39V/5W B+ dropping zener causing smoke?

Discussion in 'Amp Tech Center' started by EETStudent, Feb 11, 2020.

  1. EETStudent

    EETStudent Tele-Meister

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    I want to drop between 70 to 120 volts of B+ in my 5E3-style homebrew. I started with five 16V/5W zeners in series between the PT center tap and ground. Worked fine, dropped about 70V with no apparent issues. I then tried three 39V/5W zeners in series to drop even more voltage with fewer parts. After a few minutes with the power on, I started observing some smoke in the area of the standby switch. Couldnt tell exactly where it was originating. Left the power on for another minute until the smoke intensified. Then I used alligator clips to bypass one of the 39V zeners, giving me two in series, and the smoke returned. Switched back to the 16V zeners and no more smoke.

    What about the 39V zeners could be causing the issue?
     
  2. Dunedin2019

    Dunedin2019 TDPRI Member

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    25W (5 x 5W) versus 15W (3 x 5W) rating?
     
  3. Wally

    Wally Telefied Ad Free Member

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    V = I x R. Therefore, I = V/R. Could the increased current draw be beyond the ability of something in the circuit to handle that draw? Edit note: this is above my pay grade and beyond my experience as I don’t drop voltage this way, and I am just waking up. Therefore, this may be a train of thought that has problems. Willing to learn...


    what is your target voltage...and what is the ‘un-zenered’ voltage? You may have the wrong power transformer if you are trying to reach something in the range of 360-380VDC and need to drop 70-120 volts to get to that.
     
    Last edited: Feb 12, 2020 at 9:26 AM
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  4. Jon Snell

    Jon Snell Tele-Meister

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    If you disregards the zener wattage values for a moment and consider the power requirements of the output stage this is supplying purely in the current drawn, it will become apparent as to why the zeners burn up.
    I am not sure you meant, "I started with five 16V/5W zeners in series between the PT center tap and ground", as that means to me that you are clamping the HT at 80volts. I suspect you meant in series with the HT supply and the centre tap on the PT.

    The pair of 6V6s will consume at least 24mA at rest so 80volts at 24mA is 19.2Watts or 117v at 24mA is 28Watts, without any power output.
    A little much for a set of zeners I think.
    If you want the sound to break up sooner then place the zeners in series with the screen grids. It will have a similar effect without the overheat problem.
     
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  5. EETStudent

    EETStudent Tele-Meister

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    "Un-zenered" voltage is around 370VDC with a NOS 5Y3 rectifier and 6V6s. Fine for a standard 5E3, but I'm overhauling the power section to reduce power output to 3-5 watts. I put in some 6K6s, connected as triodes, and now I want to experiment with plate voltages between 250-300VDC.
     
  6. EETStudent

    EETStudent Tele-Meister

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    Thanks for your response. To clarify, I have the zeners positioned between the HT center tap and ground. This link explains it: https://music-electronics-forum.com/showthread.php?t=563#post52097.

    I'm no expert, but are you sure of those power dissipation numbers? Wouldn't 80V at 24mA (80*.024) be 1.92W instead of 19.2?
     
    Last edited: Feb 12, 2020 at 3:48 PM
  7. EETStudent

    EETStudent Tele-Meister

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    Correct. Seems to me that 15W is ample. I believe my power transformer is rated at 120mA max, so if I'm dropping 120V at .12A that would be 14.4W under max load. Under no load the dissipation is much lower.

    Surely I am missing something. I appreciate the help.
     
  8. Wally

    Wally Telefied Ad Free Member

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    I will say no more than this. Imho, it would be better to leave the amp’s circuit as it is and build a nice reactive load attenuator....or buy a flux density adjustable speaker. Ommv....
    IF one wants to cut the power with a voltage reduction, I prefer to see the preamp voltages left ‘as is’ and lower only the output section voltages. Again...ommv....but reducing preamp voltages rebiases all of the preamp and changes the essence of the amp, 8mho.
     
  9. EETStudent

    EETStudent Tele-Meister

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    I hear ya. This is my homebrew that is always undergoing some type of major change. I get bored easily and just like to experiment for the heck of it. By lowering B+ to around 275V, my preamp plate voltages are just above 100V. So far I am liking the softer, mushier overdrive I am getting. I think much of this is due to the 6K6s wired in triode
     
  10. dan40

    dan40 Tele-Afflicted

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    Is the circuit still a basically stock 5e3? You haven't added fixed bias or any other modifications have you?
     
  11. jimgchord

    jimgchord Tele-Afflicted

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    Honestly, its a bit silly to try and drop that much with diodes.
     
  12. EETStudent

    EETStudent Tele-Meister

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    Correct. Cathode biased 6K6s wired for triode operation, pretty vanilla PI and preamp.
     
  13. EETStudent

    EETStudent Tele-Meister

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    Why?
     
  14. Nickfl

    Nickfl Tele-Afflicted

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    You aren't leaving a lot of margin for those 39v zeners... lets say your amp is idling at 70ma, 39v * 0.07A = 2.73W, which seems fine, but that is idle current, its going to increase during use and you still want some margin after that too. I use a rule of thumb that I want the part to be rated for at least 3x idle dissipation, so you would want at least a 10W zener if you are going to drop 39v on a 5e3.
     
  15. jimgchord

    jimgchord Tele-Afflicted

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    120 volts is a lot, that energy doesnt just disapear without some consequence.
     
  16. Jon Snell

    Jon Snell Tele-Meister

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    I must get a new decimal point for my calculator!
    Loosing screen grid voltage will have the effect you want. Less power loss across your zeners or
    use a stabilized supply for the HT.
    Ampeg used a variable voltage on the drive valve to give a similar effect. The wiper of a 100k pot sat across part of the HT feeding the last anode load resistor before the phase splitter.
    As it was customer adjusted and on the front panel, I always thought a bit dodgy but hey, they used the capacitor of death for many years and I hear, only killed a few.

    Corrected part of the information. Memory not as good as it was. ...
     
  17. EETStudent

    EETStudent Tele-Meister

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    Does this hold up even when several 5W zeners are strung together in series? My understanding is the effective wattage rating is the sum of the individual parts.
     
  18. EETStudent

    EETStudent Tele-Meister

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    Thanks for the correction. I'm interesting in experimenting with lowering the screen grid voltage. Remember that my 6K6s are wired in triode so the screen and plate of each tube are connected together. Actually I have a 3W 470 ohm resistor between the plates and the screens for safety. Not sure if dropping the screens by 70V would change anything about the way the triodes operate.
     
  19. Nickfl

    Nickfl Tele-Afflicted

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    Yes, but each individual diode is still dropping its rated voltage and still dissipating the same amount whether it is in series with others or not. If you have three of them in series you are dropping around 117v, assuming 0.07 amps of idle current thats 8.2W through parts rated for 15W total. You said your PT is rated to 120ma, so if you pushed the amp hard enough to hit that you would be hitting those three zeners with 14W total, which is going to cause them to get very hot.

    I stick to 10v, 5W zeners to keep the wattage per piece in check, you could probably go up to 20v for a 5W part in a 2x 6v6 amp but I'd avoid going higher than that with 5W rated diodes. If you don't want to have to string a bunch together you can get an 80 or 100V diode, but it will have to be one of the big, 25W or higher rated ones that you have to bolt to the chassis or another heat sink.
     
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  20. Jon Snell

    Jon Snell Tele-Meister

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    I see, they will need to be wired as pentodes to reduce the screen voltage successfully.
     
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