Super Champ x2 bias

Discussion in 'Glowing Bottle Tube Amp Forum' started by Willie Johnson, Sep 16, 2020.

  1. Willie Johnson

    Willie Johnson Tele-Meister

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    Hey all. Took the plunge and got myself a multimeter today to bias my amp. I've read here and elsewhere that these come from Fender biased cold, and read 34.5 mV. Turned the bias pot to read 40 on the nose per spec (about 1 o'clock on the blue bias pot). Is there any good reason to run this hotter, or colder than that? I've got the stock GT 6v6 R's back in there (4's in their rating system), after running old, fatigued RCA grey glass/black plates for a while in there that started to lose their lustre (just out of curiosity, I popped those back in there, and with the bias turned up all the way, couldn't bring them up past 23 mV). Any advice, please let me know (besides don't shock yourself; yeah, yeah, kept my left hand behind my back the whole time). Thanks!
     
  2. archetype

    archetype Fiend of Leo's

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    If the GTs sound good at 40 mV, twist that trim pot up to 45 mV and see if you like the tone better. You're measuring at the recommended test point that sums the bias voltage: 40 mV is 20 mV per output tube. You can do some bias calculation to ensure you're not trying to dissapate more wattage than they can handle.

    NOS or used US, EU, UK, and Japanese 6V6-GTs can do 45 mV at the test point without breaking a sweat and most should sound good at that point with variations of that smooth 6V6 breakup.

    JJ 6V6S tubes are a slightly harder, brighter tone in the SCX2 and sit kind of between the 6V6 and 6L6 tones, mostly due to their longer plates. They aren't a true 6V6, but many folks like them for a cleaner tone when cranked. They're built like a battleship and should take up to 50 mV at the test point.

    BTW, your GTs won't last forever, so be thinking about another, better set. NOS or used GEs and Sylvanias are cost-effective and sound good. I've got old Tung-Sol 6V6-GTAs in mine, 45 mV at the test point, and they are great when the amp is cranked up.
     
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  3. Dan_Pomykalski

    Dan_Pomykalski Tele-Meister

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    There’s normally a few other things that need to be taken into account than just the mA, although I’m not familiar with the amp at all to say either way. You can use Rob’s bias calculator: https://robrobinette.com/Tube_Bias_Calculator.htm

    If you can’t tell a huge difference, I’d suggest running the tubes a bit cooler than hotter, but it’s not my amp.
     
  4. Willie Johnson

    Willie Johnson Tele-Meister

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    Thanks to you both. This place is so helpful--I'm so grateful for this community!
     
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