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Mission 5E3 - burnt screen resistor

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  • #1

    Mission 5E3 - burnt screen resistor

    Today, after playing my Mission 5E3 for about 2 hours, suddenly the sound went and I saw smoke rising. I pulled the plug and had a look inside. The only sign of burning I can see is a well burnt out screen 470R resistor on one of the power tubes. The resistor still reads OK though. This is a Mission 5E3 240V version with the Mission choke kit.

    I was playing through an attenuator (Weber MiniMASS) which is how I normally use it. What was different this time is that the B+ was about 375V instead of 350V which I usually have the B+ set to using a string of zener diodes. However, today I removed the zeners from the PT CT in preparation for a VVR (a form of Power Scaling) circuit I will install soon. The amp appeared to be biased fine and played fine up until the problem.

    So my question is this. What could cause a screen resistor to get this hot or what could cause so much current to flow?

    Could this be a sign of a tube failing? If the screen resistor still reads good (cold) then why did the sound stop?

    I have replaced both screen resistors with 2W ones and tried to trace out any problem. I haven't switched on yet to check the voltages in case there may be another problem.
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  • #2
    I have come to the conclusion that one of the power tubes has failed and that is what caused the screen resistor to burn. I checked all voltages without any tubes, then added back in the pre amps and checked again. All seemed OK so I then put a different pair of known working 6V6's in and the amp and it now works OK. Again all the voltages appear to check out OK.

    However, there is a worrying intermittent noise coming through the speaker. It sounds like occasional spikes of signal which in my mind I think maybe arcing somewhere. I can't see any other burnt out components or any signs of arcing. It doesn't sound good so I have switched the amp off.

    Anyone have any ideas of what to look for?

    Comment

    • #3
      Have you got a different speaker cab to use? If so, with the speaker cab well away from the amp (to isolate physical vibrations) does the problem go away? If you hammer the amp cab with you fist can you make the problem appear?
      Have you checked that the tube socket contacts are good and tight?
      Have you tried a complete different set of tubes?
      Peter
      My band:- http://www.youtube.com/user/RedwingBand

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      • #4
        Gee,

        Can you give us a report back on how the VVR implementation goes?

        I'm about to start a 5E3 build, and am going to include the VVR, so I'm curious...

        Thanks!

        -John

        Comment

        • #5
          Originally posted by haashole View Post
          Gee,

          Can you give us a report back on how the VVR implementation goes?

          I'm about to start a 5E3 build, and am going to include the VVR, so I'm curious...

          Thanks!

          -John
          I certainly will do. I was recently trying different locations for the small circuit and I am still undecided. As you probably know, the 5E3 chassis is a bit crowded. So far I have replaced the On/Off switch with a Carling On/On/Off switch to make room for a 1M pot (VVR control) where the Standby switch was.

          However, all of this is a bit on hold at the moment until I get the amp back on its feet.

          Comment

          • #6
            5E3 problem solved

            Following on from a suggestion on another board, I re tensioned the octal sockets. The sockets appeared to be OK and there were no traces of arcing. However, on examining the previous set of 6V6's (Brimar) I could see that the pins were very slightly bigger that the new JJ 6V6S's I had fitted. The JJ pins do look like they have a smaller diameter.

            After re tensioning the octal sockets, the amp is now perfect with the JJ 6V6S's. So thanks to everyone.

            Comment

            • #7
              VVR circuit in a 5E3

              Originally posted by haashole View Post
              Gee,

              Can you give us a report back on how the VVR implementation goes?

              I'm about to start a 5E3 build, and am going to include the VVR, so I'm curious...

              Thanks!

              -John
              I have the VVR (Variable Voltage Regulator) circuit working in my 5E3. It is simply amazing to be able to adjust the power output of the amp using a control..

              I fitted the VVR control in place of the Standby switch and replaced the Power switch with a Carling Off/On/On switch. The VVR circuit adjusts the amp B+ from full down to about 30V. The effect is to provide a much better way to attenuate the output. There is a slight loss of treble and more distortion and compression in a very pleasing way. I'm currently using it at home with it set to about 110VDC.

              Full details are available over at www.ppwatt.com with more details about the VVR circuit at www.18watt.com.

              Comment

              • #8
                Originally posted by Gee View Post
                I have the VVR (Variable Voltage Regulator) circuit working in my 5E3. It is simply amazing to be able to adjust the power output of the amp using a control..

                I fitted the VVR control in place of the Standby switch and replaced the Power switch with a Carling Off/On/On switch. The VVR circuit adjusts the amp B+ from full down to about 30V. The effect is to provide a much better way to attenuate the output. There is a slight loss of treble and more distortion and compression in a very pleasing way. I'm currently using it at home with it set to about 110VDC.

                Full details are available over at www.ppwatt.com with more details about the VVR circuit at www.18watt.com.
                Thanks Gee, I just finished my 5E3 build last night. It sounds great. There is still some tweaking I want to do, but the VVR circuit will be going in very soon. I'm glad to hear it works well in the 5E3.

                Did you put it between the first and second filter cap?

                -John

                Comment

                • #9
                  Originally posted by haashole View Post
                  Did you put it between the first and second filter cap?

                  -John
                  No. I put it between the Standby switch and the first cap. I also removed the 5Y3 and added a couple of diodes to the back of the 5Y3 socket to provide SS rectification. I found the 5Y3 to be providing too much sag at lower voltages.

                  Comment

                  • #10
                    Originally posted by Gee View Post
                    No. I put it between the Standby switch and the first cap. I also removed the 5Y3 and added a couple of diodes to the back of the 5Y3 socket to provide SS rectification. I found the 5Y3 to be providing too much sag at lower voltages.
                    Are you Conger over at 18 Watt? If so, I've read all the details in the "Layout for VVR mod in Lite2b" thread. It was good of you to post the followup over there, after you and Dana took the discussion to email and you got it resolved. Since I was doing a 5E3 build, I was following along with interest.

                    Comment

                    • #11
                      Originally posted by haashole View Post
                      Are you Conger over at 18 Watt? If so, I've read all the details in the "Layout for VVR mod in Lite2b" thread. It was good of you to post the followup over there, after you and Dana took the discussion to email and you got it resolved. Since I was doing a 5E3 build, I was following along with interest.
                      Correct. Dana is such a helpful guy by the way.

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