What's going on in this rectifier circuit?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Leonc, Feb 8, 2012.

  1. Leonc

    Leonc Wild Gear Hearder Gold Supporting Member

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    I replaced the filter cap can in my old PANaramic 1210 (distant cousin of the Magnatone 213 Troubadour) last night and encountered a puzzling rectifier circuit. Can someone explain this to me please.

    Here's a photo. I've added the diode symbol for the small, black diode. It's clearly marked but you can't see it in the photo. The larger "top hat" diode is not marked w/respect to polarity, but my understanding is that the cylindrical part is on the cathode side and the "brim" part is the annode side--so it is oriented in the opposite direction of the other diode.

    [​IMG]

    So this diagram represents my interpretation:

    [​IMG]

    I'm more familiar with this kind of rectification scheme, when two diodes are involved:

    [​IMG]

    So what's going on here?
     
  2. teemuk

    teemuk Member

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    What is going on is simply that your interpretation and circuit sketch is incorrect.

    As it is drawn it would practically short circuit the secondary during each positive half wave. And we know that ain't happening, right?

    Recheck and redraw the circuit and I bet it starts making more sense.
     
  3. Leonc

    Leonc Wild Gear Hearder Gold Supporting Member

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    Yeah, I agree, some of my interpretation has to be wrong...

    I guess one explanation would be that:

    - the red/yellow winding is NOT the center tap

    - the top hat diode (that connects to the red/yellow secondary) is actually oriented the same way as the other diode, so that the annodes of each diode go to the cap and the cathodes go to B+.

    Just tested the diodes and sure enough, the top hat is oriented the same as the little one...but it is absolutely connected to the red/yellow 2ndary, which is absolutely connected to ground. And the other red secondary is absolutely connected to nothing on that 2nd terminal.

    The red/yellow may not be the center tap, it could be the other end of the B+ winding...but why would it go to ground and the other red secondary go to nothing?

    I'm still left :huh
     
  4. teemuk

    teemuk Member

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    So it's a basic half wave rectifier...? So there's an unused (higher or lower) voltage tap within the secondary, voltage provided not needed in the amp...?

    All that seems perfectly normal to me.
     
  5. Leonc

    Leonc Wild Gear Hearder Gold Supporting Member

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    It's not necessarily half wave just because there's only two diodes, if that's what you mean. If the PT is center tapped (which I think it is), then you only need two diodes for full wave rectification, right? But this still doesn't explain why the top hat diode and one secondary go to ground...and why the other red secondary goes to nothing. That just makes no effing sense to me!

    Keep in mind this amp worked quite well when the photo above was taken. And I've subsequently rewired it just like this with the new cap can and it continues to sound great (though it is a little firmer/healthier sounding with the new filter caps).

    Now check out this photo of the rectifier in my old Titano 314 Custom (which is VERY similar to this amp).

    [​IMG]

    You can see quite clearly that each of the two top hat diodes goes to each of the two red secondaries and that the red/yellow secondary goes to ground. Now THAT makes sense to me. That is what you see in my 2nd hand drawing. And that is a full wave rectifier.

    So I'm still completely flummoxed.
     
  6. schmidlin

    schmidlin Member

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    Factory mistake?? Very odd indeed, never seen anything like that.

    I would measure the HV secondaries on the PT to see if that is indeed a center tap.
     
  7. Blue Strat

    Blue Strat Member

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    Also, check the actual direction of the top hat diode with a meter (disconnect one end) and if it points in the same direction it probably means that the anode side was inadvertently moved over to the CT terminal. What DC voltage is the single diode producing?
     
  8. Leonc

    Leonc Wild Gear Hearder Gold Supporting Member

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    Yep, seems like a mistake.

    Just checked AC on the two red and one red-yellow secondaries. The red lead that goes to nothing reads 263VAC. The red lead going to the little diode reads 256VAC. The red-yellow lead reads 0VAC. It's a center tap. Seeing 312VDC at the 40uF cap where the diodes are connected.

    So why does this work w/o the one red secondary connected to anything?
     
  9. Leonc

    Leonc Wild Gear Hearder Gold Supporting Member

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    Mike don't know if you saw this in one of my earlier posts:

    I didn't disconnect to test, but tested both diodes with my DMM (set to Diode reading) as follows. Connected black tip to what I assumed was the annode (side near the cap) and checked reading with red tip on other side of diode. Got about the same reading (.52) on each diode, with the DMM connected this way.

    Then tried reversing the DMM connections and a reading of 0.0 on both diodes...so they seem to be oriented the same way, yes?
     
  10. schmidlin

    schmidlin Member

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    See post #4.
    The PT is currently (no pun) using only half of it's potential current supply. Kinda surprised it didn't wear itself out using only half of the PT.
    Hook it up right and you should definitly get more punch out of it. Probably quieter, too.
     
  11. Blue Strat

    Blue Strat Member

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    I hate to say it, but you guys messing with 300+ DC should really take a basic course in electronics. As it is, it's a half wave rectifier, move the anode end of the top hat to the other red wire and it becomes a full wave rectifier. Wikipedia probably shows the difference.

    As is, you're getting 300+V peak humps every cycle. With the correct wiring (since the diodes are different, someone probably replaced the original diode with the top hat and connected it to the wrong terminal) you'll get 2 humps every cycle...just like what it is...a full wave rectified power supply.

    It's not about current so much as having the full wave to deal with instead of only half.
     
  12. Blue Strat

    Blue Strat Member

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  13. donnyjaguar

    donnyjaguar Member

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    This unit has been repaired 'cuz the black diode and Milwaukee resistor aren't original. Incorrectly repaired it would appear. :)
     
  14. CA_Dan

    CA_Dan Member

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    My thoughts also.
     
  15. Blue Strat

    Blue Strat Member

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    So, the incorrectly connected top hat diode is original? ;)
     
  16. Leonc

    Leonc Wild Gear Hearder Gold Supporting Member

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    Well, thank you all. Mystery resolved. I replaced diodes and wired it up like my 2nd diagram, i.e., a normal full-wave rectifier. Amp sounds great and it is indeed quieter. I have no idea when the earlier "repair" was done or what they intended to "repair"...I'm hardly an expert but it appeared to have been done a long time ago as everything in there had an "untouched" look to my not-terribly-experienced eye. As for two different brands of diodes...hell, I've seen all kinds of crazy **** inside old Magnatones so that in itself didn't set off any alarms at all.

    But I did recognize something not being right when I saw the second diode going to ground...

    I'm not experienced enough to quickly recognize a bad repair per se; I tend to assume other people know more about what they're doing than I do...and of course, this proves to be a BIG, BAD assumption sometimes :D
     

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