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Re: Setting Bias/voltage charts

7/31/1998 1:27 AM
Steve A.
Re: Setting Bias/voltage charts
    JD got his wrist slapped for the personal attacks in some of his posts here. So maybe we better not make personal attacks against him or we may end up being deleted, too...  
Steve Ah. . . . [deleted!]
7/28/1998 8:12 PM

I don't really understand why setting the bias is so hard to understand, especially in guitar amps. For your edification I now present my easy method for biasing guitar amps. I call it my 10/20/30 method. Works great if you don't have a scope!  
First I remove the power tubes and measure the B+ voltage, then set the bias voltage to roughly 10% of that. Then I figure the maximum current allowable given the aforementioned B+ voltage and derate it by about 20% to allow for shitty tubes. This is my adjusted maximum value.  
Next the tubes go in and I set the bias to about 30% of my max value. Check for spots on the plate and make sure everything is kosher.  
Now I usually apply a 440Hz sine wave at 100mV p-p and run the amp HARD through a dummy load. For those of you at home without a function generator you can make a recording of a sine wave to a tape deck with something like cooledit or the like. It ain't perfect but it will work.  
After about 10 minutes or so recheck the bias current and look for overheating. If all is well start to crank up the current (in small increments)! Don't exceed 70% unless you want to kill your transformer. I usually stop at 60% for customers, just to be safe.  
If you do have a function generator, let them roast a while, say about an hour, so everything gets used to the new tubes and settings.  
That's all there is to it. And I could care less what the scope or factory specs say. The factory sets values for warantee purposes and the scope is for Hi-Fi. Been using this method for a couple of years and have yet to have a problem.  
7/29/1998 1:19 AM
John Dillinger

You sound like my kind of guy.  
Hope I haven't polluted you with  
Dillinger pox...
7/23/1998 6:49 PM

Mike: Lotsa good advise from Richie and Doc. My 2-cents: What is your power line voltage? Were I live mine runs high and every reading on every amplifier is high. Perhaps some of the high plate voltages you are reading are the effect of your power company. I don't remember what Marshall designed around for the line voltage but it may be lower.  
7/14/1998 4:28 PM
anonymous Re: Setting Bias
Here's my personal way of biasing that will no doubt disgust and horrify you guys: I seem to like the sound of the amp with a lot of juice on the tubes. The sound seems big and fat with huge amounts of bass. I turn the bias pot until the plates just turn barely red, then I ease off a bit to where I get the best sound and the plates are their normal gray.(btw, it'a 5f6a)  
I never understood about lower plate voltage, and a browner sound. Sure it seems to distort easier, but at higher voltages you thump on that E string and boy, you feel it! Comments welcome.

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