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Thread: 6505+ bias

  1. #1
    Toneologist TSE's Avatar
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    Default 6505+ bias

    Ok, so I read on other forums about how cold the stock 6505+ amps come from the factory and the bias pot does not have a wide sweep and even at max the tubes are only seeing about 20-25 ma. So I just turned it to the max and and the amp sounds MUCH better. I think I got it out of crossover distortion a little bit.

    My only concern is the power tubes. When I play a note at high volume the blue(cathode current?) gets brighter and dimmer when not playing anything. Didn't check if it did this before or not, is it something I should worry about?

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    Electron Herder glassman's Avatar
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    Default Re: 6505+ bias

    Quote Originally Posted by TSE View Post
    Ok, so I read on other forums about how cold the stock 6505+ amps come from the factory and the bias pot does not have a wide sweep and even at max the tubes are only seeing about 20-25 ma. So I just turned it to the max and and the amp sounds MUCH better. I think I got it out of crossover distortion a little bit.
    That's how they run. Boogie's run in the 16 to 20 mA range. This is what gives them a tight, pecussive response; the 6505+ is much the same animal. Higher plate current will give it more of a fuller yet spongier tone...not really the tone that the design was targeting but it would be useful to some people. The small sweep of the bias control was a mistake-proofing measure; with the knowledge that many players are tweaking bias to change tone, Peavey simply limited the range of the pot to one that is in a safe range through the whole sweep of the pot.

    Quote Originally Posted by TSE View Post
    My only concern is the power tubes. When I play a note at high volume the blue(cathode current?) gets brighter and dimmer when not playing anything. Didn't check if it did this before or not, is it something I should worry about?
    The blue corona is totally normal. It is the result of a little bit of "gas" that makes its way into the glass envelope. Every tube has it to some degree. Blue is normal, magenta is bad.
    Now operating part time: Glassman Tube Amps...repairs, rebuilds, restorations & modifications of tube equipment.

    Still building: GlassMan Amplifiers (25 watt, all tube, single channel w/reverb, single 12" combo) and some more designs in the works.

    Located in Fort Wayne, IN


    Note: I've "parked" my website in case anyone has been looking for it. I moved locations and haven't updated the site to reflect this.

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    Toneologist TSE's Avatar
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    Default Re: 6505+ bias

    Hey thank you very much, I didn't really know about the magenta thing, very helpful reply. Thanks again.

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    Gear Ho Gearjoneser's Avatar
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    Default Re: 6505+ bias

    IMO, you should only worry about power tubes being over biased when you turn out all the lights, play the amp for hours, and notice that the gray plates inside have any sort of an orange glow to them. Then, you've over biased the amp. If they're just glowing bright when you dig in, you're still safe. If you've got a mA bias tool, which is what I use, I just try to keep 6L6's around 32 mA and EL-34's around 36mA- 38mA.

    Biasing by ear is not always recommended by amp techs, but a lot of guys do it. It means turning out the lights, having the chassis out, and getting the amp to sound as bold and strong as possible, listening carefully. I usually run the amp at stage volume, set where I like it, then setting the bias where the amp sounds it's very best.
    Quote Originally Posted by Boogie Bill View Post
    I've got 60 guitars...but 49 trumpets is just...INSANITY! WTF!

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    Toneologist TSE's Avatar
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    Default Re: 6505+ bias

    Is that a multi-meter to mesasure the bias with? And Can I just use the testpoints on the back of the amp? Or do I HAVE to buy an o-scope?

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    Electron Herder glassman's Avatar
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    Default Re: 6505+ bias

    Quote Originally Posted by TSE View Post
    Is that a multi-meter to mesasure the bias with? And Can I just use the testpoints on the back of the amp? Or do I HAVE to buy an o-scope?
    You can use the test pionts on the back of the amp and a VOM. I only use my scope on high end audio amps such as Fisher and McIntosh amps which have a far more complex biasing sequence (some Fisher amps have 4 pots you have to adjust). The only time I ever use a scope on a guitar amp is when I need to find a noise source within a circuit.

    Your 6505+ is designed so that the range of the pot won't allow you to set it in a range that will cause the tubes to redplate. It also keeps all potential bias settings firmly in the range that a metal amp should be (18 to 22mA). I have never set a "metal" type amp with 6L6s any higher than 25mA...ever. And lets face it, nobody buys a 6505+ to play blues or classic rock.

    Your amp could be easily modified to allow bias adjustments in the 30 to 35mA range but I wouldn't suggest it. Most guitar cabs (with a few exceptions) can't handle the bass capability of a 6L6 equipped metal amp when they are running at these levels. The bass becomes overbearing and your palm mutes will have a droning ring that follows them that iritates the crap out of most players. When setting bias on a metal amp, its all obout how your amp AND cab handle the stops and setting it so that it has the character you are looking for.
    Now operating part time: Glassman Tube Amps...repairs, rebuilds, restorations & modifications of tube equipment.

    Still building: GlassMan Amplifiers (25 watt, all tube, single channel w/reverb, single 12" combo) and some more designs in the works.

    Located in Fort Wayne, IN


    Note: I've "parked" my website in case anyone has been looking for it. I moved locations and haven't updated the site to reflect this.

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    Fudgeitallologist PVFan's Avatar
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    Default Re: 6505+ bias

    Quote Originally Posted by glassman View Post
    I have never set a "metal" type amp with 6L6s any higher than 25mA...ever. And lets face it, nobody buys a 6505+ to play blues or classic rock.
    Quote Originally Posted by glassman View Post
    Your amp could be easily modified to allow bias adjustments in the 30 to 35mA range but I wouldn't suggest it. Most guitar cabs (with a few exceptions) can't handle the bass capability of a 6L6 equipped metal amp when they are running at these levels.
    I did all of the above and am happier with all of my sounds than I've ever been with any rig borrowed, owned, stolen, snatched, tried, played, heard.

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