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  • #31
    Measured voltage drop across the zener.Amp is operating close to "normal" for the voltage I have on the plates.Biased on the colder side for now,will bring it up and drop a few volts when I get it sorted out with the new zeners.

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    • #32
      Does your meter separately measure DC from AC, and does it measure true rms, and does it have a crest factor rating for AC measurement?

      One concern, is that the zener voltage has a small duty cycle square waveform, where it is either about zero volts (not conducting diode current, but may have parasitic current waveform which may take voltage negative by pn junction), or about the zener voltage (when the diodes are conducting). The meter may accurately measure the 'DC' value of the waveform, which won't be the zener voltage due to the duty-cycle.

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      • #33
        If you want to play it real safe and easy to check, let transformer/diodes/capacitors rectify unimpeded, so ypu´ll get 1,41X the RMS voltage, and then sehd 10/30/50/whatever V **DC** from whatever +B you got.

        Absolutely no problem measuring continuous/average/RMS/peak voltage drop ... because all are the same if e are talking DC

        So if your supply is, say, 60VDC high, a 60V Zener will drop *exactly* 60V .

        Is there any problem with doing that?
        Probably: I doubt insulation between a power Zener chip and its outer casing or heatsink is rated 500/600 VDC, so I wuld not ground it.
        I would bolt/clip/epoxy it to a small but adequate heatsink and hold it away from (grounded) chassis with some standoff insulators.

        Current fade about using Zeners on the PT CT comes, of course,because of the need of keeping it close to ground potential but the problem is that current there is **anything but** sinusoidal, which complicates measurement big time.

        The perfect solution (besides getting the proper transformer) is to build a MosFet "fixed" Power Soak or whateve you want to call it, with fixed, say, 60V drop (or whatever).
        It won´t be harder to insulate it than any regular Variable Voltage Attenuator.

        Legal Notice: the undersigner, Juan Manuel Fahey, declares under oath that he´s over 60 years old, has built at least some tube stuff without using the famigerated Devil`s invention called PCB, has at last once soldered with a hammer type soldering iron heated in a flame, by the way using muriatic acid and water with some zinc clippings dissolved in, and has *built* a 30" field coil speaker in the 80`s.
        Also keeps an OT in the Family Shrine, surrounded by red and black candles.
        [signed]
        JMF
        Juan Manuel Fahey

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        • #34
          Originally posted by J M Fahey View Post
          Legal Notice: the undersigner, Juan Manuel Fahey, declares under oath that he´s over 60 years old, has built at least some tube stuff without using the famigerated Devil`s invention called PCB, has at last once soldered with a hammer type soldering iron heated in a flame, by the way using muriatic acid and water with some zinc clippings dissolved in, and has *built* a 30" field coil speaker in the 80`s.
          Also keeps an OT in the Family Shrine, surrounded by red and black candles.
          [signed]
          JMF
          Good one Juan!

          Let's visit the suburbs of Antwerp, there's lots and lots of single ended OT's lying around, they grow like mushrooms, we can ALL pack a suitcase full! Plus a couple bottles of kriek, some of that terrific chocolate, and wholesale diamonds in the city district. Should make a fun holiday.
          This isn't the future I signed up for.

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          • #35
            Anyone else have to look up what a field coil speaker was?? Just me?
            If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

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            • #36
              Originally posted by J M Fahey View Post
              If you want to play it real safe and easy to check, let transformer/diodes/capacitors rectify unimpeded, so ypu´ll get 1,41X the RMS voltage, and then sehd 10/30/50/whatever V **DC** from whatever +B you got.

              Absolutely no problem measuring continuous/average/RMS/peak voltage drop ... because all are the same if e are talking DC

              So if your supply is, say, 60VDC high, a 60V Zener will drop *exactly* 60V .
              Just to clarify things Juan, since he has already ordered those 10W 30V zeners as stated in post #29, could this work? Is this what you were getting at, at least in theory? Put the zeners in the B+ rail? Just install the zeners he has instead of the 12V 5W zeners as shown. Or if not, this could be an alternative at least instead of putting them in the PT CT.

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              Turn it up so that everything is louder than everything else.

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              • #37
                Originally posted by DRH1958 View Post
                Just to clarify things Juan, since he has already ordered those 10W 30V zeners as stated in post #29, could this work? Is this what you were getting at, at least in theory? Put the zeners in the B+ rail? Just install the zeners he has instead of the 12V 5W zeners as shown. Or if not, this could be an alternative at least instead of putting them in the PT CT.

                [ATTACH=CONFIG]43018[/ATTACH]
                Just curious, but is that a schematic for an old Ampeg?
                If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

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                • #38
                  Quite a few of us are familiar with field coil speakers. BAck in the old days, before ceramic magnets were common, a field coil was a good way to make a strong magnetic field instead of a permanent magnet. All it means is there is a large coil of wire that is enegized - an electromagnet - to form the magnetic field in the speaker. Google "field coil speaker" for more.

                  There are several threads around here discussing them, and their replacement.
                  Education is what you're left with after you have forgotten what you have learned.

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                  • #39
                    Hey Leo Gnardo, you're welcome man to come and see the old stuff lying around in fact I nearly fall over that junk here while finding a safe place to work on the oldies . The Kriek is plenty and the Duvel is next to it , I'll buy you some , together with the choclats and the original Belgien Fries ( you call it French fries , France has nothing to see with the original fries ) )
                    Now just for the sake of it .. anyone can walk to Rome by all ways ..even by going via the moon and back ..
                    I suggested a simple , fast , original and cheap solution in proposing a choke after the rectifier.. if You all guys prefer to look up first in your bloody smartphones to find an app to see where they sell computers you need to find out there exist some complicated modern technical ways to lower a DC ripled voltage .. go your way , I won't stop you .. progress is invented to make life easier .... or more difficult .....and certainly more complicated .. that's modern life now ..
                    And since some of you seem to know a stranger immediately as a nono .. without knowing his backgrounds ( I own a muzeum with at least some 40 tubeFenders all kinds , nearly all the Klemt Echolette amps that where once available ,the old Dynacord amps from the 50's-60's , the Isophon golden speakers , all the models of Hammonds since 1935 till 60 , a big lot of the leslies and cabs of that time , I should apparently never have seen a tube amp inside and certainly never worked on a guitar amp .
                    By the way the 'Fieldcoils' Jensens are still swerving around in my workshop .. need some ?? as are the JBL's D130F , the D110f's even some you probably never have seen in your life the ones with the orange aluminium Dustcap .. even, some later JBL's like the K140F and some E130 s out of JBL horncabs .
                    Why is it that you should turn against an intruder with a simple and correct idea ? is it because I am stepping into your private teritorium , this forum ? or is it because I hate destroying old stuff or upgrading it with inaproporiate parts ?? ; I should say .. the hint was there , take it or leave .. not my problem , I hold on to my antic electronics point of view .
                    I like the Legal Notice of Mr Fahey especially the use of that muriatic acid and water with some zinc clippings dissolved , pure proof he is up to my own technical level and a pure skilled electronics man ,almost a dinosaur .
                    But in fact I think we should go for a donation so the proper transformer can be bought as a gift to Mr Stokes ,to solve this heavy problem and so we can close this discution .. but then we should really miss the fun of the whole discussion isn'it ??

                    Have a nice day .. Stax

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                    • #40
                      Originally posted by stokes View Post
                      Measured voltage drop across the zener.
                      Right, that explains it. You should be measuring the B+ voltage not the zener voltage. A 60V zener in series with the transformer CT will reduce the B+ by 60V (as it would in series with the B+ rail). The meter won't measure the voltage across the zener as 60V because the waveform at that point looks like this -

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                      • #41
                        Originally posted by Dave H View Post
                        Right, that explains it. You should be measuring the B+ voltage not the zener voltage. A 60V zener in series with the transformer CT will reduce the B+ by 60V (as it would in series with the B+ rail). The meter won't measure the voltage across the zener as 60V because the waveform at that point looks like this -

                        [ATTACH=CONFIG]43025[/ATTACH]
                        I have measured both,the voltage across the zener is 29vdc and the drop in the B+ is 29vdc

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                        • #42
                          Originally posted by SoulFetish View Post
                          Just curious, but is that a schematic for an old Ampeg?
                          Good eye. Pretty sure it's from the G-15. I found it in this thread on AX84's site. The first post refers to the "Ampeg G-15 B+ too high thing" and when you click on the second link, the new tab that opens on top says "G-15_ power_ supply_ zener": AX84.com - The Cooperative Tube Guitar Amp Project
                          Turn it up so that everything is louder than everything else.

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                          • #43
                            Originally posted by stokes View Post
                            I have measured both,the voltage across the zener is 29vdc and the drop in the B+ is 29vdc
                            I officially don't understand it then

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                            • #44
                              Originally posted by Dave H View Post
                              I officially don't understand it then
                              And that is exactly why I posted this here,I didnt understand.I think R.G. and Jazz hit the nail on the head.The 50watt zeners are just too big for this circuit.If you read back,I put a 18v 5 watter in there just to see and it dropped 18vdc in the B+ and the voltage across the zener was......18v's!If you read R.G.'s post,he explains it in detail as to why the 50 watter is too big.

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                              • #45
                                Originally posted by DRH1958 View Post
                                Good eye. Pretty sure it's from the G-15.
                                my eye's not that good, I was going to guess B-15
                                If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

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