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How does using a variance on tube amp effect tube life

Discussion in 'Amp Tech Center' started by roberts67, Oct 19, 2020.

  1. roberts67

    roberts67 TDPRI Member

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    Hello! I was wondering if someone smarter than me on electronics can explain something. I like Rick Beato on YouTube a lot. He just did a video about getting the brown sound with a variac along with a great demonstration of the effect. At the very end of the video he says that dropping the voltage will decrease your tube life. I am think when you drop the voltage for sure the plate voltage is dropping but how is the bias effected? Does dropping the voltage shorten the tube life or is it the opposite? Looking for explanation of why/how? Purely for fun and a greater understanding. Thanks! Link to video that just dropped today -


    And spellcheck changed my title before I caught it. Variac, not variance.

    Robert
     
  2. JRapp

    JRapp Tele-Meister

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    Ummm, yeah...
     
  3. Dacious

    Dacious Poster Extraordinaire

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    The main reasons why are that not only are you dropping plate voltage but heater filament voltage. There's a range of maybe +/- 5% where it has minimal effect.

    Below that you get cathode stripping, where the metals that allow electron flow wear off, and the tube stops emitting. At that point, it's worn out.
     
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  4. StrangerNY

    StrangerNY Friend of Leo's Gold Supporter

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    We used to use them back in the day and we literally burned through tubes for our Marshalls. It got to the point where we put them back in the road case and just cranked it up, it was less expensive. :)

    - D
     
  5. Jon Snell

    Jon Snell Tele-Holic

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    Reducing the HT supply will make the distortion point happen at a lower level but reducing the heater voltage is definately not to be done as it reduces the life of the valve.
    A very silly thing to do in my opinion. Active adjustment of the bias voltage does the same thing and saves killing the valves.
    I sample the output level and using possitive feedback, purely at DC level, adjust the bias point on the output valves with the screen grid voltage.
     
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  6. mrriggs

    mrriggs TDPRI Member

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    Variac on heaters = bad. However, if you run the heaters off a separate transformer then driving the power transformer with a Variac does no harm at all to the tubes. If it's cathode biased then it'll automatically compensate to whatever voltage you set it to.

    I built a 5F1 with a built-in Variac on the power transformer (separate heater transformer). It was cool because turning down the voltage had no perceivable effect on the gain (until you really starved the plates) but would simply set the volume that it started to break-up. I could wail out power chords with the amp sounding like it was "turned to 11" but at a volume you could talk over.
     
  7. Fuelish

    Fuelish Tele-Meister

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    I dunno what y'all are talkin' 'bout...LOL
     
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