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Thread: DC Not Blocked in output stage. Why ?

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    Member HaroldBrooks's Avatar
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    DC Not Blocked in output stage. Why ?

    Please bear with me, as I am relatively new to fixing tube amps, and still have some fairly big 'holes' in my understanding of electronics.

    That being said, I suddenly considered the fact that there are no coupling caps after power tubes, and the plate voltages go directly to the output transformer. I realize at that point there is no grid of the 'Next' stage to worry about, but why is it important to still have the DC current go to the output transformer ? How is this beneficial to the signal path and audio output ?

    I believe the output transformer is blocking the DC, and just letting the AC audio through ? Would it help or hurt to add coupling caps after a power tube (before the OP transformer), and if so, Why or why not either way ?

    I've never taken an electronics class, and it's something I seemed to have missed !

    (Embarrassing considering the work on amps I've engaged in so far, but I believe asking even very fundamental questions will pull me up the ladder)

    Thanks for your help !

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    Supporting Member eschertron's Avatar
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    Think of it this way: the plate of the tube is not feeding the output transformer, but rather the output transformer is feeding the B+ to the tube. The output transformer (or half of it) replaces the plate resistor that you see in a preamp stage. No cap there, so no cap on the OT primary.

    The signal getting to the secondary of the OT is happy side effect of the power tube's changing conductivity (controlled by its grid), and changing the amount of current flowing through the OT. There's no current flowing from OT primary to secondary, if that's what you were thinking.

    As far as blocking DC through the tube itself, it would be hard (understatement) to bias the tube with no current. I'm not sure if that would work in class C or not?

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    Tube operation requires a load impedance to develop an output signal as well as a DC path for the plate current. In low signal signal stages both is accomplished by a plate resistor. But a plate resistor wastes a least half the available output power in dissipation. A transformer load is much more efficient (maybe 90%). Using coupling caps between output tubes and transformer is not necessary and would not allow for DC plate current without additional plate resistors wasting power.

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    Last edited by Helmholtz; 04-12-2019 at 04:27 PM.
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    Member HaroldBrooks's Avatar
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    Fantastic, and thank you.

    I forgot that critical point of the output transformer is actually feeding the B+ to the power tubes ! It's right there on the schematic, but I keep ignoring it because the output transformer looks so small, and before I started fixing amps, I never realized the big voltage travels through that small transformer (only considered the power transformer as the Big Daddy), but it's burned into my brain now.

    Also as you stated, there is the resistance of the transformer replacing the plate resistor. Now I see how the AC will still pass through, and the output transformer acts as the 'Block' for the DC while supplying it in the first place, and regulating the voltage via it's natural resistance.

    Once again, thank you eschertron for setting me straight on this.

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    Old Timer J M Fahey's Avatar
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    Shorter answer: transformers do not pass DC, so there is nothing to block.

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    Juan Manuel Fahey

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    Member HaroldBrooks's Avatar
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    Great, thank you for going further Helmholtz ! I won't forget the lesson regarding the output transformer's role now. No need for plate resistor or coupling cap I see, as it would waste signal energy and is not needed.

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    Member HaroldBrooks's Avatar
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    Thank You JM Fahey !

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    Supporting Member Jazz P Bass's Avatar
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    The 'True' role of the output transformer is to match the output tubes specified plate impedance to the speaker impedance.
    Roughly speaking you are looking at something like a 2K to 8 ratio.

    Click image for larger version. 

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    Here is a good but rather lengthy read:
    http://education.lenardaudio.com/en/...ve_amps_5.html

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    Yes, the OT provides the necessary DC path and steps up the the speaker impedance to a value that allows the tube deliver max. output power (800 Ohm to 2.5 K per tube for PP amps and up to 10k for single ended amps).

    It also isolates the speaker output from high voltages.

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    Last edited by Helmholtz; 04-12-2019 at 06:08 PM.
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