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TOTOTOB 发表于 2013-10-23 14:52 
首先，感谢你的关注，你追求真知的态度和精神值得我学习，在这里以我的肤浅的知识与你一起互相学习 ...
Vancouveraudio 果然一语导破啊! 听起上来并不合理, 但原来真的是这样, 终於解开了我长期以来一个未能解决的问题.
好开心! 多谢指点!
http://www.valvewizard.co.uk/se.html
这是我找到的资料, 相信对大家都很有帮助的.
Choosing a bias point:
Because we are using a reactive load rather than a resistive one (i.e., a transformer and not an anode resistor), the bias is not chosen in the same way as for a preamp stage.
The transformer primary has very low DC resistance so only a few volts will be dropped across it, which we usually ignore. Therefore, the quiescent anode voltage will be the SAME as the HT, no matter how much quiescent anode current we choose. We could draw a vertical line at the HT voltage and know that the bias point must be somewhere on it. You can now see that with the load line in its current position, the valve would be biased at cut-off and could only operate in class B!
So, instead of selecting a bias point somewhere on the load line, we slide the load line up the graph, whilst maintaining its gradient. The grey lines show this process. The load line must not go beyond the maximum dissipation curve, and it would be wise to choose a position slightly below the curve for safe running. Load lines for other impedances are drawn in the same way.
We decide that the quiescent anode current we want in this case is 75mA (purple dot), just below max dissipation, and you can see the valve is now operating close to centre-biased Class A (i.e., the bias point is roughly in the centre of the load line)
If you are wondering why it appears that the signal voltage can now swing higher than the HT voltage, it is because this is exactly what happens! Inductances abhor changes in current. When current through the transformer increases it stores energy, which is released when the current falls again, allowing up to twice the HT voltage to be developed. Because of this, the HT in a Class A amp must never be more than half the maximum peak anode voltage rating of the valve, given on the data sheet. For the EL34 this is 2000V so we are well within safe limits!
Now we have the load line we need to set the screen voltage before we can continue.
这清楚说明了 vancouveraudio 所说的, 能量会储存在输出牛那里, 而且这能量能达 B+ (HT) 的2倍, 所以我们选择静态工作点的屏压时不应超过该管子最大屏压的一半.
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楼主
发表于 2013-10-21 14:24
