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6505+ blows output fuse on standby. Does not with no power tubes. (

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  • #31
    It looks like an input grid resistor (2K2 1/4 watt). The other of the same side is also deteriorated by the heat. The third band (red) looks brown.

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    • #32
      Follow the power traces measuring currents. If you don.t have resistors inserted you need to temporary mount some(big enough to dissipate the expected requested power). That.s a easy way to isolate and spot on where you problem commin from
      "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

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      • #33
        Hmm.. nice catch, guys! When I get home after work, I'll take a closer look at those two.

        So to confirm, the one at the far left and the one in the middle should be identical to the two red/red/red/gold resistors at the far right?

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        • #34
          The 'burnt' resistor is R204. It is the grid resistor (2.2K/ 1/4 watt) for the outboard tube V8.

          PV 5150-2_6505+ Output Board.pdf

          There are only 4 small resistors on that board.
          They are all the same: 2.2K/ 1/4 watt.
          One for each tube bias control grid. (Pin #5).
          Last edited by Jazz P Bass; 11-21-2017, 06:31 PM.

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          • #35
            If a grid resistor is burned open, there will be no bias voltage on the associated tube, causing the symptoms you have. I still doubt the socket is bad. Going back to my post #21, check voltages on the sockets and see. I see no reason to replace a socket without first checking the voltages on socket pins. It's the best way to figure out what's going on. Or, we could just replace parts until it works.
            "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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            • #36
              Originally posted by The Dude View Post
              If a grid resistor is burned open, there will be no bias voltage on the associated tube, causing the symptoms you have. I still doubt the socket is bad. Going back to my post #21, check voltages on the sockets and see. I see no reason to replace a socket without first checking the voltages on socket pins. It's the best way to figure out what's going on. Or, we could just replace parts until it works.
              Do you happen to know which tube pins are for the bias voltage? Or what values I should be seeing at each pin?

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              • #37
                The bias will be pin 5 (control grid) of the output tube sockets. Compare the sockets working to nonworking. It will be around negative 50 volts (give or take). The exact voltage isn't as important as is that it's there. Also check all of those grid resistors. There could be more than one bad.
                "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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                • #38
                  I just did a quick re-read of the thread and noticed that, in your post #26, one of your sockets smoked even with no tube in it. I hadn't caught that post before I wrote the above. That said, there must also be something wrong with that socket or underneath it. My apologies for missing that post. It's still a good idea to check voltages on the socket and those control grid resistors. It may point you to where the short is.
                  "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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                  • #39
                    Three tubes worked and the bad socket smoked. You pretty much have to have an arc in that socket. Turn off all the room lights and position yourself so you can see that socket, and see if you spot a blue spark or glow under there.

                    generally once a socket arcs, you need to replace it.

                    As to unused pins, I think if you look, some pins that would be unused were used as just a pathway for copper traces. They may or may not have pin 1 connected on any socket. 6L6 doesn't use pin 1. Sometimes they need to run a copper trace through a socket area. Instead of trying to sneak it between pins 4 and 5 or something, they bring it right through the unused pin 1 to keep it away from other pins.
                    Education is what you're left with after you have forgotten what you have learned.

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                    • #40
                      The shitty thing about this amp is that to test the board while live, I have to completely reinstall it and screw it back in. The ribbon and other connectors are too short to use with the board half-hanging out of the chassis. And when it's fully installed, all you get to see is the underside/trace side of the board. Not very useful for finding an arc or seeing the resistors or other components for smoke.

                      So this evening, I didn't put it back in to test the voltages at each pin one by one, but I did test the resistors and got the following:

                      This is the very burned/discoloured resistor of socket 1 (good socket). The resistance still checks out ok:


                      This is the partially discoloured resistor of socket 2 (the "bad" socket). The resistance also still checks out ok:


                      These two images are for sockets 3 and 4, the resistors are not discoloured (both sockets are good). Both resistors also check out ok:



                      I also checked the two discoloured resistors from their socket pin to the ribbon pin at the other end of the board. There are no other components in the path, so I didn't need to lift these resistors to test them. Both checked out ok from end to end:

                      Socket #1, a good socket, the very discoloured resistor:


                      Socket #2, the bad socket, the partially discoloured resistor:


                      I also re-checked the white block screen grid resistors again. The resistor on the bad socket seems to have gotten worse. It was reading the proper 100ohms a few days ago (along with the rest), but I guess all of the arcing, smoke and everything else during testing gave that resistor a beating. I still have replacements, so I'm going to swap it out.


                      This isn't the resistor that I changed out earlier in the thread. The one I swapped is on socket 3. That one, along with 4 & 1 also still read ok at 100ohm.

                      Tomorrow I'll throw the board back in and check the pin voltages, but as it stands, if all of the resistors are ok, minus the SGR of the bad socket being down around 7 ohms, do you guys think that this further points to the socket itself being toast?

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                      • #41
                        I hope you saw my post #38. I had missed your post about the socket smoking even without the tube installed and I again apologize for missing that. So, yes, I believe the socket is bad. Or there could be something lodged underneath like a blob of solder, lock washer, etc.
                        "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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                        • #42
                          Those 2K2 grid resistors are very critical. Replace the discoloured ones, it's not worth risking loss of bias for a couple cents.
                          When you have the bad socket removed, post a pic of the underside of the socket, and the board area under where the socket was.
                          Originally posted by Enzo
                          I have a sign in my shop that says, "Never think up reasons not to check something."


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                          • #43
                            Originally posted by The Dude View Post
                            I hope you saw my post #38. I had missed your post about the socket smoking even without the tube installed and I again apologize for missing that. So, yes, I believe the socket is bad. Or there could be something lodged underneath like a blob of solder, lock washer, etc.
                            No need to apologize! You guys are the ones helping me.

                            I did see your post, yes, and that's what I'm leaning towards as well. I ordered a new socket yesterday, which should arrive on Thursday. In the meantime, I figured I'd at least check out the rest of the traces/resistors before throwing in a new socket, only to have it short out and die again. But I'm not really seeing anything else on this board that could cause it. There really aren't that many components on this power board in the first place, but it's amazing how finicky it can be with even a single resistor being out.

                            Originally posted by g1 View Post
                            Those 2K2 grid resistors are very critical. Replace the discoloured ones, it's not worth risking loss of bias for a couple cents.
                            When you have the bad socket removed, post a pic of the underside of the socket, and the board area under where the socket was.
                            Will do. I'll have to go to a shop and pick up some 2K2 resistors though.

                            I could swear these were not discoloured a few days ago when I began troubleshooting. They must have begun to cook during all of my testing/arcing/smoking.

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                            • #44
                              If the burned 2K2 is the gridstop and it has gone open circuit then there is no bias being applied for that tube. It will conduct max current and blow a fuse.
                              Replace it.
                              Cheers,
                              Ian

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                              • #45
                                Originally posted by Gingertube View Post
                                If the burned 2K2 is the gridstop and it has gone open circuit then there is no bias being applied for that tube. It will conduct max current and blow a fuse.
                                Replace it.
                                Cheers,
                                Ian
                                Note, that resistor is still holding its value, despite being burned. And it's connected to one of the good, working tube sockets. I'm still going to replace both of them though.

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