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So here’s a fun one (AD-100 clone woes)

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  • #31
    Also, where does the OT get it's supply from, the same node as the screens? With no common dropper resistor between the OT and screen feeds?
    As you mentioned, disconnecting one of the 100uF's would be the quickest method of dropping that node capacitance.
    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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    • #32
      As you mentioned, disconnecting one of the 100uF's would be the quickest method of dropping that node capacitance.
      Yes, but I don't think less capacitance will noticeably increase sag and relieve the screens. KT88s should be safe with a stiff screen supply voltage of 400V and individual screen resistors >1.5k.

      This said, a reservoir cap of 100µ should be completely sufficient.
      Last edited by Helmholtz; 01-18-2020, 07:45 PM.
      - Own Opinions Only -

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      • #33
        Originally posted by Mr_bibbles View Post
        could a loss of bias cause a heater to plate short though?
        For my small experience a plate to heater short is usually caused by a bad tube. But it's hard to say what will happen in a tube if it's stressed and overheated. A short in a power tube is usually an arc. That requires a high differential in voltage. So I would imagine a short from plate to anywhere else requires a voltage spike. This can happen under a few different conditions, but a load that's too high or an open load would seem the most likely scenario.


        Originally posted by Mr_bibbles View Post
        Here’s a picture of the speaker jacks, are these already installed dummy load resistors you’re suggesting?
        Those would be "dummy load" resistors... If they were 220 ohms! At 4.7k they won't drop enough current across their load to affect the voltage through the OT and on the plates significantly at all. My personal amp (I have several, but the one I play through most), for example, is 20W. It has a "line out" circuit that includes a 2.7k 1/2W resistor and a 500 ohm pot rated about the same I'll guess. There has been more than one occasion when testing and other bench shenanigans that I've run the amp into an open load WITH signal at the input and the amp cranked. Fortunate for me I used a burly OT and there was no damage, BUT... The only load on the amp would have been the line out circuit at something like 3k total and rated for less than 1W with both components in series (considering the pot was probably at five?). That circuit did not roast. Which it should have were it dropping more than a couple of watts of the amps total power across it's relatively high resistance components. Those resistors are doing basically nothing. You should change them out for the suggested 220 ohm value.

        Originally posted by Mr_bibbles View Post
        The PI caps are .1, I could try dropping hem to .047 if I don’t find a vibration fault?
        That depends on a couple of things. For one, with eight kt88 tubes the grid load resistance is probably quite small. A small resistance here would require a larger cap value for an acceptable knee frequency. But even if your grid loads are something like 100k a .047u coupling cap value would have a -3dB of something like 25(ish)Hz. So your customer isn't likely to hear a difference or notice at all. Unless he's trying to communicate with submarines . A reduction in extreme LF can't hurt since it's just useless current affecting the power tubes.

        Originally posted by Mr_bibbles View Post
        What changes would you recommend to the filters? Just disconnect one of the 100’s?
        g1 already covered this, but I'll back him here in the interest of completing my response. Disconnecting one cap should be fine.
        "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

        "If you're not interested in opinions and the experience of others, why even start a thread?
        You can't just expect consent." Helmholtz

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        • #34
          Helmholtz,

          Thanks for your reply!

          I’ve meticulously cleaned the sockets and replaced the one that arced, it had melted a bit.
          The sockets are almost new do not look like they need retensioning. Each socket showed healthy current draw with a bias probe last time it was on the bench.

          I actually do mean DC for the transformer leads, they’re tied to the rectifier. That measurement is with standby off.

          Also, I took a closer look and it’s actually 1n4005, but these are only for the screen supply. There is a string of 8x 5408’s tied to the primaries, does this make a difference in the acceptability of the 1n400x series?

          Finally, Good thinking on the fuse i’ll drop it down.
          Last edited by Mr_bibbles; 01-18-2020, 07:57 PM.

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          • #35
            The OT is supplied by 2x 250m cap cans in series, tied to 8 5408’s (is this a dual rectifier??)
            Pics:
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            The red leads from the cap go to B+ of the OT, and the 5408’s as seen in the pic.

            The black lead from the diodes is tied to ground via standby switch, and the two red leads in the center are HV secondary from transformer

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            • #36
              Also, I took a closer look and it’s actually 1n4005, but these are only for the screen supply. There is a string of 8x 5408’s tied to the primaries, does this make a difference in the acceptability of the 1n400x series?
              Oh, this is new info. So you have separate screen and HT supplies? Please try to post a drawing of both.
              - Own Opinions Only -

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              • #37
                Click image for larger version

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                Here is HV supply- the red wires attached to the diodes lead to HV secondary transformer wires. The red lead from the filters lead to OT B+.

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                • #38
                  If these pictures don’t do it for you I can sketch the supply! Those caps are 2x250m in series

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                  • #39
                    I can sketch the supply
                    Yes, please sketch both (screen and plate) supplies showing filter caps and voltages.

                    (Please try to not mix up m (=milli) and µ (=micro). 1mF equals 1000µF.)
                    - Own Opinions Only -

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                    • #40
                      Chuck I missed this earlier (my phone is not the most effective forum reading device), but thank you very much for your input! Great stuff here.

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                      • #41
                        Here are the supplies, sketched. There is a diagram of the KT88 too from earlierClick image for larger version

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                        • #42
                          Originally posted by Mr_bibbles View Post
                          Here are the supplies, sketched. There is a diagram of the KT88 too from earlier[ATTACH=CONFIG]56737[/ATTACH][ATTACH=CONFIG]56738[/ATTACH]
                          Ok, thanks.
                          The 6A screen fuse and 200µ screen reservoir capacitance make absolutely no sense. Add to this a missing HT fuse.
                          This all shows that the "designer" had no idea what he was doing.

                          Nevertheless it gives no clue to the failures.

                          I still suspect an intermitting load connection or plate pin contact.
                          - Own Opinions Only -

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                          • #43
                            Thank you for your reply- after your comment about the fusing system it led me to examine the three fuses... one is a 8a mains attached to primaries, one is a 6a screen fuse.... and one is a 2a fuse attached to the ground through the 1 ohm resistors.. I had already measured the rest of the voltages for vibrational disconnect under duress.... but I never measured the ground connection! It was being disconnected at the fuse holder with even slight vibration. His has to be the issue, right???


                            I’m considering replacing that fuse holder and converting it to a HV fuse, hardwiring the ground to avoid any further issues. Can anyone tell me how to determine fuse ratings via the voltage, or if this is a fools errands?

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                            • #44
                              I thought about a way to calculate the appropriate rating of the screen supply fuse and came up with the following:

                              The (absolute) max screen dissipation of a KT88 is 8W. Times 8 (tubes) makes 64W. Dividing by screen supply voltage (400V) gives the max allowable total screen current as 0.16A. So a 200mA fuse should be fine.

                              Comments welcome.
                              Last edited by Helmholtz; 01-24-2020, 08:57 PM.
                              - Own Opinions Only -

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                              • #45
                                Originally posted by Helmholtz View Post
                                I thought about a way to calculate the appropriate rating of the screen supply fuse and came up with the following:

                                The (absolute) max screen dissipation of a KT88 is 8W. Times 8 (tubes) makes 64W. Dividing by screen supply voltage (400V) gives the max allowable total screen current as 0.16A. So a 200mA fuse should be fine.

                                Comments welcome.
                                My only consideration might be the tendency for overdissipation to be balanced with periods of much lower dissipation WRT clipping circuits. Which tubes handle rather gracefully. We see this a lot in guitar amps. Since fuses in these locations tend to be regular fast blow type I might be concerned about popping fuses whenever the amp is clipping.?.
                                "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

                                "If you're not interested in opinions and the experience of others, why even start a thread?
                                You can't just expect consent." Helmholtz

                                Comment

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