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

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  • #46
    It was being disconnected at the fuse holder with even slight vibration. His has to be the issue, right???
    Actually, don't ask that. Clearly it needs to be corrected, so correct it. THEN tell us if it made the difference.
    Education is what you're left with after you have forgotten what you have learned.

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    • #47
      Originally posted by g1 View Post
      Multiple arcing issues make me immediately suspicious of too high cab impedance or speaker cable going open.
      Did the 1st replaement PT fail or was the 2nd replacement just to try with tamer voltages?
      Working my way through this thread so I will see how it all ended, but G1 your intuition sounds good to me in any event.

      So far I've been lucky with OT transformers or tube shorts, but my bandmate had a scare when we were playing a gig and the crimp speaker connection fried at the speaker wire connection, and he played for a perhaps 10-15 seconds that way full tilt, as he could barely hear himself over the drums. Turns out no carnage, he dodged the bullit.

      Since then I have been using 625v Varistors between each leg of my push pull plates and center tap, just incase a thing like that happens to me. It's supposed to dump voltage and current back to ground and protect things, for a while at least, and may buy you time before things light up.

      You have the best guys helping you out on this, but I think you know that. Hope the ground was the issue !
      Last edited by HaroldBrooks; 01-25-2020, 06:19 AM.
      " Things change, not always for the better. " - Leo_Gnardo

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      • #48
        How many amps have a fused screen supply? I can't recall one right now. Fusing the entire tube set off one fuse gives a problem in identifying if a single tube blew the fuse. If so, which one? I would rather use individual fusible resistors (FF type) for each tube screen as Fender does. It's always easy to determine if a single tube or location is problematic. The peak screen current can sometimes be momentarily higher than the specified maximum in some amps (as Chuck points out) which leads builders to either increase the wattage of screen resistors or use carbon comp screen resistors for their better surge characteristics. Usually these amps run higher screen volttges than I would like. Dropping screen voltage can give prolonged service life for tubes without a huge impact on output.

        Fusible resistors are quite useful. Marshall uses FF1R resistors between cathode and ground in some amps, as well as overall fuse protection for the HT circuit. A benefit is a 4-tube amp will keep running in the event of a single tube failure. The downside is the same.....

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        • #49
          I don't recall ever seeing a fused screen supply except in cases where they were separate supplies, such as a 700v plate supply and a 300v screen sup-ply. generally screens are just a node on the B+ string.
          Education is what you're left with after you have forgotten what you have learned.

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          • #50
            Originally posted by Chuck H View Post
            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.?.
            I don't think the balancing of overdissipation works for screens as they have much lower thermal inertia than plates and heat up very fast.

            My proposal above already considers some abuse:

            - The individual screen resistor reduce actual screen voltage and dissipation at screen current peaks.
            - Peak screen currents are supplied from the large filter cap and the fuse only "sees" total average current.
            - I was going to propose a slow blow fuse, because a fast fuse might trigger by the the turn-on inrush current of the filter cap. A SB fuse opens after 2 minutes of 2.1 times rated current

            This said, the question remains, what a common screen fuse is actually meant to protect. It is not able to protect individual screens, but may provide some protection for the PT screen winding.

            I did this exercise mainly to demonstrate that a 6A screen supply fuse is ridiculous and to get a feeling what realistic (averaged) max screen currents could be expected.
            Last edited by Helmholtz; 01-25-2020, 03:48 PM.
            - Own Opinions Only -

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            • #51
              So the ground disconnect was verifiable, I removed the ground fuse and hardwired it to ground. I guess before I was asking if a disconnected ground reference would account for all the previous failures- I’m fairly certain 8 kt88’s with no ground reference would experience some ‘slight’ issues .

              Although it seems that a ‘global’ screen fuse is not the right way to go, I’m probably going to leave it in rated slightly above Heimholtz’s great recommendation, if not only for protection against direct shorts from the way-too-close-together tube rails. After all it’s already installed...

              So if you can calculate screen fusing using screen voltage and screen dissipation, you could so the same for a Hv fuse yes? 8 kt88’s at 40w a piece, (320) divided by he plate current of 500? .64 amps at maximum dissipation?... corrections appreciated

              And lastly, I have some 270 ohm 10w on hand, this should work for dummy loads, yes? At least work better than the 4.7ks?

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              • #52
                So if you can calculate screen fusing using screen voltage and screen dissipation, you could so the same for a Hv fuse yes? 8 kt88’s at 40w a piece, (320) divided by he plate current of 500? .64 amps at maximum dissipation?... corrections appreciated
                Plates are much more rugged than screens and can take (and probably see) more abuse. So although tube data suggests something like a 1A HV fuse, I would go with a 3A fuse HV fuse as used in similar high power amps. Also as said before a supply fuse will not protect single tubes but rather the PT.
                - Own Opinions Only -

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                • #53
                  Thanks! 3A seems reasonable and I will go with that. Is there an accompanying explanation of WHY they see abuse above the chart ratings?

                  Also, can you give any insight about the 270 ohm dummy loads, or point me to a reference on how to calculate and select dummy load protection?

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                  • #54
                    I think Helmholtz sort of covered this responding to me earlier. The screens, both ideally and in their circuits, don't see the same variation in current that the plates do. So the concession I was suggesting for the screen fuse rating, while not ideally applicable for screens, is applicable for plates. That is, you want to bump the rating a little for those peaks of higher dissipation.

                    And 270 ohms should be fine. But I believe your 4.7k resistor safety loads in there now are wired in parallel, right? So it's actually a 2.35k safety load. Still too high, but my point is that if you were to wire the 270 ohm resistors the same way the you'll have a 135 ohm safety load. Which seems a bit low and might noticeably affect performance. If you just used one 270 ohm at 10W I think the rating would be a little low as that resistor will probably dissipate a little more than that intermittently if no speaker is used. That could open the resistor and render the safety load moot. So, to wire two resistors like it is now you would replace the 4.7k resistors with 470/10W resistors or a single 220 ohm 20W resistor.
                    Last edited by Chuck H; 01-25-2020, 08:09 PM.
                    "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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                    • #55
                      Aha! So it’s a matter of ‘peaks’ through normal
                      operation? The screens will remain more constant while the draw of the plates will vary with a wider degree? This is honestly beyond my understanding but this project has been illuminating.

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                      • #56
                        Is there an accompanying explanation of WHY they see abuse above the chart ratings?
                        Tube data is always for linear unclipped operation and specified optimum load. Clipping and deviation from optimum load can strongly increase tube dissipation.


                        Also, can you give any insight about the 270 ohm dummy loads, or point me to a reference on how to calculate and select dummy load protection?
                        I think what you call dummy loads are base loads that protect the OT from arcing over in case of load-drop (open output). AFAIK, values of 470R and lower work ok.
                        Too low values dissipate and steal a lot of power. I don't know of a way to calculate an optimum resistance.
                        Such base loads will not prevent melting of tube screens if the amp is operated without speakers connected.
                        - Own Opinions Only -

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                        • #57
                          This was very helpful. Thank you all for your knowledgeable replies. Chuck, good catch! With no jack plugged in, it would be in parallel, however with the jack’s shorting lug lifted, it only engages one, so I think 470r would be a great fit because as y’al mentioned, 220-470 is the acceptable range.

                          EDIT: at no time are these resistors paralleled, with zero, one, or two jacks plugged in the resistance is always 4.7k.


                          I’ll update you if anything else happens, but I feel much more confident that this will run smoothly and have better protection against failure and misuse in the future. Thank you all for your help, as always.

                          Long live the forum.

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                          • #58
                            Originally posted by Mr_bibbles View Post
                            With no jack plugged in, it would be in parallel, however with the jack’s shorting lug lifted, it only engages one,..
                            I'm trying to logic this out and coming up short. I think the idea must be to prevent loading when a speaker is in use. But this fails to provide equal protection should a speaker system fail. Like an open cord/jack, blown speaker, etc. Which seems just as likely as anything else. I would absolutely wire it up for a single 220 ohm 20W resistor parallel to the output at all times. The amp will suffer a loss of about 7W peak performance. Your guy would never notice any difference between 200W and 193W. I promise.
                            "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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                            • #59
                              Why 200W?
                              With 8 (!) KT88s and 500V plate voltage, 400V screen voltage this amp should deliver in excess of 400W (provided that PT and OT support that power).
                              - Own Opinions Only -

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                              • #60
                                The largest matched assembly of 10watters I have on hand is 2x350ohm, for a combined 175 ohm 20watt. I also have a 2.7k/270r pair of 10 Watts I could use, for a total of 245r, 20w. Would his be safe considering the imbalanced resistance of the pair? That would change the handling of each individual resistor, correct?

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