Reverse Polarity Zener diode

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by AL1, Dec 24, 2005.


  1. AL1

    AL1 Member

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    Can anyone tell me which zener diode I would need to lower the voltage on a Black faced Princeton Reverb about 15 to 30 volts. Could I use one rated 50 watts ? Thanks.
     
  2. tonezoneonline

    tonezoneonline Member

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    What are you trying to accomplish with the zener diode?They are not rated by watts but by amps and the volts that they will drop.I don't think Weber sells zeners but Mouser does or I have some around the shop.I'd be glad to drop you a couple in the mail if I knew what you were doing with them and I could be sure you would not do damage to yourself or your amp.Not trying to be a pain but you don't sound real knowledgable about this.
    Let me know what your trying to do and I'll try and help.
     
  3. AL1

    AL1 Member

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    Tonezoneonline, I am getting this info from a book by Dan Torres. He mentions a 50volt 50watt reverse polarity zener diode to lower the voltage from the power transformer. Since the Princeton is only about 18 watts, do I need a 50 watt zener? Mouser does have them in various voltages and 10 and 50 watts. I am just wondering if the 50 watt one would be too much. Thanks.
     
  4. VacuumVoodoo

    VacuumVoodoo Member

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    The wattage rating of aZener diode tells you how much power it can dissipate into the air not how much power it will draw, this power draw is set by the voltage rating and the current flowing through the diode: Power=Voltage x Current. Now, a certain minimum current must flow through a Zener diode to make it reach the specified Zener voltage.

    When you connect a Zener diode between the PT's secondary windings center tap and ground all of the B+ current will pass through the diode (it must somehow return to ground and does so through the diode).

    Total B+ current draw in the Princeton is about 100mA, or more depending how hard you drive the power tubes, which in a 30V Zener diode will dissipate over 3W.

    Since the temperature inside the amp is quite a bit above room temp one must use a diode that can dissipate 3W efficiently enough so it dosn't overheat nad selfdestroy. Unless you can mount a diode so it gets proper cooling you have to use a diode with a much higher power rating. A safe way is to go 5 to 10 times actual dissipated power so in this case a diode rated for 15-30W or higher should do the job. I would say a 20W rated Zener at least to be safe.
     
  5. tonezoneonline

    tonezoneonline Member

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    Alex has given you a very good explanation of how this works and I've offered to send you a couple suitable diodes.I'm still trying to figure out why you and Mr. Torres think you need to drop some voltage in your Princeton?
    I've used zeners several times to drop some voltage in scratch builds (prototypes)but it's usually not the best solution .Why do you need to drop some voltage in your Princeton?I'm just curious.
     
  6. AL1

    AL1 Member

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    To lower voltages to the what they are on the schematics.
     
  7. acorkos

    acorkos Member

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    i've used 10 watt zeners made by NTE....they drop your voltage by 5-200 volts. nice design...you drill a hole in your chassis near the PT and bolt it in like a jack with a nut....the solder tab sticks up into the chassis and you solder your high voltage CT to it...easy and clean.
     
  8. TheAmpNerd

    TheAmpNerd Member

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    Yeah but he's got a nice (assuming) BF Princeton Reverb amp.
    I wouldn't go drilling holes in it.

    I also wouldn't drop the voltage either.
    15 to 30 volts on the b+ is no big deal when the
    schematics say there is a 20 percent tolerance.
    Depending on when he measure it...and
    the load of the hood he lives in.

    If your amp sounds fine, I wouldn't mess with it.
    if you want to tinker around, find something similar.
    or buy a kit, then go to town on it.

    Put a different rectifier in it to lower the voltage.


     
  9. ChrisGS

    ChrisGS Member

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    Hi AL1 ,
    I used a 50 watt - 50 volt reverse polarity zener to drop my B+ from about 460v to about 415v on a scratch built project...it works .

    In some thread somewhere...I think on the weber board , guys were saying that Fender used the same technique on some bandmaster amps here and there. They posted pictures of a couple of Bandmaster amps that appeared to have the stud mounted reverse zener as original factory installed .

    NTE makes the diodes and I think they are available from Mouser , and I think they cost about $10.

    NTE5257AK is a 50watt - 15volt reverse zener
    NTE5268AK is a 50watt - 30volt reverse zener
    NTE5263AK is a 50watt -22 volt reverse zener

    Also , a while back , Weber advertised Zener Copper Cap rectifiers where you could specify a voltage drop AND a sag voltage...I dont know if they are still available with the Zener voltage drop feature , but check with Weber . This way you wont need to drill a hole in your chassis .

    Also, you may want to try just lowering the B+ to the preamp by increasing the size of the B+ dropping resistors , and see if that gets you what you want .

    Good luck with your project ,
    Chris .
     
  10. PaulC

    PaulC Member

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    If it's a black face, and you want to get the voltages down to vintage levels you should try and reduce the AC mains voltage, and not the secondary high B+. Chances are your heaters are a little high right now also.

    You can use a variac to do this, or make a simple "bucking" transformer box. That takes a small 6.3-12.6 volt pwr transformer like what you can get at radio shack. You wire it up in a strange parallel/series wiring that will actually reduce the line voltage by the amount of the transformer. So if you're a little over coming out of the wall (we're 124vac all the time here) it'll subtract 6.3 volts or 12.6 getting you closer to what you're looking for. One voltage that stays pretty much a constant in amps is the 6.3 volts heaters. The high B+ can be all over the place, but the heaters should be the same. So if you had a variac you'd adjust it untill you got 6.3vac under load. Then that would tell you about what line voltage you should have. Most of these I've seen came out to about 115vac. You can put in a fuse and switch with the bucking transformer, or get really fancy and put in some line noise filters. These things are great, and a must for some vintage amps.

    Later, PaulC
     
  11. AL1

    AL1 Member

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    I will be trying this with a David Allen tp25 transformer. It has a red/yellow center tap, a green/ yellow center tap and an orange shield ground wire. All three of these are together and grounded to the same spot. Do all three of these go to the zener diode?
     
  12. VacuumVoodoo

    VacuumVoodoo Member

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    No. Shield winding wire MUST be connected directly to the safety ground. (center prong on the mains connector/cable) If you have only a two-prong mains connector then chield goes to main chassis ground.
    Heater winding center tap connects to chassis ground. (green/yellow*)
    Only the HV center tap connects to the Zener diode. (red/yellow*)

    * assuming standard color coding but pls check
     
  13. acorkos

    acorkos Member

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    only the red-yellow goes to the zener. the others go directly to ground.
     
  14. hasserl

    hasserl Member

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    Unfortunately Weber quit carrying this item, I already checked about this for one of my projects.
     

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