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  • Tuesday report :
    I don't have much time to work on the amp (1 hour max, I'm home at around 8.30pm back from work) but even tough I tried to experiment.
    I have a question about the tremolo footswitch : when no footswitch is plugged in, is it supposed to act like a normally open switch ? Should the wire that goes to the footswitch be linked to ground in order to make the tremolo work or is it the opposite ?
    I reflowed every solder and now the reverb is not working any more (but I believe it can easily be fixed so I don't really care for the moment).
    The main problem that remains is that loss of volume which I believe is rather a gain loss. I tried different tube with no success. Pulling V1 brings back a little of the missing volume thanks to the added gain. I still have some breakup but way less than stock which makes me think the power section is fine and the problem resides in the preamp section. Everytime I try something new, even if it as simple as moving or reflowing a single solder a new problem appears, haha and that's for real.
    I'll try to post the pins voltages of the preamp tubes (except for the plate resistors and pin 3 of power tubes, everytime I touch them there's a blue electric arc popping somewhere, randomly, in the amp.
    I have to mention, I've been trough every component, every jumper, every wire,... everything is at the right place at the right value.
    Have a nice day !

    Comment


    • Originally posted by Wil View Post
      (except for the plate resistors and pin 3 of power tubes, everytime I touch them there's a blue electric arc popping somewhere, randomly, in the amp.
      This should be dealt with first. Who knows, it may even solve your problem.
      "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

      Comment


      • Originally posted by gui_tarzan View Post
        Truth! I learned that the hard way.
        That made me laugh out loud! Hahahaha! Yes, graphite is a pretty good conductor!
        When I was about 10, I read about carbon arcs. So my [older] brother and I got some lamp cord, and whittled the wood away so we could wrap the cord around the pencil leads. He got the plug end, and I got the pencils. We were of course hiding back in a tiny enclosed dark cubby hole.

        Amazing how, now matter how spectacular, potentially dangerous, or even near death experience: the VERY first thought in my head was "My parents are gonna kill me!!" (The next thought was "I'M BLIND!!!") Fortunately, neither was true.
        The prince and the count always insist on tubes being healthy before they're broken

        Comment


        • Originally posted by g-one View Post
          This should be dealt with first. Who knows, it may even solve your problem.
          Maybe my meter is wrong. But when I try to measure voltages on the 100k plate resistors (at the end toward the tubes), I have arcing between the solder and the meter wire. Once it even made a hole trough the shielded that cover the twisted wire of the OT that go to pin 3 of the power tube. And on my last try, it started a mini blue fire at pin 3 of V7. If you have any idea on how to resolve this, that'd be great :-)
          (next time I'll try with a crocodile clip and setting the standby on and off each time).
          Have a nice day,

          Comment


          • I tried the amp with only V1 and V6 plugged in and still I have that lack of volume. (I swapped several tubes with no improvment)
            Bias current goes from idle (19mA) to 70/75mA when cranked and hit hard, which makes me believe the power section is fine.
            I changed the PI and coupling caps,... and still that problem.
            Tried with a 5U4GB, no improvment, tried other 6V6, no improvment...
            Now I will try to scratch between the eyelets to be sure nothing shorts to ground or to something it should not be connected.

            Comment


            • Originally posted by Wil View Post
              I tried the amp with only V1 and V6 plugged in and still I have that lack of volume. (I swapped several tubes with no improvment)
              Bias current goes from idle (19mA) to 70/75mA when cranked and hit hard, which makes me believe the power section is fine.
              I changed the PI and coupling caps,... and still that problem.
              Tried with a 5U4GB, no improvment, tried other 6V6, no improvment...
              Now I will try to scratch between the eyelets to be sure nothing shorts to ground or to something it should not be connected.
              "70/75mA when cranked and hit hard"

              Open (bad) speaker, bad speaker cable, speaker jack(s) broken / disconnected, impedance selector switch bad, secondary of output transformer is open?
              It seems like the amp is working and the speaker is disconnected or blown somehow.

              Un plug the speaker from the amp and touch a 9V battery to the speaker connections and speaker plug.
              The speaker should make a loud POP when the battery is touched to it, or the speaker cable plug too.
              You need a good fresh battery to test this. A weak battery won't do it.

              Take the speaker out, check to see that the cone is moving in/out freely.
              A blown speaker might be rubbing the voice coil against the magnet.

              This stops the speaker from moving in and out, it's frozen. This causes very low volume.
              A blown speaker will have very low volume, or no volume.

              Move the speaker cone in and out gently, with your fingers. Does it scrape? OR is it free to move?

              You have checked that the voice coil is free?
              You have checked with the battery that the speaker works?
              You have checked with the battery that the speaker cable works?
              Those 3 are good tests when you have low volume / no volume.

              Hook the amp to a different speaker. Use a different speaker cable.

              Comment


              • Thank you soundguruman.
                Your answer match with a "discovery" I've just made.
                The speaker is a brand new WGS G12C so I hope it's not already blown.
                But there is connectivity between the black and the green wire of the OT. Is that normal ?
                (I twisted them, also. Shouldn't I ?)
                I have a eminence swamp thang which I know is in proper working condition. I'll try that one later !

                Comment


                • Originally posted by Wil View Post
                  Thank you soundguruman.
                  Your answer match with a "discovery" I've just made.
                  The speaker is a brand new WGS G12C so I hope it's not already blown.
                  But there is connectivity between the black and the green wire of the OT. Is that normal ?
                  (I twisted them, also. Shouldn't I ?)
                  I have a eminence swamp thang which I know is in proper working condition. I'll try that one later !
                  yes black to green is a winding and should have continuity.
                  seems like it's a speaker disconnected somewhere?

                  Comment


                  • Originally posted by soundguruman View Post
                    yes black to green is a winding and should have continuity.
                    seems like it's a speaker disconnected somewhere?
                    I rewired the speaker with no improvment, unfortunately.
                    I will try changing the filter caps once again and see what it does... They're brand new so they should work properly.

                    Comment


                    • Instead of changing them, test them.
                      They either remove Vac ripple or they don't.

                      Comment


                      • Hi everyone,
                        Remember me? 4 years ago you helped me build my first amp. An Hoffman style Deluxe reverb. It worked for some time then I ruined it by experimenting a couple of stuff. A lot of professional work prevented me from fixing this amp but now I'm back at it (and obviously need your tremendous help again, haha!).

                        Here's my situation. The problem:
                        - The amp sounds ok even if it's a little noisy
                        - Tremolo doesn't work
                        - The amp breaks up, the power section seems to work as it should
                        BUT, it has as much volume as a 2W amp.
                        When I push the amp for a couple of minutes, the bias becomes really inconsistent

                        Currently, what I have tried is:

                        - Check all the resistors : everything is good
                        - Swap tubes : found one bad 12AX7 but apart from this, everything is good.
                        - Swap speaker : from a WGS G12C to a Eminence Swamp Thang. The Eminence was obvisouly louder but still not loud enough.

                        Here's a couple of measurement I took:

                        Output Transformer :
                        B+ to V7 plate : 138 ohms (traces of fire on the pin/cable)
                        B+ to V8 plate : 125 ohms

                        all to ground : over 2M

                        PT
                        between primaries : 8ohm
                        red/white to ground : 10 ohm

                        rectifier filament : 1ohm
                        rectifier filament to chassis : infinite

                        Unloaded :
                        V9 pin 8 to chassis (B+) : 5V = V8 pin 3 = V7 pin 3
                        This is the weirdest. I tried with two different (rather cheap) multimeters and the displayed the same thing.

                        If you have any idea, I'm all ears!
                        It doesn't seem to come from the power section, neither does it come from the tremolo/reverb section (tried without the associated tubes with no success)... Next thing I'm going to check is, as simple as it is, the input.

                        Ready to take a look at the worst build ever? There you go! (It once used to be pretty ok but experiment led me to this... I'm going to put the filters caps back in the dog house!)
                        Thank you in advance.

                        Click image for larger version

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                        Comment


                        • You say you ruined it by experimenting with stuff, so I guess you mean modifications. Have you put it back to normal stock circuit?
                          Is that layout paper in the photo the layout you are using? Can you post it so we can see it better?
                          For your B+ where you are measuring 5V, is your meter on DC volts?
                          What is the red/white wire of the PT, and why is it measuring 10 ohms to ground?
                          Why is there no resistance to ground from the heater winding? Is the green/yellow center tap disconnected?
                          "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

                          Comment


                          • Originally posted by g1 View Post
                            You say you ruined it by experimenting with stuff, so I guess you mean modifications. Have you put it back to normal stock circuit?
                            Is that layout paper in the photo the layout you are using? Can you post it so we can see it better?
                            For your B+ where you are measuring 5V, is your meter on DC volts?
                            What is the red/white wire of the PT, and why is it measuring 10 ohms to ground?
                            Why is there no resistance to ground from the heater winding? Is the green/yellow center tap disconnected?
                            Hi,
                            Thank you for the answer.
                            The diagram I used is the following:
                            http://el34world.com/Hoffman/images/Hoffman_AB763.pdf

                            Yes the meter is set on 600VDC.
                            By "modifications" read playing with different values, adding a variable NFB resistor, putting filter caps on the main board (which is super stupid in this case but I remember I saw someone doing this with some success)...
                            I think I tried everything to put it back to original without any improvement.
                            For B+ I measure 5VDC at pin 8 of V9 (JJ Rectifier = GZ34), but also at the positive end of my filters caps (right after my stand-by switch).
                            The Red White wire of the PT is the red/blue on the schematics and goes to the bias range resistor. And it actually is measuring infinite ohm to ground (sorry for the mistake).
                            The center tap of the heater wires is disconnected and isolated.

                            Tubes do get hot as they should some I don't think the filaments are to blame.
                            Forget what I said with the 5VDC, I guess I will need to re-learn how to properly work on amps.
                            Plate voltage of both power tubes is 455VDC as well as the B+ (455VDC). This is with the speaker and the reverb tank removed, as well as V1 V3 and V5 (testing as is because I don't think the problem comes from the normal channel or the reverb and vibrato themselves).
                            I can't seem to manage to get any bias reading though... 0.00ma.

                            This thing is a mess. I think I'll start over again. I just want to check the transformers are fine.
                            Apart from what's on GEOFEX is there any test I can run to be sure they're ok?

                            Thanks.
                            Have a nice day,
                            RW

                            Comment


                            • Forget testing bias current. Do you have bias voltage? And when you say the speaker is removed, do you mean there is no load on the amp at all or are you using a resistive load?
                              "Take two placebos, works twice as well." Enzo

                              "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

                              "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

                              Comment


                              • Originally posted by Chuck H View Post
                                Forget testing bias current. Do you have bias voltage? And when you say the speaker is removed, do you mean there is no load on the amp at all or are you using a resistive load?
                                I couldn't get any neither. I'm at work now so I can't take measurements.
                                But yesterday evening, the amp wouln't produce any sound at all.

                                To avoid any headache and avoid wasting your time as well with such a messy build, I'm gonna start from scratch.
                                I'll keep you updated on my progress :-)

                                Comment

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