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发表于 2014-1-17 18:55:04
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就我目前的经验和体会，我不知道为什么是70%。我也不想告诉蝎子先生什么，人家没问过我，我也不够格。我的话没有分量，但我可以搬运。我知道国外很多讨论这个问题的资深网友，并不认为70%是个放之四海而皆准的最好比率。比如下面这段话：
“%ratings are over rated. Numbers don't hear and 70% is generally WAY too high. Try between 50 and 60% and use your ears. On 6V6 amps 20mA seems to be a good number Percentage wise that's usually 70-80% but still fine in Deluxe Reverbs. ”
这段话是一个卖管的专业人士说的。大意是：比率的重要性被过分夸大了。要知道数字并不是声音，而70%一般来说太过高了。尝试用耳朵在50%~60%之间寻找合适的比率。但是6v6是个例外。在deluxe reverbs上6v6可以适应20mA，而这个已经达到70%~80%的比率了。
他还说："70% is unnecessarily high for 6L6s and EL34s and usually used when someone has an amp that's more powerful than he needs, attempting to make it break up sooner.
It also leads to shorter tube life (good for me, right?  ). Generally, I doubt many people can hear the difference between 60 and 70% in a BLIND test. No other tests are really valid."
随便翻翻：70%对6L6和EL34来说，是一种过度使用。有一些人拥有着超过自己需要的大瓦数箱头，他们试图用这种高比率来让自己的箱头更容易破声。而这会让管子短命（对我有好处不是啊？）。一般来说，我很怀疑人们能在盲听中分辨出60%和70%的区别。
还有另外一个资深高手说的：
“I think people get too concerned about exact bias point.
Yes it's important, as everyone knows bias too hot they burn up, too cold and crossover distortion becomes excessive. I agree w/ Mike and Alex about what most can hear in a blind test. No offence to those with GOLDEN EARS but it's scary how high the bias can get before it "SOUNDS ABOUT RIGHT".
Almost all pentode class AB1 amps have a NFB loop which reduces crossover distortion and gives you a little more leeway
to bias colder w/o too much sacrifice in tone, 60% or lower, and on the other hand if you never push the amp hard then your dissapation stays in the low range so you can get by hotter without too much worry, 70% or higher, but there are other factors.
Plate voltage is significant.
If it's a high voltage amp then of course you must bias at a lower current to reduce plate diss as everyone knows but what gets overlooked is that now you are biased further toward class B which means as you drive the amp hard the increase in dissapation from quiesent to full signal is even greater and 70% may not be safe especially if you drive it hard.
Another thing that doesn't get mentioned much is screen dissapation. It's usually the screens that go and as you bias hotter, so also goes the screen diss.
Amps with no screen resistors are more at risk. I like to use 1K on my 6L6 builds.
Related to bias is OT DC balance. With most amps all you can do is used matched pairs so on amps w/o balance provision you can measure the DC resistance of the OT pri. halves then power up and measure voltage drop across them to find the dissapation through each pri side, swap the tubes into the most balanced positions since the DC resistance of the OT pri windings will never be exactly matched.
This helps to minimize hum and higher order odd harmonic distortion generated in the OT.
Before you're done measure voltage drop on the screen resistors then cathode current and you have a pretty good idea what's going on in the power stage.
OT Pri dissapation should be the same as the cathode and screen dissapations added together
My general rule of thumb is 35ma - 40ma cathode current with most 6L6CG amps.
Mike K is always very conservative on his bias recomendations which is kind of funny since he sells tubes, but I would take his suggestions to heart.
”
太长，懒得翻译了。抱歉。 |
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