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Zener -- dropping voltage oddity-- 5F11

Discussion in 'Shock Brother's DIY Amps' started by JuneauMike, Feb 13, 2018.

  1. nasdak

    nasdak Tele-Meister

    119
    Mar 22, 2013
    france
    Always be sure of what you're doing before moding and amp... You're lucky the power tube went to cut off an not to 200pct bias !
     

  2. Bendyha

    Bendyha Tele-Afflicted Ad Free Member

    Mar 26, 2014
    Northern Germany
    You could shift your negative bias supply from the winding tap you are now using, to the zener on the center-tap to solve the problem.
    A good description of the combination of B+ drop and neg. bias supply is given by Aiken, Back-biasing with a zener. http:what-is-back-biasing.
     
    Last edited: Feb 14, 2018
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  3. JuneauMike

    JuneauMike Tele-Afflicted

    May 5, 2015
    Alaska

    Awesome, thank you! It was obvious to me last night that those diodes couldn't stay on the ground lug, its great to know precisely why now. I'll look into this to get a better understanding of what is happening here. If I'm attempting to shave off 40 volts I'll obviously be adding quite a bit of sag by putting the voltage dropping diodes in front of the filter caps, correct?

    I should have asked this voltage dropping question earlier in my 5F11 build thread since the Vibrolux is kind of an odd duck compared to the armies of Champs and 5E3s out there. As you explain it, it makes sense. Unfortunately, the Vibrolux hasn't been as well picked over by folks as the above examples. Wouldn't this also go for the 5G9 Tremolux as well, their bias circuits seem somewhat similar, not brothers but more like cousins?
     

  4. JuneauMike

    JuneauMike Tele-Afflicted

    May 5, 2015
    Alaska
    Foul language like that has no place around here.

    th.jpg
     
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  5. RLee77

    RLee77 Friend of Leo's

    May 15, 2016
    Silicon Valley
    .
    There’s no difference in the performance of the zeners between being in the CT or in front of the filter caps. Either way it’s in series with your supply, and does the exact same thing.
    The only reason the diodes were ever put in the CT to begin with was to allow a steel-cased zener to be easily bolted to the chassis for heat sinking.
     
    Last edited: Feb 14, 2018
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  6. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    You don't get voltage sag with zener diodes. Once they start conducting there's no additional "drag" as current increases like you get with resistors.
     
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  7. RLee77

    RLee77 Friend of Leo's

    May 15, 2016
    Silicon Valley
    Yes, any amp that uses a negative bias supply referenced to ground would behave the same.

    BTW, what do your 6.3 and 5vac filament windings measure under load? If it turns out that all your PT voltages are high, you could benefit from making a simple voltage step-down box to go from 120ac to like 110ac, which will drop about 8% across the board. Handy project info here: http://www.instructables.com/id/VIntage-Voltage-for-old-Equipment/
     
    Last edited: Feb 14, 2018

  8. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    It's a good thing there are people around here who know what they're doing! So with a fixed bias amp, you've either got to put the zeners between the rectifier and the first filter cap (RLee) or you could shift your negative bias supply over to where you've got the zener now (Bendyha). Either way, the zeners don't produce "drag" the way a dropping resistor does after the rectifier (RobRob). Cool!
     

  9. JuneauMike

    JuneauMike Tele-Afflicted

    May 5, 2015
    Alaska
    Man, am I glad I started this thread. Thanks for the info all.
     

  10. JuneauMike

    JuneauMike Tele-Afflicted

    May 5, 2015
    Alaska
    You know what, they're high too. My heaters were 6.7vac and my HT AC is 5.4vac.

    That would be a slick solution to lowering B+ and would put B+2 right at the sweet spot 332vac. But I'd be worried that my AC would be too low.

    Heaters would be 6.16vac and HT AC would be 4.97. That would be a problem, right? Especially the filaments running that low.

    Hmm, aiming for 6 percent looks pretty nice.
     
    Last edited: Feb 14, 2018

  11. dan40

    dan40 Tele-Holic

    712
    Aug 19, 2015
    Richmond Va
    Those slightly lower filament voltages would not be a problem and you may get a smidge more life out of your tubes.
     
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  12. tubeswell

    tubeswell Friend of Leo's

    Jul 1, 2008
    NZ
    386 is not a bad plate voltage for a 5F11 running a pair of fixed-bias 6V6s. Similar to a tremolux 5G9.
     

  13. JuneauMike

    JuneauMike Tele-Afflicted

    May 5, 2015
    Alaska
    Hmm. Not the results I was expecting.

    A string of three 12v 5w zener diodes between the rectifier and B+1 cut exactly 13vdc. A string of three 15v 5w zeners cut 16vdc. Just for the heck of it, I turned the zener around so the cathode was pointing back at the rectifier and it cut 35 volts.

    I snipped the B+ connection and alligator clipped various zeners.
    43v 5w zener: anode to rectifier: 12v. Cathode to rectifier: 38v.
    Two 25v 5w zeners: anode to rect: 14v. Cathode to rect. 25v
    Two 43v 5w zeners: cathode to rect. 43vdc.

    This has not worked as advertised. I was expecting much higher voltage drops. I was expecting to see a 1:0.85 drop. Three 15v zener should have gotten me roughly 38vdc when placed between the rectifier and B+ in a fixed bias amp. Any ideas?
     
    Last edited: Feb 19, 2018 at 2:16 AM

  14. RLee77

    RLee77 Friend of Leo's

    May 15, 2016
    Silicon Valley
    Yes, the cathode *should* connect to the rectifier. Zeners must be reverse biased to function as zeners. Earlier in post #20 I had indicated ‘anode to cap’, which would put the cathode to the rectifier.
     
    Last edited: Feb 19, 2018 at 2:44 AM

  15. JuneauMike

    JuneauMike Tele-Afflicted

    May 5, 2015
    Alaska
    Ah yes, i overlooked that. Sorry.

    Even so, they don't appear to be behaving the way I expected. I was expecting a voltage drop by the value of the zener less 10-15%. I'm using 1N53xxB zeners. I have no experience with zeners so I'd be grateful for any insight.

    This is the first time I've tried to manipulate B+, so I'd really like to come out of this smarter for the effort. Even if I ultimately decide to just buy a 300-0-3000 transformer and be done with it.
     

  16. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    Looking over your shoulder here, trying to learn. To recap:

    On a cathode-biased amp, one can use zeners between the high-voltage center tap and the ground for the B+. One installs them in a string, anode side connected toward the CT wire (red-yellow) and cathode (striped) side of the zeners connected to ground together with the B+. Thus: CT --> +zener- +zener- +zener- --> B+ ground.

    On a fixed-bias amp, this method will not work. Instead, one can connect the zener string between the rectifier and the B+, as one would a dropping resistor. Unlike a dropping resistor, however, zeners don't sag. And unlike the installation in a cathode-biased situation, one installs zeners between the rectifier and the B+ cathode to anode, rather than anode to cathode. That is, rectifier --> -zener+ -zener+ -zener+ --> B+.

    The zeners should typically drop about 85% of their rated value and should be rated at 5W, of the series 1N53xxB.

    I hope that's all correct. If it is, I still don't understand what's going on, @JuneauMike . Can you post another picture of how you've done it most recently? That might help other folks who can troubleshoot this better than I can.
     

  17. JuneauMike

    JuneauMike Tele-Afflicted

    May 5, 2015
    Alaska
    Professor, I'll post pics tomorrow. But basically, this experiment hasnt really gone as expected at all.

    B+ = 386vdc.
    3x12v string of zeners brought it to 370vdc. 16vdc drop.
    One 12v zener = 382vdc. 4vdc drop.

    2x25v zeners = 361vdc. 25vdc drop.
    One 25v zener = 377vdc. 9vdc drop.

    One 43v zener = 366vdc. 20vdc.
    2x43v zener = 343vdc. 43vdc drop.

    When I reversed the zeners I actually saw a voltage increase, on the order of 2-3 volts. Voltages should have dropped by like .6 vdc in those instances.

    I tried 3 watt metal comp resistors too and the results were more predictable. Voltages dropped commensurate to the resistor value and the resistor got hot as fast as you'd expect.

    Someone on another forum suggested that it may be related to the pulsing DC source coming off the rectifier tube making the results f4om the zeners more unpredictable. This is all above my knowledge level.
     

  18. RLee77

    RLee77 Friend of Leo's

    May 15, 2016
    Silicon Valley
    Seemed like they were measuring close to their expected voltage when the orientation was corrected. If not, then you’ll need to describe how you are measuring the drop, and also you may as well measure the dc current through the zener string. If you’ve removed the output tubes while experimenting with the zeners, for example, the zener current would be too low to create the rated zener drop.
    For all measurements, the side closest to the rectifier output will be the most positive.
     
    Last edited: Feb 19, 2018 at 4:38 PM

  19. JuneauMike

    JuneauMike Tele-Afflicted

    May 5, 2015
    Alaska
    And yes, these are all1N53xxB zeners.
     

  20. JuneauMike

    JuneauMike Tele-Afflicted

    May 5, 2015
    Alaska
    Wouldn't a 12v zener oriented correctly drop DC by about 10.n volts? I saw 4vdc drop. That doesn't seem right.

    Oh, and I'm measuring DC to ground at the B+1 node. Probes are clipped on. Zeners jumpered in place, amp turned on. Amp turned off, caps drained, new zener config jumpered in. And so on, and so on.
     

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