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Installing SS Rectifiers for Tube Rectification

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  • #16
    Originally posted by trobbins View Post
    I would also recommend using one 250VAC fuse in the CT, rather than 2 in the secondary arms, as there may be collateral damage if one fuse fails.
    What concerns me about a single fuse in the CT, is the fuse will only offer protection if a fault occurs in the rectified circuit, or if a leg of the secondary shorts to ground.
    If there is a fault in which the AC side two secondaries short together, a fuse in the CT is of no use. Right?
    If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

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    • #17
      Correct - and is why adding ss diodes in series with valve diodes is so worthwhile, as the ss diodes pretty much eliminates valve diode arcing and failing, and the collateral damage to the PT that seems to be commonly associated.

      A CT fuse (or winding arm fuses) are only really practical for faults on the B+, or the output stage, as any dropping resistance for preamp B+ supplies will typically restrain a fault current to being much lower than the fuse rating. Even with a fuse, if its rating and type are not appropriate, it is likely to just sit there and not blow at all for even a typical fault like an output stage valve being full on 24/7.

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      • #18
        Ahhh, I see what you mean. The silicone diodes protect the common mode of failure in tube rectifiers. So, in this application, most all the other common faults occur in the rectified circuit (tube shorts, filter cap failure, OT, etc)
        Whereas, solid state rectifies are more likely to fall short.
        If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

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        • #19
          Originally posted by Helmholtz View Post
          Or use single sufficiently rated (1250V/1A or higher) diodes like BY127/227/133/255 without the caps.

          https://www.donberg.fr/catalogue/sem...bz/page_2.html
          Ordered some RY255 diodes yesterday morning... So yea, caps eliminated.

          @trobbins
          Merlin seems to suggest, unless I misunderstand, that silicon will fault before a tube does. Even though in 30 years I have never seen either of them fault, I have used more SS rectified amps than tube rectified.

          I suppose at this point with adding the RY255 (or any diodes that can handle the HT) before the rectifier tubes, the tubes are nothing more than a glorified vdrop resistor... ?

          Perhaps at this juncture, having amended and fixed my schematic, someone could take a look over the whole power section to see if there are any other faults??

          Click image for larger version

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          "'He who first proclaims to have golden ears is the only one in the argument who can truly have golden ears.' The opponent, therefore, must, by the rules, have tin ears, since there can only be one golden-eared person per argument." - Randall Aiken

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          • #20
            There is inherent balancing of PIV if any leakage from one ss diode starts to occur
            Good point!

            After all, straight series wiring of rectifier diodes was quite common in the early days when high enough PIV diodes were not available.
            Last edited by Helmholtz; 08-20-2019, 01:41 PM.
            - Own Opinions Only -

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            • #21
              Originally posted by Gtr0 View Post
              @trobbins
              Merlin seems to suggest, unless I misunderstand, that silicon will fault before a tube does.
              All parts have their own failure modes and failure rates. There is no general comment that is applicable here. Understanding comes from assessment, so it would be good to locate the comments/sections you have read.

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              • #22
                Another thought occurred to me yesterday, if the CT should be fused in the above case, where does one fuse a full wave bridge setup where the CT goes between the first series set of caps? Still the CT?
                "'He who first proclaims to have golden ears is the only one in the argument who can truly have golden ears.' The opponent, therefore, must, by the rules, have tin ears, since there can only be one golden-eared person per argument." - Randall Aiken

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                • #23
                  A circuit configuration like that needs a variety of situations to be assessed, and there also needs to be some definition of what the load or loads are.

                  For example, it could be powering a valve amplifier with the 50% B+ connected to the screen(s) of an output stage, and the 100% B+ connected to the OT/anodes. In that case you don't want a fuse to somehow allow the screens to be powered if the plates have no power.

                  If you fused the secondary arms, and one fuse blew, then you would effectively have a doubler supply still power the load(s). So then you have to categorise why the fuse may have blown - if it was from a heavy load across the 100% output, then the load would likely soon take out the other fuse.

                  If you fused the CT only, and it blew, then you would still have a 100% rail, but the the 50% point may change depending on what loads were being applied and where.

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                  • #24
                    Yes, I was looking into adding fusing for the HT winding of a Fender 75 a while back, and decided it was too complicated. The CT feeds the half voltage output used for the low 1/4 power mode.
                    https://el34world.com/charts/Schemat.../Fender_75.pdf
                    Both red HT winding legs and the red/yellow CT would seem to need fusing but would there be any deliterious consequences should one of them blow spuriously? eg might one the stacked reservoir caps potentially get reverse biased?

                    Regarding fusing the HT CT return to 0V of a regular biphase arrangement, my understanding is that it shouldn’t be done for HT windings with a bias supply tap, as per eg TR AB763, as when the fuse blows, large voltage can appear the bias supply, potentially damaging components there. I’ve not breadboarded it out (yet) though.
                    Last edited by pdf64; 08-22-2019, 08:36 PM.
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                    • #25
                      Some real smart amp wizards posted that the resistance between clips and fuse pose a problem when fusing the HV. i read and read, and found somebody that solders wire to the fuse caps to get around that problem. Knowing my soldering skills so far, I gave up at that point and left the HV windings unfused.
                      The only good solid state amp is a dead solid state amp. Unless it sounds really good, then its OK.

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                      • #26
                        Originally posted by trobbins View Post
                        A circuit configuration like that needs a variety of situations to be assessed, and there also needs to be some definition of what the load or loads are.

                        For example, it could be powering a valve amplifier with the 50% B+ connected to the screen(s) of an output stage, and the 100% B+ connected to the OT/anodes. In that case you don't want a fuse to somehow allow the screens to be powered if the plates have no power.

                        If you fused the secondary arms, and one fuse blew, then you would effectively have a doubler supply still power the load(s). So then you have to categorise why the fuse may have blown - if it was from a heavy load across the 100% output, then the load would likely soon take out the other fuse.

                        If you fused the CT only, and it blew, then you would still have a 100% rail, but the the 50% point may change depending on what loads were being applied and where.
                        I got to that point and gave up. There's an image on an amp tech guys blog on facebook of a Marshall amp that was sold in Denmark or Netherlands, one of those places. The amp has a board loaded with fuses. Like 6 or 8. I'll try to find the post and get a copy of the photo. You guys have probably seen something like that, but wow, I never have. That amp must have so many odd fail modes.
                        The only good solid state amp is a dead solid state amp. Unless it sounds really good, then its OK.

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                        • #27
                          Fuses can be purchased with pigtails (for pcb mounting).

                          The contact surface area made by most fuse holders with miniature cartridge glass fuses would have to be insignificant for valve amplifier type applications, but perhaps a concern for big hi-fi ss amps where fuse ratings may be over 10A.

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                          • #28
                            Yes the typical bias supply circuit powered from a HT tapping can allow a charging loop to still exist if a CT fuse blows - and that charging loop can cause the bias filter capacitor(s) to exceed voltage ratings.

                            The Fender 75 just uses the 250V and 500V nodes for switched OT supply of UL output stage, so no concern about output stage screens being damaged. So you could fuse the CT feed to the 250V node, and the bridge output feed to the 500V node, as those wires normally pass pulsing current (where the current falls to zero each mains half-cycle) and an AC rated fuse can typically blow ok (although the time when current is zero reduces as load fault current increases). That type of fusing would aim to protect against filter and amp circuit over-current faults - which would be the most likely region for faults to originate.

                            I've used PSUD2 in the link below to assist in identifying the most appropriate fuse value and type to use when hoping to improve amp protection, but it starts to get quite detailed for design, whereas many would use a more pragmatic approach.

                            https://dalmura.com.au/static/Valve%20amp%20fusing.pdf

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                            • #29
                              What trobbins said ^^^.

                              With the proliferation of "fuse everything" going on in the last couple decades, it's easy to get corn-fused. Especially if you're a "real smart amp wizzard" dispensing advice on dodgy websites.

                              Hi voltage isn't so much a problem, no need for special handling, except of course the usual safety concerns.

                              What's a problem in modern tube amps - the filament circuit is often fused. The interface area between fuse and holder eventually becomes corroded, then current movement to the tubes is compromised as the contact between fuse and holder gets worse with the current surge on every power-up. But that's not what we're addressing, is it. Perhaps it is what the wizzards are addressing, but they don't know their high voltage from their filament circuits, they're so busy muttering incantations and casting spells. And typing up nonsense based on their confusion.
                              This isn't the future I signed up for.

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                              • #30
                                Originally posted by mikepukmel View Post
                                Some real smart amp wizards posted that the resistance between clips and fuse pose a problem when fusing the HV. i read and read, and found somebody that solders wire to the fuse caps to get around that problem. Knowing my soldering skills so far, I gave up at that point and left the HV windings unfused.
                                You may be getting mixed up between HT and heater fusing?
                                I seem to remember RG Keen recommending the use of automotive blade type fusing for protecting heater windings as, unlike regular cartridge fuse systems, they are specifically designed for low voltage, high current applications.
                                I've not tried it out yet.

                                The key issue driving the inclusion of heater circuit overcurent protection is that due to the low capacity of the heater winding (relative to the PT as a whole), a fault in the heater circuit might overheat and damage the winding, without ever drawing enough primary current to blow the mains fuse.

                                Thankfully, as Leo points out, despite the historical absence of heater circuit protection, such a failure mode seems somewhat unusual.
                                Last edited by pdf64; 08-23-2019, 10:44 AM.
                                My band:- http://www.youtube.com/user/RedwingBand

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