8080 tube circuit question

Discussion in 'The Workbench' started by luvmy8080, Oct 20, 2020.

  1. luvmy8080

    luvmy8080 New Member

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    I had this guy apart for a while trying to figure out why I had low dirty signal with master volume off. Figured that out, but after spending way too much time looking at the schematic, I noticed that the signal from the second plate doesn't go anywhere.

    The way I had always understood it, grid->plate->grid->plate->rest of the amp. This one doesn't seem to do that. What's the operating principle?
     
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  2. william vogel

    william vogel Well-Known Member

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    It’s called a cathode follower. The first triode is a gain stage and the second triode is DC coupled. It’s used to drive the tonestack. Notice that the load resistor (100k) is on the cathode of the second triode. The signal from the second triode is taken from the cathode before the load resistor. It’s a non inverting stage and it’s gain is slightly less than unity (<1). Okay, you say why would this be so good? The output impedance of a typical inverting 12ax7 gain stage is about 38k bypassed with a cathode capacitor or 65-69k unbypassed. In a cathode follower the output impedance is really low, like less than 800 ohms. It can drive a tonestack. A tonestack depending on how the pots are turned can have high or very low impedance and the output voltage swing will be dragged way down unless it’s driven from a low impedance stage. I know this is technical but your reckoning for the condition from your amp has nothing to do with the cathode follower stage.
     
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  3. luvmy8080

    luvmy8080 New Member

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    Thank you for that. (My amp problem was a leaky cap in the power supply. Found signal on the +15v rail, replaced C40, 41, 61, and 62. Problem solved)
     
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  4. SkyMonkey

    SkyMonkey Well-Known Member

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    Glad you got your 8080 sorted @luvmy8080 and welcome to the MF :welcome:

    I had one of those back in the 90's and still wonder why I sold it (for a Peavey Transfex 208s!!!). :shrug:
    Keeping the Valvestates alive!!!!:hbang:
     
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