5f1 zener diode question

Discussion in 'Shock Brother's DIY Amps' started by Linkjr, Jan 8, 2019.

  1. Linkjr

    Linkjr Tele-Meister

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    Hi guys,

    I would like to lower the voltage by 50v in my 5f1 so that my b+ is close to 340-350v and after researching I decided on zener diodes, I tried leaving long leads and tried a string of 12v 5w diodes but seems like every time I have used them they make the tone great but have then overheated and eventually failed.

    Sources I found say that they have used them without a problem for years but mine have lasted for a few sessions of playing

    Am I placing them in the correct location in the circuit?

    Please see below

    IMG_1094.jpg
     
  2. King Fan

    King Fan Poster Extraordinaire

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  3. JuneauMike

    JuneauMike Poster Extraordinaire

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    Yep. Your example is the approach to take for fixed bias. In that case you'd dial in your level of bias, note the plate voltage (I think it was plate), add the zener and then dial down the bias to get the plate voltage back to where it was pre-zener. Then poof, your voltage drop appears commensurate with the zener value. For cathode bias, you just do what is listed above.

    And, as a stray asside, more is better than less in terms of heat dissipation. So 2 25s would be better than 1 50. But of course, 1 50 would work. And try to gin up an attachment that leaves lots of zener lead in tact, again for dissipation.

    I don't know what kind of current you are going to have across the zeners, but 5 watts ought to be adequate. I think ....
     
    Last edited: Jan 8, 2019
  4. RLee77

    RLee77 Friend of Leo's

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    If you're using 4 of the 12v 5w zeners for a 48v drop, it seems unusual that they would have heat problems. I think the 5f1 would not draw more than 50ma from the B+ (just to estimate high), and at that current, 48v of drop would be 2.4w total across all 4 of them, so each would see about 0.6w, which they should be able to easily handle. As JuneauMike mentions, leaving some lead length helps with dissipation, like 3/8" or so of lead. But if they're getting really super hot and failing, something strange is occurring. You might want to measure (a) the voltage drop across each diode, and (b) the total current through the string.
     
  5. JuneauMike

    JuneauMike Poster Extraordinaire

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    Rlee, you don't think it has something to do with inserting them between the B+ and Rectifier? This is a cathode bias amp.
     
  6. Linkjr

    Linkjr Tele-Meister

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    Thanks for the suggestions guys, so wired the way they are in my first post above would not cause the problem I'm getting?

    But what you are saying is that I should wire them like this:
    IMG_1094.jpg
     
  7. JuneauMike

    JuneauMike Poster Extraordinaire

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    If by "this" you mean one end of the zener string is attached to the PT center tap and the other end is attached to the ground, then yes. The CT ground reference needs to flow through the zener to ground. Very important.
     
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  8. Linkjr

    Linkjr Tele-Meister

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    Thanks JuneauMike, I did toy with the idea of doing this but wondered if it might mess up something else as the design layout I'm using has this connection minus the zeners anyway like this:
    IMG_1100.jpg
     
  9. JuneauMike

    JuneauMike Poster Extraordinaire

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  10. Linkjr

    Linkjr Tele-Meister

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    Thanks JuneauMike, I'm definitely a newbie!

    I did read that thread but I'm worried about the placement of my zener string in the circuit. Since there will be no connection from the power transformer ct to the top of the first filter cap which is in the circuit layout I followed I'm worried wiring the ct to the zeners and then to ground will upset something else.
     
  11. JuneauMike

    JuneauMike Poster Extraordinaire

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    Wait, what? The PT center tap goes to ground.

    https://robrobinette.com/images/Guitar/HowAmpsWork/5F1_Layout_large.png

    Does your layout essentially look like this?

    Your B+ supply comes from the 5v going through the rectifier tube. A barber pole colored CT wire goes to ground. Could be red with yellow stripe, or red with brown stripe, whatever. But it is conspicuously different than all the other solid colored wires.
     
    Last edited: Jan 8, 2019
  12. RLee77

    RLee77 Friend of Leo's

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    It's hard to tell from a layout diagram what's being connected where... a schematic is the way to show such circuit changes.
     
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  13. Linkjr

    Linkjr Tele-Meister

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    Thanks for looking into this JuneauMike,

    Here is my power transformer spec sheet if I'm correct in thinking my ct is the black wire
    IMG_1095.jpg
     
  14. JuneauMike

    JuneauMike Poster Extraordinaire

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    Funky, but yes. You have two center taps. One is the secondary supply and the other is the 6.3v filaments (that one is handy so you don't need to elevate your heaters with 100ohm resistors to ground.) Both CTs go to a grounding point on the chassis (usually people elect to ground it to a PT bolt.)

    You're in the UK so you are using the 240v mains supply?
     
    Last edited: Jan 8, 2019
  15. Linkjr

    Linkjr Tele-Meister

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    Yes that makes sense my layout says the circuit is grounded to the chassis at the input jack sockets, there is a "safety ground" to chassis from the power supply also, as here in the U.K. we have a 3 prong grounded power supply. yes im using the 240v mains supply
     
  16. JuneauMike

    JuneauMike Poster Extraordinaire

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    Yep, perfect. I make my safety ground separate from the circuit ground. Also, you wire the safety ground with enough slack so that if the cord is jerked out of the chassis the ground wire is the last one to break. But that's not essential, so however you've grounded that will probably work.
     
  17. dan40

    dan40 Tele-Afflicted

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    Since we don't see the entire turret board in your pic, does the ground buss run from the main filter cap negative all the way across the board and then connect to ground at the input end of the chassis? It's perfectly fine to ground the PT secondary centertap to the main caps negative lead but ultimately that must connect to the chassis. Some grounding layouts split the ground buss into two halves...one for the main cap, screen supply cap and cathode resistor and then another ground buss for the preamp cap and other preamp components. Running a single ground buss is also fine if it connects to ground at the preamp end of the chassis.
     
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  18. JuneauMike

    JuneauMike Poster Extraordinaire

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    The link provided above shows a "split bus" grounding scheme for theoretical quieter operation. That's the one I'm familiar with and prefer. But a single bus terminating at the input jack works too. That's what's being described above.
     
  19. Linkjr

    Linkjr Tele-Meister

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    Do you think putting the zeners between the ct and filtercap are causing the premature failure?
     
  20. JuneauMike

    JuneauMike Poster Extraordinaire

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    I don't know.

    At some point here you are going to have to share some pics, voltages and more specifics about how you've wired everything.

    So it sounds like you first built the amp stock and it worked? Is that correct? What did the diode do to your B+ when you turned it on? Did it drop at all? Did it drop by the value you should expect? I agree that it the 5W diode string should not be burning up.
     
    Last edited: Jan 9, 2019
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