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Zener -- dropping voltage oddity-- 5F11

Discussion in 'Shock Brother's DIY Amps' started by JuneauMike, Feb 13, 2018 at 4:45 PM.

  1. JuneauMike

    JuneauMike Tele-Afflicted

    May 5, 2015
    Alaska
    This is the first time I've tried to use a zener diode to drop B+ voltages in my 5F11 build, and it didn't work. My voltages are actually higher now. How is that possible.

    Previously my voltages were:
    B+1=386v
    B+2=361v
    B+3=282v

    Wanting to drop about 40 volts, I installed a single 43v 5w zener at the ground lug. And connected the CT to the anode side.

    zener43v.gif

    zenerinstalled.gif

    After powering it up, I got the following voltages.
    B+1 = 407v
    B+2 = 385v
    B+3 = 306v

    These voltages track with my first 5F11 build. I'm using a 350-0-50-350 PT.

    This doesn't make sense to me. (I posted this in my build thread, but its getting long enough that I don't think people are checking it much. Was hoping to get some suggestions more quickly as to what is causing this problem.)
     

  2. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    Hey @JuneauMike - I'm having a hard time telling what's going on from the pics.

    The way I did it was to put it in-between the high-voltage CT wire and ground. The high-voltage CT wire goes first to the anode side of the zener, then you have the zener, then the other side of the zener (the cathode side) goes to ground. Here's a pic of mine. The bottom left of the photo is where the CT comes directly out of the PT. The it goes into the anode side of the zener. Then you can see just a bit of red under the heater wires at the top right of the zener string. That's where I re-used a piece of the red-yellow CT wire and sent it to ground, along with the grounded side of my B+.

    IMG_9594.JPG
     

  3. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    I think I see now in your second pic that your terminal lug is connected by ground to one of your PT bolts, and that your B+ wire is going over to that lug as well. It does seem like that should work. Are you sure that middle lug where you've got the cathode side of the zener soldered is going to ground?
     

  4. RLee77

    RLee77 Friend of Leo's

    May 15, 2016
    Silicon Valley
    I'd measure the voltage directly across the zener, to see if it's actually zening (yeah i just made that word up).
     
    JuneauMike likes this.

  5. JuneauMike

    JuneauMike Tele-Afflicted

    May 5, 2015
    Alaska
    Yep. Got it. If you examine the first picture in my post, the zener is going from the outside lug to the ground lug. The CT is attached to the anode side. I think I'm doing the same thing as you, I just used 1 zener, instead of a string of them.

    And yeah, the center lug on these three post strips go to ground. It's unmistakable.

    I remember your thread, you were able to drop your voltages that way.

    Did you use 3 zeners because that was what you had? Any problem with taking all your voltage drop in one gulp by using a single zener? As I said, I've never tried this before so I'm not sure of all the downstream effects of using various zener values.

    I'm trying to do this whole thing with one terminal strip to avoid drilling any holes in the chassis. This was my original idea.
    zener-string2.gif
     
    Last edited: Feb 13, 2018 at 5:16 PM

  6. JuneauMike

    JuneauMike Tele-Afflicted

    May 5, 2015
    Alaska
    I think I tried to do that last night on both the zeners installed on my amp (bias circuit uses a 1N4007). I don't think it registered at all, but maybe I was doing it wrong. It's DC voltage, right?
     

  7. RLee77

    RLee77 Friend of Leo's

    May 15, 2016
    Silicon Valley
    Yes, dc. Well, pulsing dc. And the 4007 isn't a zener, it's a standard rectifier diode, so the most you'll see across it is about 0.7 vdc when it's conducting.
    But for the 43 v zener, you should see about 43 vdc right across it, if it's working properly. And your meter will show a bit less as it is pulsing dc voltage.
     
    Last edited: Feb 13, 2018 at 5:47 PM
    JuneauMike likes this.

  8. JuneauMike

    JuneauMike Tele-Afflicted

    May 5, 2015
    Alaska
    Well, thanks. That gives me something to go on. I hadn't even considered that the diode could be faulty. So out of curiosity, I check the voltages and find something moving through it. Is there other possibilities?
     

  9. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    Aren't zeners normally reverse biased?

    Edit: nevermind, trying to work through a headache I woke up with. Carry on. :)
     
    Last edited: Feb 13, 2018 at 6:21 PM
    dogmeat likes this.

  10. RLee77

    RLee77 Friend of Leo's

    May 15, 2016
    Silicon Valley
    You can easily just clip a test lead across the diode to bypass it (short it), and your voltages should go back to the original values before the zener installation. If they don't, then something else is wrong.
     
    Andy B likes this.

  11. RLee77

    RLee77 Friend of Leo's

    May 15, 2016
    Silicon Valley
    The main concern is heat. With one zener, you'll be dissipating around .08 x 43 = 3.4 watts (using 80ma as the current, which is maybe on the high side, not sure). A 5w zener is going to get *very* toasty running 3 watts or so continuously, in free air. That's why multiple zeners are often used... to spread that heat around a bit.
    Some use the steel-packaged chassis mount parts, that bolt directly to the chassis which provides a great heat sink. (You have to order them correctly with the cathode as stud, however.)
    [​IMG]
     
    Last edited: Feb 13, 2018 at 5:49 PM

  12. JuneauMike

    JuneauMike Tele-Afflicted

    May 5, 2015
    Alaska

    Ahhh, interesting. So my PT secondaries are at 75mA, so I'd be at about 3.2 watts. If I used two 20v zeners in series I'd cut that to about 1.5 watts each?
     

    Attached Files:


  13. RLee77

    RLee77 Friend of Leo's

    May 15, 2016
    Silicon Valley
    Yes. Although you do have the cathode of that zener soldered directly to a ground lug that bolts to the chassis, so that gives you some decent heat sinking. It may be ok.
     
    JuneauMike likes this.

  14. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    That rules.
     

  15. dogmeat

    dogmeat Tele-Meister

    Age:
    67
    223
    Oct 12, 2017
    Alaska
    ahhhhhhh.... the pic in post 5..... no zening going on there... thats a dead short to ground (well except for the 1.4 drop of 2 diodes in series). I mean they appear to be forward biased & therefore simply acting like regular diodes
     

  16. RLee77

    RLee77 Friend of Leo's

    May 15, 2016
    Silicon Valley
    Nah they are reverse biased -- the CT is the most negative point in that series-to-ground chain.
     

  17. dogmeat

    dogmeat Tele-Meister

    Age:
    67
    223
    Oct 12, 2017
    Alaska
    doh'.... not thinkin. now about the watts.... how much will the current drop due to the diodes? thats the current you need to use for calculating the wattage.
     

  18. JuneauMike

    JuneauMike Tele-Afflicted

    May 5, 2015
    Alaska
    Ok, no luck. Hmm. I measured DC across the zener pictured above and got 14 volts. It should be 40something.

    I jumpered the zener to ground and my B+ returned to normal.

    Removed it, installed two 25v 5w zener in series, same as pictured with anode toward the CT and cathode towards ground and my voltages went up again.

    B+1 UL (meter jumping. Sometimes I got 406-408vdc)
    B+2 386.7vdc
    B+3 300vdc

    As you can see, it's not dropping voltage.
    Read across these zener and got 12VDC.


    The following readings are based on 10k bias pot CCW to CW.

    V4
    P3 407v to 419v
    P4 389v to 400v
    Cathode to ground 4.2ma to 4.9-3.1 (erratic readings that never settled)

    V3
    P3 407v to 419v
    P4 388v to 407v
    Cathode to ground 3.7ma to ?

    Any chance I'm installing them backwards? Hard to believe that 100% of the diodes I've used are defective. I've got to be doing something wrong. I'll post pics tomorrow.
     
    Last edited: Feb 14, 2018 at 2:31 AM

  19. RLee77

    RLee77 Friend of Leo's

    May 15, 2016
    Silicon Valley
    .
     
    Last edited: Feb 14, 2018 at 6:38 AM

  20. RLee77

    RLee77 Friend of Leo's

    May 15, 2016
    Silicon Valley
    Ah, ok I see the problem... I didn’t realize you were doing this to a fixed bias amp. Putting the diodes in the CT won’t work. That raises the cathodes of your power tubes more positive by the voltage of the zener, so they are biased into cutoff. The negative bias supply itself isn’t affected by the zener offset since the bias supply current forward biases the zener, only losing 0.7v. End result is about 40v more negative bias on the grids. (Less in this case, since the zener ends up being a bit current-starved, which is why you only see 12v.)
    With the power tubes in cutoff, your supply voltage would of course go up.

    You’ll need to move the diodes in between the connection from the rectifier tube to the first filter cap, with anode to the cap. That way it only affects B+, not the bias reference.

    I’d also recommend using the 2 diodes, leaving the leads somewhat long for better heat dissipation, rather than the single diode. Or even three 15v diodes similar to what theprof posted. Remember that 5w rating goes down as ambient air temp goes up, and amps are not exactly cool inside!
     
    Last edited: Feb 14, 2018 at 6:40 AM
    Mr Ridesglide likes this.

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