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  • amp hum after mod - help!

    Okay, so I modded my amp (DSL401) - adding birght switches to both of the channels, removing a unnessasary capacitor that was limiting the bass response and added pin jack to the back of the amp so I could check the bias without taking the amp apart.

    So i put it all back together and fired it up - no sound - then the sound faded in and there was all this crackling and stuff and then I watched on the 1/2W screen resistor burn up!! YIKES!! I looked around and found that I hadn't connected the HT to the output transformer - it was way in the back and close enough to the place it was supposed to go that it looked cennected when I double checked.

    Well, I was only running my amp with two tubes anyway, so I put the two tubes in the outer sockets. I turned it on and then I see lots of blue sparks in the tube that was in the socket when the screen resistor burned up. WOAH!! Well, I have like six spare tubes so I put in another set and everything seems ALMOST alright....

    One strange thing is that when I turn down the master volume all the way now, I still get a signal coming through the speakers..
    there is also a permanant hum. The master volume seems to funstion more as a presence knob now, because when I turn it down the sound get thin and when I turn it up it gets really think and bassy....

    I am assuming that somewhow somewhere the signal from the phase inverter is sneaking past the master volume and into the grids...

    one more odd thing.. I was cheking the differences in the circuitry between the two tubes. and noticed that when I connected the lead of my multimeter to pin 2 of the leftmost tube there was a loud pop. When I connected it to pin 2 of the rightmost tube there wasn't a pop. This was the same for the inner sockets that contained no tubes (including the one that had failed previously) The left one popped and the right one didn't..

    anybody have any suggestion as to what might be the problem here? It is baffling me!! Thanks for any input

  • #2
    those tube sockets are wired in parallel. if you bugger something up on one of the left ones, then the other left one probably isn't gonna work right either.

    the problem is you're getting in over your head. take it to a tech before you kill it.

    Comment


    • #3
      yup, they are wired in parallel alright.... thats why it still works!! The burned out resistor isn't connected to anything and therefore shouldn't effect anything.

      For what its worth the bias is 12V on the left side and 17V on the right - another odd symptom...

      anybody have any ideas as to what could be happening here?

      Comment


      • #4
        After poking around with my multimeter I found something VERY different between the left and right sockets. Measuring the resistance from the contol grid to ground on the lft side gave 9-10M ohms while the right side it measured between 25K -200K (by varying the master volume control) It may be a bad master volume pot, or just a bad solder joint. That PC board is pretty flimbsy when you pull it out of the chassis...........

        Looks like I just have to find where it isn't connected and fix it, while I'm at I'll I'll replace the screen grid resistor, too, and it should be good as new...........

        Comment


        • #5
          You're in way over your head. Time for a tech. You're going to end up costing yourself way more than you think you're saving by doing it yourself.
          Jerry
          Not here anymore. Find me at my forum or Rigtalk.
          FJA Mods Forum and Tech Help FJA Mods Forum
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          Comment


          • #6
            I appreciate the "help" so far, but let me just let ya'll know that I am going to work this out myslef. I have been studying tube circuits and such for over a year and am just about to graduate in Electrical Engineering, so this IS up my alley.... and I've already traced the problem to a specific area.... I AM gogin to fix this myself and I see absolutely no danger in trying to do so.... If I screw up my amp, so be it. It would be worth the cost to learn from the experience......

            Unfortunately I don't have too much time to do a lot of diagnostic work because I have a lot of projects at school, too... so What I am looking for is someone who may be able to give me some information that could quicken the process - since I won't be able to fix it for another week or so.... but at this point I think I've done all I can do without taking it apart and finding the bad connection.

            Comment


            • #7
              Originally posted by gusfinley
              I appreciate the "help" so far, but let me just let ya'll know that I am going to work this out myslef. I have been studying tube circuits and such for over a year and am just about to graduate in Electrical Engineering, so this IS up my alley.... and I've already traced the problem to a specific area.... I AM gogin to fix this myself and I see absolutely no danger in trying to do so.... If I screw up my amp, so be it. It would be worth the cost to learn from the experience......

              Unfortunately I don't have too much time to do a lot of diagnostic work because I have a lot of projects at school, too... so What I am looking for is someone who may be able to give me some information that could quicken the process - since I won't be able to fix it for another week or so.... but at this point I think I've done all I can do without taking it apart and finding the bad connection.


              If you want my help please post a schematic and use it to explain the changes you have made and as reference to where you are taking your measurements. For now if the amp had no serious problem before your mod I would switch it back and go from there. Also a good thing to remember if you are doing more than one mod to an amp, do one mod at a time and check results before going on.

              Comment


              • #8
                good luck. let us know how much the repairs end up costing.

                Comment


                • #9
                  I'm sure that the shipping on any repairs will cost more than the repairs themselves.... a few cents for a new resistor and about 50 cents each for the little caps I'm going to replace...

                  I've decided that I don't like the new bright cap anyway... I'm going to just put 1 pin terminals (socket, aka "headers") where the cap leads where and then I can easily switch back and forth between mod/stock values and see which one I want to put on the switch.

                  I'll post the schematic a bit later.... the main thing I will need to do is figure out how all this stuff is connected on the PC board traces... there are a lot of "links" running from one trace to another so Its hard to follow the circuit path on the upper portion of the board...

                  I'll also post measured values on the effected part of the amp.. branch values for voltage, capacitance, etc... but it maight take a day or two....

                  THANKS for the offer of your help guitarzan!!!

                  Comment


                  • #10
                    Originally posted by gusfinley
                    I'm sure that the shipping on any repairs will cost more than the repairs themselves.... a few cents for a new resistor and about 50 cents each for the little caps I'm going to replace...

                    I've decided that I don't like the new bright cap anyway... I'm going to just put 1 pin terminals (socket, aka "headers") where the cap leads where and then I can easily switch back and forth between mod/stock values and see which one I want to put on the switch.

                    I'll post the schematic a bit later.... the main thing I will need to do is figure out how all this stuff is connected on the PC board traces... there are a lot of "links" running from one trace to another so Its hard to follow the circuit path on the upper portion of the board...

                    I'll also post measured values on the effected part of the amp.. branch values for voltage, capacitance, etc... but it maight take a day or two....

                    THANKS for the offer of your help guitarzan!!!


                    You are most welcome! I check in most everyday at about noon(EST). If you post your schematic I will be sure to help any way I can. I will not go into all my experience but lets just say my work forces me to play with voltages that make a tube guitar amp look minuscule and that my own DIY amp is pretty much perfect except to look at : ) One day I will improve on that too.
                    Remember safety first, always!

                    Comment


                    • #11
                      Originally posted by potaetoes
                      good luck. let us know how much the repairs end up costing.


                      How is he to learn if he just jobs out? I think DIY is a great way to understand more about your amp, as long as you follow basic safety procedures.

                      Comment


                      • #12
                        Originally posted by guitarzan


                        How is he to learn if he just jobs out? I think DIY is a great way to understand more about your amp, as long as you follow basic safety procedures.


                        DIY is a great thing if you have a clue about what you're doing. To go in recklessly and start clipping out caps because you don't think they should be there is foolish. He took out caps that limited his bass and then added bright switches. Sounds like those two things work against each other. Those caps are there for a reason. To change them you should at least know their function first and then make changes because you have an idea of what the results will be. Watching changes on a scope and with meters is the right way to mod amps, not clipping things and watching for smoke.
                        Then he powered up an amp with a known failed resistor without any care or thought about what more damage that would cause. I'm sorry, I'm all for people learning and trying things but it seems quite obvious to me that this guy is in too deep. I help people everyday when I can and it doesn't look like they're gonna get hurt or destroy their amps.
                        Jerry
                        Not here anymore. Find me at my forum or Rigtalk.
                        FJA Mods Forum and Tech Help FJA Mods Forum
                        FJA Mods Website FJA Mods
                        FJA YouTube Mod Videos

                        Comment


                        • #13
                          For those of you have been cheering me on rather than scorning me, you'll be happy to know that I tracked it down to the master volume pot itself. There is an open circuit somewhere in there (bad solder joint probally) and when I take it apart again I'll be able to diagnose it

                          "Then he powered up an amp with a known failed resistor without any care or thought about what more damage that would cause"

                          If this person would have paid attention to my post they would have seen that I changed the tubes over so they were operateing on different sockets.... so the failed resistor wasn't in the circuit at that point....

                          ChiLL people!!

                          Let the kids learn the hard way!!

                          "To go in recklessly and start clipping out caps because you don't think they should be there is foolish. He took out caps that limited his bass and then added bright switches. Sounds like those two things work against each other. Those caps are there for a reason. To change them you should at least know their function first and then make changes because you have an idea of what the results will be. "

                          Again, I had a knowledge of what the caps were doing and which were unecessary, I didn't just hack at it with a chainsaw as you seem to be implying......

                          Everything is gonna be okay..... just have a little patience!!

                          Comment


                          • #14
                            Good luck fellow Finley! Be safe and live to mod another day!

                            Comment


                            • #15
                              Originally posted by gusfinley
                              For those of you have been cheering me on rather than scorning me, you'll be happy to know that I tracked it down to the master volume pot itself.


                              Good for you! See it wasn't rocket science after all
                              First,
                              To be sure it is not a great ID for anyone to poke around in an amp.
                              Second,
                              To be sure you will not cover tube therory in EE class except maybe one afternoon outlining history.
                              Third,
                              Be sure to pick up some worthy text. At least an old RCA tube manual. The info in that would be far better than most you PU on the net. Not refering to JerryP, he is usually right on but several others I see have little or no ID what they are talking about. Suggestions for mods given out on the net can be very wrong and some are down right stupid.
                              Lastly,
                              Mistakes can be costly as you know but the ones that kill are the **************** to always look out for. Always work with one hand only in a live amp. Always wear shoes, runners are fine. Never under estimate the damage 500ma @ 450V can do to your heart and other organs. No matter what anyone tells you the CURRENT in a tube amp is enough to kill you! Voltage just sits there, that's why it is defined as potential difference but if you become part of the circuit and allow that much current threw you it will hurt a lot, if not kill

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