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Re: Which bias method is the most accurate?


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4/27/1999 7:10 AM
Liam
Re: Which bias method is the most accurate?
Sorry Reid,  
 
What I meant is all "inductances" are short circuits, and so don't need to be considered for DC biasing. Resistance obviously does need considering.  
 
AC ripple should be negligible, as it will appear as hum over the speaker.  
 
Thanks for pointing out my errors in explaining this.  
 
Liam
 
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4/27/1999 4:32 PM
Reid Kneeland

No apology necessary!  
 
quote:
"AC ripple should be negligible, as it will appear as hum over the speaker."
 
 
Not in a reasonably well-balanced push-pull output stage (that's why I was specific about that). You can have fairly severe ripple on the B+ (which would be totally unacceptable in a single-ended design) and still have a quiet output. The ripple gets inverted and summed with itself in the OT primary, nulling it out, but it's still there on the B+ and on each leg of the primary.  
 
Reid
 
4/23/1999 8:11 AM
Bruce

Generally, this is time better spent on other tweaks IMO. Either method is close enough.  
Don't beat your head against the wall on this folks!  
I use the transformer shunt method when doing repairs and such. Tricky and I keep a tray of fuses for my meter nearby!  
It is very reliable and pretty accurate with a halfway decent meter though.  
But, when I build an amp that uses fixed bias, I use 1% 10 ohm 1/2w resistors, (to decrease the error factors ), and install them on the cathodes. You would read tenths of a volt instead of hundreths to interpolate the ma of current.  
You will be reading the plate current and the screen current so adjust accordingly and be withing a few % of right on the money.  
I think + or - a few miliamps either way is insignificant in the real world and don't fall for the guru hype about matched grid load resistors and caps and all that rot!  
Some of the best sounding amps I've heard startled me when I looked at the bias of each tube and all the slightly off value parts surrounding the driver and output tubes.  
The amp should have a personality and color the sound a bit, not reproduce it flawlessly.  
Liams + or- 3% to 5% is way good!  
Besides, depending on the manufacturer, once the tubes gets hoter then heck a few times from having driven the crap out of them, they aren't perfectly matched anymore.  
 
Bruce
 
4/23/1999 12:25 PM
Dave Stork

Re: Which bias method is the most accurate? (don't lose sleep over it)
Generally, this is time better spent on other tweaks IMO. Either method is  
close enough.  
Don't beat your head against the wall on this folks!
 
 
I think + or - a few miliamps either way is insignificant in the real world and  
don't fall for the guru hype about matched grid load resistors and caps and all  
that rot!
 
 
I'm with Bruce. People worry way too much about these things. Biasing is a ballpark adjustment anyway because of the unregulated supplies found in most amps and the changes in output tubes characteristics over time. I have never found a "discrepancy" of a couple of milliamps of plate current to make any difference in operation. I don't even bother with buying matched tubes. I've found that most decent quality tubes from the same batch will be close enough. Of course, I always verify this because there are some rare exceptions. If the mismatch is bad enough you can usually hear it manifested as hum.
 
4/23/1999 2:00 PM
TAP

Dave, what is your "cutoff" for tubes being to far off (in current matching) 5ma's,10ma's?  
 
tap
 
4/24/1999 12:43 AM
Dave Stork


I don't have a "figure" that I use. I test the amp thoroughly including power output and THD measurements and listening tests. I try different output tubes if the amp doesn't perform to my satisfaction. As I mentioned, an increase in hum is a fairly obvious sign that the output tubes you've just installed are badly mismatched.  
 
My attitude is that current draw figures are OK for getting you in the ballpark but the proof is in the performance. That's why I get a little annoyed when I hear people talking about how so-and-so says to bias 6L6s for X milliamps or whatever...
 
4/24/1999 1:08 PM
SpeedRacer

Dave - I'm in your bias camp 100%  
(of course, I'm imbalanced to begin with..)  
Some of my best amp-moments have been with tubes that were "way out" by some standards.  
It's great that there is such an awareness of the need to bias but IMO People need to LISTEN first, and sweat the theoretical maths later.. (this is about music and tone first right?) bias is just a static point (just like idle on a car) and is not some absolute setting that will ensure that everything else will go perfectly when the "revs come up", if you get my automotive drift. (your secondaries open, and if your car is old enough, raw gas may come out your tail pipe..) With NOS tubes in a Fender, I will tend to stick to the (original 1960's) factory setting of 40mA, but even then I'll more it around if the sound is not up to par, or swap out the tubes. Different tubes in different amps act differently. My sovteks kick sand on my NOS in some amps, and visa versa. IMO The only absolute is that there are no absolutes. :-) Go into the whole set up thing with open eyes and be amazed at what you'll see.  
(ok, next soap-boxer..)
 

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