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  • #46
    Originally posted by stokes View Post
    ...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.
    I concur. For additional supporting information one can look up the detailed specifications for a component. You didn't mention the part number of the 62V / 50 W Zener Diode that you used so I just used the data for the first one I could locate, a Microsemi 1N3335B. There is a parameter listed as "Zener Test Current (IZT)" and it is 200ma. I also looked up the specifications for a 1N4964 which is an 18V / 5W zener and the IZT for that component is 62 ma. The IZT is specified to insure that the part is operating over the knee of the curve when the voltage reading is taken. This data is the supporting evidence for the explanations previously posted by R.G. and Jazz and your actual fix confirmed it. In general, the high the power rating, the higher the current required to be flowing through the zener to achieve the rated zener voltage.
    Cheers,
    Tom

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    • #47
      Thanks for the data Tom. I was always hesitant about using 'overkill' zeners in low voltage circuits for this reason, like a 5W in a 1/2W position for +/-15V etc. circuits. I'd suppose using 1W where 1/2W is spec'd is probably ok, but I guess you would really need to look close at the actual numbers when in use.
      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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      • #48
        Originally posted by staxmotownjunkie View Post
        I suggested a simple , fast , original and cheap solution in proposing a choke after the rectifier..
        The criticisms of your posts is because you have completely ignored all the practical considerations that are required for the solution.
        Must use very little space
        Must not add much weight
        Must be cheap (1 or 2 dollars for new components)

        Using a new choke for the application requires that it can handle the full current from B+. These are large, heavy, and expensive. Subbing an OT instead of a choke is larger, heavier, and expensive.
        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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        • #49
          Originally posted by g1 View Post
          The criticisms of your posts is because you have completely ignored all the practical considerations that are required for the solution.

          Using a new choke for the application requires that it can handle the full current from B+. These are large, heavy, and expensive. Subbing an OT instead of a choke is larger, heavier, and expensive.
          And the OT solution requires a big gapped core designed for class A operation i.e that can handle DC. A push-pull OT cannot take much DC.
          Experience is something you get, just after you really needed it.

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          • #50
            Originally posted by staxmotownjunkie View Post
            And since some of you seem to know a stranger immediately as a nono .. without knowing his backgrounds
            or is it because I hate destroying old stuff
            .. Stax
            As a matter of fact, we WELCOME newcomers as a general rule. But we also are looking for people who can read the posts, interpret them and THEN offer solutions within the guidelines of the Original Poster (see g1's post #48). And since I see that you like to save old useful parts like so many of us do, I would think you would commend stokes on re-using old iron. Instead, you wonder why he used the "wrong" Power Tranny in lines 3-6 of your post #24 and add fuel to the fire at the end of your post #39 suggesting that we take up donations to get him the "proper" PT.
            Turn it up so that everything is louder than everything else.

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            • #51
              Originally posted by g1 View Post
              The criticisms of your posts is because you have completely ignored all the practical considerations that are required for the solution.
              Must use very little space
              Must not add much weight
              Must be cheap (1 or 2 dollars for new components)

              Using a new choke for the application requires that it can handle the full current from B+. These are large, heavy, and expensive. Subbing an OT instead of a choke is larger, heavier, and expensive.
              +1,only thing is I'm not so concerned about "Must be cheap",actually cost me $25 for the two zeners,but much cheaper than an OT big enough to handle the job.

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              • #52
                Originally posted by SoulFetish View Post
                Anyone else have to look up what a field coil speaker was?? Just me?
                Just you ...... youngster !!!!!!!!!!





                Juan Manuel Fahey

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                • #53
                  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]
                  Almost
                  You are not passing DC through them (yet) but full wave rectified AC, not the same thing.
                  It may work, and the 5W ones can just float in the air, on terminal strips or something, but you know I am a minimalist and want to reduce problems to their minimal expression if possible.

                  IF I have , say, 450VDC and add ,say, 60V worth of zeners in series, I *know* I will end up with 390V (450-60) ... period.
                  No Ifs or Buts or Whens or Whys DC is DC.

                  If I add them at a non sinusoidal waveform point I have to start thinking RMS/Peak voltage, Duty cycle, dynamic Impedance, etc.

                  Not saying they will not work, just not sure what exact voltage will I get at the end.

                  Hope this clarifies my simplistic ways
                  Juan Manuel Fahey

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                  • #54
                    Ok,I put the 2-30v 10 watters in.I got the plate voltage down to an acceptable 360v when I biased it up using a 5Y3 rect.Only problem is the zeners are getting so hot I'm afraid the solder holding the 2 in series is going to melt.I had it running about 10 mins while I djusted the bias and took measuremnets and then about 5 mins or so looking at the oscope with a signal running thru the amp.I have a smaller PT I'm thinking about using,I'm just not sure the current rating on the HT is enough.I dont know for sure but I guesstimate it to be 70 to 90ma's.Its an old Thordarson,I emailed them to see if they have any info.

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                    • #55
                      Originally posted by stokes View Post
                      Ok,I put the 2-30v 10 watters in.I got the plate voltage down to an acceptable 360v when I biased it up using a 5Y3 rect.Only problem is the zeners are getting so hot I'm afraid the solder holding the 2 in series is going to melt.I had it running about 10 mins while I djusted the bias and took measuremnets and then about 5 mins or so looking at the oscope with a signal running thru the amp.I have a smaller PT I'm thinking about using,I'm just not sure the current rating on the HT is enough.I dont know for sure but I guesstimate it to be 70 to 90ma's.Its an old Thordarson,I emailed them to see if they have any info.
                      The two diodes are thermally as well as electrically coupled and in total dissipating something like 5W at idle. You are going to need a small heatsink to bring the temperature down. It should take too much, just a few sq in of aluminum will probably be enough. If these are stud types don't forget about isolation of the heatsink from other parts.
                      Experience is something you get, just after you really needed it.

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                      • #56
                        The problem is that I have one zener mounted to the chassis with a heat sink,the second is soldered to the first just "floating",this one got so hot I couldnt touch it.Got a response from Thordarson about the other PT,said he had to go into the archives to find the ratings,will get back to me tomorrow.

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                        • #57
                          Ok,just to let you youngsters who contributed here know,I went and ordered a Hammond 200 series PT and put the vintage one back on the shelf for some future project.The only other time I've used a zener was a singled ended dual 7581a output I built about ten yrs ago.The PT I had was some Canadian brand,forget the label,but it looked to be a re-labeled Hammond.Was putting a little over 500v's on the plates.I used a 50 watt 62v zener and it worked fine.In this case R.G. and Jazz pointed out the fact that if there isnt enough current the zener wont work,to put it simply.That was the problem here.The next lower current value was 10 watt,and stringing 2 of them caused them to get too hot for my comfort zone so I bit the bullet and bought a suitable tranny.Probly what I should have done to begin with but I've held onto these vintage trannies for a reason and I will use it some time.Thanks to all involved here.

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