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Blues Deville low distorted sound

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  • Blues Deville low distorted sound

    Blues Deville PR247 is in for a dirty sounding, and seemingly not loud enough sound. First I replaced the 12AT7 he had in the PI to 12AX7, and things got a little better. Tried different cab, 6L6s, probed around to no avail. Then I went from a GK bass head preamp into the power amp in jack, still breaks up and doesn't sound loud enough. New tubes are idling at 41 and 39 mA with 492 plate volts. Screens are a few volts below that. I have -51v on the grids. Feeding a sine wave I only get 17vac max into 8 ohms on my NOT true RMS meter, but by then the wave is very spiked.

    Output transformer perhaps? If so can I use a 4 ohm 35watt OT for testing?

    https://www.thetubestore.com/lib/the...-Schematic.pdf
    It's weird, because it WAS working fine.....

  • #2
    Originally posted by Randall View Post
    Blues Deville PR247 is in for a dirty sounding, and seemingly not loud enough sound. First I replaced the 12AT7 he had in the PI to 12AX7, and things got a little better. Tried different cab, 6L6s, probed around to no avail. Then I went from a GK bass head preamp into the power amp in jack, still breaks up and doesn't sound loud enough. New tubes are idling at 41 and 39 mA with 492 plate volts. Screens are a few volts below that. I have -51v on the grids. Feeding a sine wave I only get 17vac max into 8 ohms on my NOT true RMS meter, but by then the wave is very spiked.

    Output transformer perhaps? If so can I use a 4 ohm 35watt OT for testing?

    https://www.thetubestore.com/lib/the...-Schematic.pdf
    Check those resistors around the phase invertor for being open.

    nosaj
    soldering stuff that's broken, breaking stuff that works, Yeah!

    Comment


    • #3
      Check the PI plate resistors R51 (91k) and R52 (100k) on the tube PCB. Both are 1/2 watt.

      Comment


      • #4
        If not those, next check R55 and R56 (1.5K) also on the tube PCB.

        Comment


        • #5
          Well, I'll be darned, that's exactly what it was, R52! Now that's some magic sh!t right there! Is this a common thing with these amps?

          Thank you guys so much for that deadly aim, I really didn't want a long and drawn out battle with this thing.
          It's weird, because it WAS working fine.....

          Comment


          • #6
            Originally posted by Randall View Post
            Well, I'll be darned, that's exactly what it was, R52! Now that's some magic sh!t right there! Is this a common thing with these amps?

            Thank you guys so much for that deadly aim, I really didn't want a long and drawn out battle with this thing.
            no, just been there before with a traynor amp and went round early in my training. Some things you just remember when they bite you real good.
            For me the hints were, power amp section in low output and the power tubes looked ok. So right before the power tubes is the invertor which splits the signal = low power 1/2 signal

            nosaj
            nosaj
            soldering stuff that's broken, breaking stuff that works, Yeah!

            Comment


            • #7
              I have seen a few of the earlier built Deville/Hot Rod/Deluxe/Whatever that had only 1/4W resistors installed there. Not sure if it was just wrong parts or a production change, but I replaced them with 1/2W when I found them open.
              "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

              Comment


              • #8
                Originally posted by Randall View Post
                Is this a common thing with these amps?
                Yes. Now you know a common trouble spot to check in future encounters with them. I usually replace with 1W parts.
                This isn't the future I signed up for.

                Comment


                • #9
                  Originally posted by Randall View Post
                  Well, I'll be darned, that's exactly what it was, R52! Now that's some magic sh!t right there! Is this a common thing with these amps?

                  Thank you guys so much for that deadly aim, I really didn't want a long and drawn out battle with this thing.
                  Change all four while you're in there.

                  Comment


                  • #10
                    Yes, agree with Leo it is fairly common in this series. If complaint has to do with weak or distorted the PI resistors are the first thing I check with these.
                    Also, a bad power tube screen resistor will create a similar symptom, for any make or model.
                    Originally posted by Enzo
                    I have a sign in my shop that says, "Never think up reasons not to check something."


                    Comment


                    • #11
                      Originally posted by g1 View Post
                      Yes, agree with Leo it is fairly common in this series. If complaint has to do with weak or distorted the PI resistors are the first thing I check with these.
                      Also, a bad power tube screen resistor will create a similar symptom, for any make or model.
                      Yes, R55 and R56

                      Comment


                      • #12
                        every time I had this type of a problem, about 3 times in 15 years, it was the output transformer, but that is my experience

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                        • #13
                          ''every time I had this type of a problem, about 3 times in 15 years, it was the output transformer, but that is my experience.''

                          sorry, I take that back. Now that I remember, although in all these cases the output transformer WAS bad, there was low output but the sound was NOT distorted.

                          It's been a while.

                          Comment


                          • #14
                            Originally posted by Tone Meister View Post
                            Yes, R55 and R56
                            Those are the control grid resistors, the screen grid R's, R57 and R58 were the ones I meant.

                            Not to argue though, if you have seen multiple failures of the control grid R's, I'll take your word for it. Just have never seen it myself.
                            Originally posted by Enzo
                            I have a sign in my shop that says, "Never think up reasons not to check something."


                            Comment


                            • #15
                              Originally posted by g1 View Post
                              Those are the control grid resistors, the screen grid R's, R57 and R58 were the ones I meant.

                              Not to argue though, if you have seen multiple failures of the control grid R's, I'll take your word for it. Just have never seen it myself.
                              Indeed I have seen it a few times, prevents the power tubes from drawing current, thus low, distorted output when one is open.

                              Comment

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