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70s Ampeg V4B Blows Fuses

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  • Originally posted by vintagekiki View Post
    Read as it is written.
    Don't philosophize.
    Three(3) power tubes measured 1.2mv on pin#8, 4th power tube burnt up R28. Replacing R28 or can I leave R28 out to test more power tubes?

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    • Originally posted by ca7922303 View Post
      Three(3) power tubes measured 1.2mv on pin#8, 4th power tube burnt up R28. Replacing R28 or can I leave R28 out to test more power tubes?
      Burn R28 (cathode resistor V4 / 12AU7) 1k5 has nothing to do with measure 4th power tube (6L6).
      Replace R28.
      If measured voltages as in your post #180 [# 3 (586v), # 4 (596v), # 5 (-63v)] and on pin # 8 tube V5 you have 1.2mv these are unusable tubes .
      You should expect a voltage > 20mVDC on pin # 8 of tube V5.
      What is the value of the sensor at pin # 8 tube V5?
      Please upload photo V5 tube socket.
      Who does not know and knows that he does not know - teach him Confucius)
      Who knows and does not know that he knows - wake him Confucius)

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      • Originally posted by vintagekiki View Post
        Burn R28 (cathode resistor V4 / 12AU7) 1k5 has nothing to do with measure 4th power tube (6L6).
        Replace R28.
        If measured voltages as in your post #180 [# 3 (586v), # 4 (596v), # 5 (-63v)] and on pin # 8 tube V5 you have 1.2mv these are unusable tubes .
        You should expect a voltage > 20mVDC on pin # 8 of tube V5.
        What is the value of the sensor at pin # 8 tube V5?
        Please upload photo V5 tube socket.
        Any idea why 4th power tube tested, caused R28 to burn?

        Comment


        • Originally posted by ca7922303 View Post

          Any idea why 4th power tube tested, caused R28 to burn?
          leave philosophy. read #197

          1)

          How much did you help to make R28 burn.
          Last edited by vintagekiki; 05-02-2021, 12:33 AM. Reason: 1)
          Who does not know and knows that he does not know - teach him Confucius)
          Who knows and does not know that he knows - wake him Confucius)

          Comment


          • Originally posted by vintagekiki View Post
            Burn R28 (cathode resistor V4 / 12AU7) 1k5 has nothing to do with measure 4th power tube (6L6).
            Replace R28.
            If measured voltages as in your post #180 [# 3 (586v), # 4 (596v), # 5 (-63v)] and on pin # 8 tube V5 you have 1.2mv these are unusable tubes .
            You should expect a voltage > 20mVDC on pin # 8 of tube V5.
            What is the value of the sensor at pin # 8 tube V5?
            Please upload photo V5 tube socket.
            Are you asking me to get a voltage measurement without tube installed on V5 pin#8?

            Comment


            • Originally posted by vintagekiki View Post
              leave philosophy. read #197

              1)

              How much did you help to make R28 burn.
              The three(3) power tubes that I tested were tested in a tube tester at a amplifier service in Nashville, TN before I began working on this V4B. They tested good. Three(3) other power tubes I had with me also tested bad. I'm curious what could have caused them to be bad now?

              Comment


              • Originally posted by g1 View Post
                After you get that socket working, try this test again to see if power amp & supply are still basically working: https://music-electronics-forum.com/...556#post929556
                Had tubes in V6&V7, ran signal to ext. amp jack, have output. Have a loud hum, but can hear signal. Used two tubes that we're used in one at a time test in V5 today.

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                • Also tested two(2) tubes in V5&V8, same result, can hear signal thru ext. amp jack and loud hum.

                  Comment


                  • Originally posted by ca7922303 View Post
                    Are you asking me to get a voltage measurement without tube installed on V5 pin#8?
                    Where is sensor resistor (1 Ohm) from pin 8 (cathode) tube V5 to the chassis???

                    Click image for larger version

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                    Did you look at the schematic.
                    Who does not know and knows that he does not know - teach him Confucius)
                    Who knows and does not know that he knows - wake him Confucius)

                    Comment


                    • Originally posted by ca7922303 View Post
                      I'm curious what could have caused them to be bad now?
                      Let go of philosophizing.
                      Where is resistor 1 Ohm ?
                      Do you have any idea what you measuring ?

                      If you are really interested in the cause, get educated, study, read technical books.
                      This is just a waste of time.


                      Who does not know and knows that he does not know - teach him Confucius)
                      Who knows and does not know that he knows - wake him Confucius)

                      Comment


                      • Originally posted by ca7922303 View Post
                        Also tested two(2) tubes in V5&V8, same result, can hear signal thru ext. amp jack and loud hum.
                        You're wrong somewhere in test.
                        Who does not know and knows that he does not know - teach him Confucius)
                        Who knows and does not know that he knows - wake him Confucius)

                        Comment


                        • Originally posted by vintagekiki View Post
                          Let go of philosophizing.
                          Where is resistor 1 Ohm ?
                          Do you have any idea what you measuring ?
                          1ohm resistor? I have been trying to follow this thread but have found it difficult to pinpoint exactly what we are fixing. Anyhow did we advise to install 1 ohm precision resistor(s) for testing? In my head I was picturing a huge SVT with those 1 ohm resistors on the board somewhere but this V4 beast does not have the same setup.
                          When the going gets weird... The weird turn pro!

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                          • Originally posted by DrGonz78 View Post
                            1ohm resistor?
                            Only one 1 ohm resistor, temporarily in the air ...
                            https://music-electronics-forum.com/filedata/fetch?id=930419&d=1619849061
                            Who does not know and knows that he does not know - teach him Confucius)
                            Who knows and does not know that he knows - wake him Confucius)

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                            • Originally posted by vintagekiki View Post
                              Only one 1 ohm resistor, temporarily in the air ...
                              https://music-electronics-forum.com/filedata/fetch?id=930419&d=1619849061
                              Oh okay I missed that schematic with the suggested install of the resistor and testing points. Pretty long thread and tough to navigate what is going on. Thanks Vintagekiki!
                              When the going gets weird... The weird turn pro!

                              Comment


                              • Originally posted by vintagekiki View Post
                                Only one 1 ohm resistor, temporarily in the air ...
                                https://music-electronics-forum.com/filedata/fetch?id=930419&d=1619849061
                                I don't know how you expected him to understand this. It is beyond his experience level. I didn't even catch it til I blew up that schematic. It's just a tiny thumbnail that looks like a copy of the regular schematic.
                                Why are you chasing power tube quality, it is not important now. It was determined earlier (post #97 & 98) that the power amp was working and no signal was getting through the preamp.
                                Originally posted by ca7922303 View Post
                                Appreciate that. Ran signal generator to ext. amp jack and am getting output. So preamp is at fault.
                                Originally posted by ca7922303 View Post
                                Unable to hear signal from V4b ext. Jack to other amps input.
                                "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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