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Thread: Self-Biasing

  1. #1
    Old Timer
    Join Date
    Jun 2007
    Pensacola, FL


    So I often find that self-biased amps need Rk increased. Typically I'm replacing a toasted set of power tubes and increasing Rk. Why do these companies run the power tubes so dang hot?

    Common amps with this issue are...
    Egnater Tweaker
    Blues Jr
    Crate VC30

    Working on a tweaker now. One 6V6 is conducting 82ma!!! With a new pair of tubes it's down to 44ma at 360v. About 360v... About 16w dissipation.

    Double checked that...

    Vgrid is 0v and Rk is close enough to the spec'd 220... reads 215.

  2. #2
    Supporting Member Chuck H's Avatar
    Join Date
    May 2007
    pacific north west
    Such amps are biased class A (probably for the street cred) and remember that you need to subtract screen current to know the actual plate figure. I find this especially important with the little bottles. In reality these amps are driven into AB due to excessive grid signal. This can indeed have them biased too hot for their operating conditions and I've changed a few myself. I guess you just have to take it on a case by case basis because there seems to be no end to this misunderstanding on the part of designers. Then again, El84's are cheap. 6V6's aren't prohibitively expensive. El84's just go microphonic prematurely. I don't know what 6V6's do because I haven't worked with them a lot. IMHE they pop actually

    So, yes, I've noticed it too.
    lowell likes this.
    "So I acquired it for the purpose of fixing it up - in case I run out of things to do with the rest of my life..." tubeswell

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  3. #3
    Supporting Member
    Join Date
    Oct 2011
    Manchester, UK
    I recently built a 2xEL34 amp running at 380V with a 4k primary, loosely based on the hot cat 30 power amp and gingertube's junkbox trainwreck. The badcat uses individual 270r bias resistors and it notorious for chewing through tubes so I increased to 360r per tube. I'll eventually drop this to 330r when I can be bothered. I figure potentially losing a little bit of tone is favourable to binning a set of bulbs every few months, especially since I use this as my main recording amp with a reactive load box. The upshot here is that it's not really that intimidating to near dime it and slam the front end with an overdrive when you can control the monitor levels in the DAW! Now in the room through a cab that'd be quite the thing.
    lowell likes this.

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