.
    5. removed grin wire at V1-7 and added a 150k series grid resistor.

    Initially also added a 470pF across the Clean Vol pot, and a 1000pF across the Limiter Vol Pot to smmoth the harshness at all vol levels in either Clean or Limiter mode.
    The amp is much more useable and it definitely earned it's keep at last weekends gig. BobW

  • #2
    No replies? I guess no one else has an RD-50?


    • #3

      I took your post and added the schematic, and voila! magically converted it into a PDF file:

      If that doesn't meet with your approval I can delete it.

      Thanks for posting your mods!

      Steve Ahola
      The Blue Guitar
      Some recordings:


      • #4
        For all the schematics you've shared with everyone Steve? No problemo, I'm glad to share this one. btw, my amp was an early REV A version.
        thanks, Bob


        • #5
          hey -- i'm interested!

          I just checked the musicman site and saw that someone had posted a reply to my posting suggesting I should look at your mods.

          From looking at your mods, it appears that you have toned down the gain in the limiter channel? and, brightened the amp?

          My biggest concern with the amp is that I find the limiter channel to be loose in the lows. I've modified my tone stack to mirror the clean channel, which addressed the amount of mids and i've changed the filter caps from 60uf to 100uf and 2uf to 4uf. My lastest mod makes my limiter channel like the specs on the last rd50 model. I like this best of all with a 12dw7 tube rather than 12ax7. But, I still think the lows have flabby.

          I'd like to know more about why you made the changes you did. I think i follow the idea. Otherwise, I love this amp.


          • #6
            Buckeye, since I don't play metal music, I find the gritty highs and lack of tone in the limiter unuseable. The stock Limiter channel Plate voltages (especially at V1-b) are too low, and that's why the gains are set so high in this stage. This results in a less than optimum tone, hence the gritty highs. In addition, the input impedance in to V1-b is all out of whack.
            I jumpered the R10 to set both plates at the same voltage. With a higher voltage now at the V1=a plate, the gain stages don't need to be that high. Using the lower OD gain values from a Dumble gave the channel more definition. A 150k (1ook here is ok too) grid series resistor was added at V1-7, to keep the stage input impedance stable at lower volume levels. It also helps prevent against grid blocking at higher signal levels. Lastly a 390pF was placed across the limiter volume to normalize the bass vs treble controls. This mods also works well for my style of playing. I don't use a pick and before the amp had an ice pick in you ears attack, it now has better definition playing a whole chord w/o getting muddy. The amp now also has a better touch response and has been used on about the last 10 gigs w/o any tone issues. I like the small size and is perfect for a small gig, however if you want a lot more bass, you may want to try an extension cab. hth BobW


            • #7
              Thanks for the reply. No metal here either -- mostly blues, classic rock, some country. I like this amp a lot -- the size is great, the clean sound is very good, but it would be perfect if I could get a usable limiter channel. Can get like an ice pick -- i appreciate the tip on the cap across the volume pot -- I assume the outer two lugs of the pots? I currently have an Eminence Texas Heat in it, but have an Eminence Cannibas Rex on order. I understand this a smooth speaker. My amp was made in December 1980, or so it written on the board - so it's the oldest model, I think.

              I initially did very similar mods to mine including the r10 removal and the values of the plate and cathode resistors/caps. I played with several values including as big as 25uf bypass cap and used 150k/1.5k plate/cath resistors as a starting place. I just never got a usable overdrive. Too muddy and no definition. the bottom farted at me! I've beefed up of the filter caps to get more power from 60uf to 100uf and 2uf to 4uf. Just this past weekend, I took the chasis out and made my limiter channel mirror the last revision MM made which uses pretty high values for the resistors and a 2uf bypass cap. I tried it and still not real usable. I tried a JJ 12AT7 tube, kind of better. I them tried a JJ 12DW7, this sounded the best I ever tried -- almost usable. But still a flabby bottom. It's interesting that you've talek away gain, as I did with the 12DW7 (gain factor of 20 on pins 1-3/GF 100 pins 6-8) and have a better sound.

              Do you have a resistor/cap at C10 & R12 now? i wasn't clear on that. What is the purpose of adding a resistor to V1-7? I suppose this cools down the signal from V1A to V1B? Which is what I've done with the 12DW7 tube. What tube are you using? Ed Goforth, who I understand is kind of a guru with these amps,has suggested changing the rectifier diodes to FREDs. I guess this would get more power converted quicker for the power demand? Have you tried anything like this?

              These amps aren't too common, in fact mine is the only one I've ever heard. Nice to see some else working the same issues.


              • #8
                "I assume the outer two lugs of the pots?" Yes

                "I played with several values including as big as 25uf bypass cap and used 150k/1.5k plate/cath resistors as a starting place." I may not have made myself clear, You also have to lift V1-8 and place Rk/Ck values 1.8k/5uF or as high as 20uF. At V1-3, Rk/Ck = 2.7k/2uF
                "What is the purpose of adding a resistor to V1-7?"
                "A 150k (1ook here is ok too) grid series resistor was added at V1-7, to keep the stage input impedance stable at lower volume levels. It also helps prevent against grid blocking at higher signal levels. "

                On the way to Colorado about 6 yrs ago, I got lucky in an Amarillio pawn shop and walked out with this amp for under $60.
                Attached Files


                • #9
                  Great information and what a deal on your amp. I think I'll play with a grid resistor on V1-7. I understand from other reading as well that this can halt the harshness and buzziness and limit inaudable oscilations. What size resistor did you use, 1/4 watt?


                  • #10
                    There is a very small amount of current flow into the grid, and 1/4W is sufficient.


                    • #11
                      I just got my rd50 112 out of storage and this looks interesting to me. I don't see the V references on the schematic, are they on the circuit board ? I will probably add the mid switch, since the footswitch does the clean/gain thing. Is it easy to get at the circuit board ? (never modded an amp, althogh I've built some simple effects from a book and rewired my guitar). Any further progress buckeye ? BTW, I've heard the Barber Direct Drive is a good pedal to use with the clean side, instead of using the gain channel.


                      • #12
                        Hi all,
                        I have modded several 50A versions to the later 50B revision E with good sucess. The limiter ch is now nore coherent and smoother.
                        John G


                        • #13

                          Great thread, very informative. I've been given an RD-50A rev. B; I've always liked MM amps and intend to keep this one. I'm ready to start re-capping it, but before I set about the limiter channel with a soldering iron, I'd be grateful if you could check over the cheeky schematic I've hidden away on my website - and make sure I've got the mods on this thread right - plus any thoughts and wisdom the group have:


                          (If it doesn't meet with your approval BobW I can soon delete it).
                          Many thanks


                          • #14
                            No problem Jooshtin, Thanks for posting the schematic mod.


                            • #15
                              Hi all, Hi @BobW,

                              I'm wondering if I can make an On/Off switch for the EQ of the Clean channel of the RD50 (mine is the 10" version, but same amp).
                              In fact, I would like to use the Normal/Bright switch for this purpose, if possible, because I never use the Bright mode (although I guess that a DPDT switch will be required).
                              My question is, if possible, which portion of the circuit should be switched?
                              This is the schematic I found:

                              I just wish the have the clean channel as flat as possible, when necessary, to use with other preamp.

                              Another question is - can is use directly the reverb tank connectors as Send/Return without modification?

                              I don't quite understand the schematic, but I can handle the modification if I know where to touch.

                              Any help will be highly appreciated!


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