# Biasing for Dummies.



## dwagar (Mar 6, 2006)

Wild Bill or someone else please?

I figured it was about time I learned how to Bias. I'm dusting off the old JCM800 (clipped the treble cap, switched the 470k/470uf to 68k while I was at it).

Bought a "matched pair" of EL34L JJs. Bought an Amp Head Bias Tester off the net, I didn't trust myself sticking probes in there.

I'm reading a plate voltage of 465v on both sockets.
so, at 60%*25/465 I should be at 32.25
at 70%*25/465 I should be at 37.63. I should set under this, correct?

It was reading a tad hot so I rolled it back a bit. I'm reading 36.5 on one, 32.5 on the other. 4 Ma out isn't exactly matched, but I guess it's okay.

So, I'm in between 60% and 70%. Is this how it should be set? Or am I supposed to set just the highest or just the lowest value?

The other thing, I've read you should burn them in for about 50 hours. Should I recheck the bias after that time, or does it matter? Oh, and does "burn in" mean play time, or just leave the amp on for that amount of time?


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## Wild Bill (May 3, 2006)

dwagar said:


> Wild Bill or someone else please?
> 
> I figured it was about time I learned how to Bias. I'm dusting off the old JCM800 (clipped the treble cap, switched the 470k/470uf to 68k while I was at it).
> 
> ...


Sounds good to me! Bias meters work by putting a 1 ohm resistor in series with the cathode to ground. This means a tiny voltage develops across the resistor that is directly proportional to the combined plate and screen currents.

The screen current at idle is only a couple of ma. so usually we ignore it. That way we're always slightly low, which is safer for the tubes and still "close enough for rock & roll"!

I agree, a 4 ma spread is not that big a deal but I wouldn't like to see over 5 ma. You then start to worry about unequal power sharing, which can be hard on the tubes. A bit of spread like you have here will actually make for a bit more harmonic "swirl" to give a better guitar sound, IMHO. I also agree with setting for the hottest tube. Only common sense - why set by the lowest and have the other run too hot?

I've never bothered with 50 hour burn ins. If you buy from reputable people like thetubestore.com they burn them for a day or so anyway, as part of their matching process. That's enough 99.999999% of the time to weed out flaky tubes.

I've had amps come back for new problems months after I've biased them and I've never found them to drift more than a ma. or two. Never anything serious. I've had a few where the amp had over two years on the output tubes and the idle current spread between the tubes had gotten much bigger, to the point where I recommended a new set as the old ones were getting on in age anyway.

Much of the "anal" factor about biasing comes from the audiophile world, where they BS each other that they can hear the difference between 33 ma. and 33.5 ma. of idle current!:smile:

:food-smiley-004:


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## dwagar (Mar 6, 2006)

thanks man! 

With the little tester, it wasn't hard at all. The hardest part was figuring out exactly what I was supposed to be doing.

And of course, being extremely careful cause I'm scared shitless of high voltages.


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