Power Transformer HT Winding Question?

Discussion in 'The Workbench' started by RickyLee, Aug 6, 2011.

  1. RickyLee

    RickyLee Well-Known Member VIP Member

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    Hopefully I will be explaining this correctly in writing what I am thinking in my Mind.

    :D

    I have a power transformer with a secondary HT winding that has no center tap - no reference to Ground. I need a bias supply winding and would like to tap off one leg of the HT winding like some older amps used to employ, like this Traynor for example:

    http://www.lynx.bc.ca/~jc/661021_YBA1.gif

    But this Traynor's HT winding has the center tap. Question is can I wire up a similar HT and bias suply circuit to this Traynor using a HT winding with no center tap?
     
  2. RickyLee

    RickyLee Well-Known Member VIP Member

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    OK, Dumb Question #2:

    Would it be possible to wire up a bias supply circuit tapping into the 120V MAINS primary feed instead of a dedicated bias supply secondary winding if there was not the latter on a secondary of a power transformer? With a dedicated fuse and all components adjusted to convert the 120VAC into the negative DC range nedded for a bias supply circuit?

    Is this also a case of there would be too much voltage flucuation with MAINS power variation - control?
     
  3. Lane Sparber

    Lane Sparber Well-Known Member VIP Member

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    You can, in fact do this if you use a capacitive divider circuit like the 900s had. Note that this circuit has a negative (ground) reference built in:

    http://www.drtube.com/schematics/marshall/cd0192-100-iss1.pdf

    Also observe that there's no center tap on that B+ winding, either. ;) Your resistance values may vary depending on your B+ voltage and how much negative voltage you'll need to bias the power tubes...experiment and see what works best for your individual amp.

    Finally, these capacitive divider circuits work best if there's a constant load on them...or else the capacitor can start to degrade over time. Does this amp's B+ topography have any totem pole filter caps with bleeder resistors across them? That would do the trick.

    -Lane
     
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  4. MajorNut1967

    MajorNut1967 New Member

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    Oh ya!
     
  5. RickyLee

    RickyLee Well-Known Member VIP Member

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    Thanks Lane.

    Actually there is no amp I am referring to, just a power transformer(s) I have from some old parts and such. I am looking at some things to possibly put a FrankenAmp together eventually.

    Yes, that 900 schematic and the secondary HT winding has no center tap - just as the DSL/TSL amps as well.

    What would be an aprox. AC voltage measured across that 900's HT legs? Could you then get an AC reference to Ground on each leg of that HT winding due to the tie in to the bias supply going to Ground/negative?
     
  6. Lane Sparber

    Lane Sparber Well-Known Member VIP Member

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    The ground reference int the capacitative divider circuit is independent of any ground reference for the B+ windings and can't be used that way. The full wave bride rectifier used for the HT circuit has it's own built in ground reference, so it all works itself out. :)

    -Lane
     
  7. jcmjmp

    jcmjmp Well-Known Member

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    Check out the DSL schematic too. No center tap on the B+ (but the bias comes out of the 22-0-22 taps).
     
  8. Wilder Amplification

    Wilder Amplification New Member

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    Yes the capacitive divider circuit will work and as Lane mentioned, the main B+ supply will need a constant load on it at all times, which bleeder resistors across stacked filter caps will provide. Otherwise in standby the capacitor in the divider becomes a charge pump that can and will blow the capacitor eventually.
     
  9. RickyLee

    RickyLee Well-Known Member VIP Member

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    What I was thinking here was, could you get an AC measurement reference to Ground/negative on each HT leg with the amp in standby mode?




    On this 900 schematic you posted (I am figuring this is the MKIII ?), the power transformer secondary seems to have the same windings/taps as the DSL/TSL amps. But the DSL/TSL bias supply uses the 20-0-20 taps. What is this 900 amp using the 20-0-20 taps for?

    And I do not see the bleeder resistor/totem pole style filter caps on this 900 schematic either. So I am confused on what you guys are referring to as the constant load while in STANDBY? So would R7 56K be the constant load you are referring to? I have not seen this topology in a bias circuit before - R7 & C15 where they are BEFORE/UPSTREAM of the reverse biased diode. Is that due to the transformer windings having no center tap to negative/Ground?

    And one more thing to clarify as I am still not getting that light bulb in my brain to light up just yet.

    :D

    This capacitive divider you are referring to is the bias circuit C13 C14 C15?

    I am also confused to how amplifiers using the totem pole style filter caps with their bleeder resistors are providing a constant load in STANDBY - as there is no voltage potential to the filter caps in STANDBY?
     
    Last edited: Aug 7, 2011
  10. Lane Sparber

    Lane Sparber Well-Known Member VIP Member

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    This all depends upon where you put the standby switch in the circuit (see the last part of this post). As you're doing a "ground up" build here, you can do whatever you want. The short answer is yes.

    Mostly the preamp FX Loop IC circuits.

    You are correct. The 900s DID NOT HAVE these totem pole stacks with bleeder resistors. That was their main problem and why their bias supplies sometimes just up and fail after a while - that capacitor dies slowly over time. When the standby switch is "off," the bias supply is connected directly to the HT windings...WITH NO LOAD. ;) The load (the rest of the HT circuit) only "appears" when the standby switch is flipped on. In a "totem pole" arrangement, those "bleeder" resistors provide this load. To answer the rest of your question, R7 and C17 ARE the capacitative divider circuit. See my next response for details.

    No, the capacitative divider circuit is simply created by C15/R7...everything after that is just the regular bias supply circuit. Those two components create the lower level AC voltage that's then rectified, smoothed, adjusted and "fed" to the power tubes by the rest of the bias circuitry.

    Here's how I'd wire it up: HT windings to the bridge rectifier to the first totem pole filter caps to the standby switch. Attach the capacitive divider circuit to the output of the totem pole filters...BEFORE the standby switch. That way, the bias circuit is always "on" (WITH a load on it) and providing the negative bias voltage to the power tubes - even in standby. Check out how the JMP 1959s were in their initial HT rectification and filtering (ignoring where the standby switch is here) - that might clear things up for you; you'd essentially be combing the following schematic with the 900 scheme I posted above. This would quite literally make this amp the "Frankenstein" you're after ;):

    http://www.drtube.com/schematics/marshall/1959mk2u.gif


    -Lane
     
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  11. RickyLee

    RickyLee Well-Known Member VIP Member

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    Outstanding info as usual Lane - Thanks again.

    The Light Bulb is getting a bit brighter now for sure . . .


    :D
     
  12. RickyLee

    RickyLee Well-Known Member VIP Member

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    That 1959 schematic is interesting for sure. New to me again is the HT winding center tap splitting the first series tied filter caps.

    Unless there is a glitch in the picture/print, it looks as if the bottom leg of the bias winding is tied to the top leg of the HT winding??
     
  13. Lane Sparber

    Lane Sparber Well-Known Member VIP Member

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    It's a glitch. In that amp, there's a separate 100vac PT secondary winding for the bias supply. One end is grounded, and the other end goes to the bias circuit. The bias winding and the HT winding are not connected.

    -Lane
     

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