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最近翻译的一篇文章及原文……关于吉他音箱的问答(更新)

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Q: What is the best type of resistor to use - metal film, carbon film, or carbon composition?

金膜电阻,碳膜电阻以及碳合成电阻哪一种最好?
A: It depends on what you are trying to accomplish. 具体使用哪种电阻,取决于你想要得到什么样的音色。 If you want the absolute lowest noise, metal film is the only way to go.如果你想把噪音降低到最小,金膜电阻肯定是你不二之选。  However, some people prefer the tone of carbon composition resistors, which they say are subjectively "warmer".  然而,也有人刻意使用碳合成电阻,以便得到一种比较温暖的音色。Unfortunately, carbon composition resistors have notoriously poor drift and noise characteristics, and are responsible for most of the "crackling" and "frying" sounds heard in most old Fenders and other type amps.不幸的是,碳合成电阻在漂移和噪音方面臭名昭著,老式的Fender和其他一些旧型号的品牌专箱也都因为使用碳合成电阻而有裂边和噼啪声。  Carbon film resistors are a good compromise between the low noise and excellent stability of metal film and the subjective "warmth" of carbon composition.碳膜电阻是折衷之选,既有金膜电阻的低噪声和稳定性,又有碳合成电阻的温暖音色。  Larger wattage resistors are quieter than smaller ones, due to the geometry of the part, so you should always use the highest power rating you can find, particularly in carbon composition resistors. 大功率的电阻因为其内部构造的原因,会比小功率电阻噪音小。所以你选用电阻最好尽量选用大功率的,尤其是使用碳合成电阻的时候。 In addition, most lower power resistors, such as 1/2W and 1W units, are only rated for a maximum of 250V to 350V. 另外需要注意的是,大多数1/2W 和W电阻耐压仅有250V 到 350V 。I recommend using only 2W 750V resistors for plate load resistors and other locations where large voltage swings or high turn-on voltage surges are expected. 所以我建议,作为屏极负载的电阻,以及其他可能出现高电压或感应高压的场合,一定要使用2W 750V 的电阻, The first stage of the amplifier and other low-level stages, such as reverb recovery amps, are the most critical for noise, because the signal level is lowest at that point, so proper attention to the type of resistor used will have the most impact on overall amplifier noise performance. 对于第一级前放,以及回声反馈级这样的低电平放大级,由于信号电压很小,抑制噪音电平会显得尤其重要。在这种场合选取哪一种合适类型的电阻,对于后面放大级的噪音抑制会有很大的影响。

Q: What kind of resistors do you recommend for power supply decoupling use?

电源的去耦电阻你推荐使用哪一种类型比较好呢?
A: I recommend 5W (or a higher power rating if needed) 1% 750V metal oxide resistors for this application.这种场合下,我建议使用额定功率 5W 精度 1% 耐压750V 的金属氧化物电阻,如果有必要还可以取更大的额定功率。  They have higher surge/overload capability than the film types, excellent long-term stability, and flameproof construction. 这种类型的电阻比膜型电阻有更高的抗高压浪涌以及抗过载的能力,而且有很优良的长期稳定性,以及阻燃性能。 Note that lower-power metal oxides, such as 1W and 2W are usually only rated for 350V and should be avoided for this application.注意,比较低额定功率的金属氧化物电阻由于耐压不够,尽量不要使用在这种场合。 Even though the voltage across the resistor in normal operation is only 50 volts or so, it can be much higher at turn on, or if a tube fails.尽管正常情况下这个电阻上的压降可能只有50V,但是在开机瞬间,或者电子管损坏的情况下很可能会出现高压。  Another good type to use is the Ohmite OX/OY series ceramic composition resistors.这种场合使用Ohmite OX/OY 系列陶瓷合成电阻也不错。  These don't have the tight temperature coefficient or tolerance (typically 10%) that metal oxides do, but they are designed for heavy surge use. 尽管陶瓷合成电阻不像金属氧化物电阻那样具有很低的温度系数和精度公差(典型值为10%),但是他们却有更高的抗过载能力。

Q: What about screen grid resistors?

那么屏栅极电阻呢?
A:  I recommend 5W 1% 750V  metal oxide flameproof resistors for this application.我还是建议使用额定功率 5W 精度 1% 耐压750V 的阻燃型金属氧化物电阻  The flameproof "cement box" resistors are also good, but are usually only available in 5% and 10%, and must be rated for at least 7W or 10W in order to get an appropriate voltage rating, because the 5W and lower units are usually only rated for 350V.阻燃型水泥电阻也不错,但是通常这种电阻的精度不高,另外这种电阻额定功率5W时耐压仅有350V,所以要选用7W 甚至10W的电阻才行。  In normal operation, the screen grid resistor only has a few volts dropped across it, so a 350V resistor is overkill. 通常情况下,屏栅极电阻的压降仅区区几伏而已,350V的耐压简直是太大材小用了。 However, in the event of an internal tube short, the voltage drop can be much higher, up to the level of the supply, and the current through the screen resistor will increase dramatically, increasing the voltage drop across it. 然而,当电子管内部短路时,这个电阻上的压降会达到和电源电压一样高,并且伴随着高压还会发生电流的急剧上升 Normally, the fuse will blow fairly quickly, but sometimes not before the screen resistor is toasted. 通常这个时候保险丝会断路保护,但是屏栅极电阻有时会在保护之前就已经烧焦了。 There are two possible failure modes, over-voltage and over-dissipation due to the increased current from the short. 过电压和伴随电流急剧增加引起的过载是两种电阻破坏机理 If the voltage rating is too low, the resistor element will flash over and either burn up or carbon track and change value, leading to eventual failure down the road.如果电阻额定耐压太低,在这种情况下,电阻会产生内部碳粒单元的燃烧或者膜层之间打火,从而改变电阻值,继而引起该通路的一系列破坏行为。  If the voltage rating is made higher than the supply voltage, there is only one possible failure mode of the resistor, over-dissipation. 如果当初选择的电阻耐压足够高,那么发生电阻破坏的机理就只可能是过电流引起的过载。 The metal oxide flameproofs or OX/OY ceramics can usually take a higher-than-rated surge for a short time before they burn up. 金属氧化物电阻或者陶瓷化合物电阻的阻燃性能,足以使得他们经得起短时间的过载 If the fuse is appropriately rated, it will blow before this over-dissipation can harm the resistor. 如果保险丝选择得合理的话,那么电阻就不会有事。 A quick fuse and tube swap and the amp is back in business, instead of having to replace the screen resistors. 这样仅仅更换一个保险丝和电子管就可以了,而不必大修。



Q: Why do you recommend polypropylene film/foil capacitors?

你为什么喜欢使用聚丙烯膜电容?
A: Because they are the best, in terms of temperature stability, and, subjectively, tone.因为他们最好呗,他们的温度稳定性能真的是没的说,因此出来的音色也是极佳。  The interior of a guitar amplifier chassis tends to get very hot, particularly in a combo, where the tubes are hanging upside down beneath the chassis and the heat rises upward.  吉他专箱的内部在正常工作时的温度是很高的,尤其是一体式的专箱,因为电子管都是倒挂在箱体内部,产生的大量热气向上流动。 Polyester and metallized film capacitors have very poor temperature characteristics compared to polypropylene film/foil, which means the amplifier tone will change as the amplifier heats up.聚酯电容和MKT电容的热稳定性能明显不如聚丙烯电容,他们会因机箱内部发热改变音色。 A 5% or 10% drift in value can produce a very noticeable shift in tone, depending upon how the capacitor is used in the circuit, and all caps will drift in the same direction, which is determined by the characteristics of the dielectric material, so the effect is multiplied.由于电容值会因为发热向同一个方向漂移,所以整体电路工作点的漂移效果是因此而翻倍的,仅仅5%-10%的容值偏移就会明显的影响到音色的变化。  Also, polypropylene capacitors are far better in terms of dissipation factor and dielectric absorption, which some people say makes them sound better.  另外,聚丙烯电容在热耗散以及偶极子吸收方面也是性能超群,难怪有人说:早晚用丙烯,声音就是好!I use them because they have the best characteristics, even though they cost quite a bit more than the cheaper polyester caps. 尽管聚丙烯电容价格比聚酯电容高,但我还是觉得从性能方面考虑值得一用。 The long-term temperature stability and quality construction makes them worth it, in my opinion.因为我更在意长期的热稳定性和整机质量。  The most commonly used coupling cap, the Sprague Orange Drop, is available in polypropylene film/foil, but only in a radial-lead package. 最常见的耦合电容大概算是丝碧的橙色经典了,虽然他们有聚丙烯电容,但是都是同轴切向引线封装的 These are fine for PC board amplifiers, but some manufacturers use them for point-to-point boards by bending the leads out at the body of the capacitor. 这种封装形式只适合在PCB板上使用,而在搭棚焊接时,有人只好把引线从外壳中掰弯曲以便安装。 This usually leads to cracking of the epoxy seal around the lead entry points, which can allow moisture into the capacitor, leading to drift and potential failure. 这样做会使封装外壳的环氧树脂出现裂缝,继而导致湿气进入电容,引起电容值的偏移和其他潜在的危险。 I use only axial-leaded capacitors, because they are designed for point-to-point style mounting, and the epoxy end seals do not crack when they are mounted on the board. 我只使用同轴径向封装的电容,就不会发生这种事情了。

Q: What type capacitor do you recommend for small values in the pF range, such as in tone controls?

在音调控制时经常用到很小容值的PF量级的电容,应该选择那种类型的电容好呢?
A: Silver mica or polystyrene film capacitors are the best choices. 镀银云母电容或者聚苯乙烯膜电容在这里是最佳选择   They have the best temperature stability and most transparent tone. 他们有极佳的热稳定性能,可以给出非常亮丽的音色However, the polystyrene film caps in the pF range usually have very tiny leads, which may not be robust enough to stand up to the rigors of road use.但是PF量级的聚苯乙烯膜电容引线一般都太细,不适合在电子管音箱内使用。  Ceramic capacitors should be avoided, as they are usually horribly microphonic, have very high odd-order harmonic distortion (the "bad, or harsh-sounding" kind), typically mostly 3rd and 5th and 7th  harmonics, and have poor temperature stability.千万不要使用陶瓷电容,他们通常会引起强烈的麦克风效应,还会引入大量的奇次谐波,而且热稳定性能欠佳。 If ceramics must be used, the COG/NPO type has the best temperature stability and much, much lower harmonic distortion - stay away from X7R and Z5U types, and ceramic disk capacitors, in particular. 如果必须使用陶瓷电容,也是用COG/NPO型的陶瓷电容,千万不要用X7R 和 Z5U 型,尤其是圆盘陶瓷电容绝对不要使用。

Q: What kind of transformers do you recommend?

请你说说那种变压器比较好用啊?
A: I have my transformers custom-made for the amplifier they are to be used in.我通常都是根据功放来订做变压器的  I typically use Heyboer or TMI transformers. 我经常用Heyboer 或者 TMI的变压器。 For most general purpose home-building, Hammond transformers are readily available and reasonably priced.不过对于大多数喜欢自己DIY的人来说,去买一个Hammond的成品变压器更好,他们总是价格便宜量又足,童叟无欺,乃居家旅行必备之神器。

Q: Do I need to replace my driver tube every time I replace my power tubes?

我每次更换后级电子管的时候是不是必须把前级管也一起换?
A: No.  This is an "old wive's tale" propagated by the tube vendors who want to sell you more tubes.不是的,那些建议你这样做的电子管供货商不过是想多赚你的钱罢了,不要听信他们的话,  The "driver" tube, or phase inverter as it is more appropriately known, is under no more stress than any of the other preamp tubes.驱动管或者倒相管,在前置放大中承受的压力普遍都是最小的  The output stage of a normal guitar amplifier is AC-coupled, class AB1 or class A1.吉他功放输出级一般采用AB1型或者A1型放大级,交流耦合
。 The "1" suffix indicates that the tubes do not draw grid current during any portion of the input cycle. 在这里的数字1代表了在交流耦合的任何阶段,电子管栅极都不应该有电流。 A "2" suffix indicates grid current during some portion of the cycle, as in class AB2.数字2则代表有可能在某个阶段是有栅流的  Since almost all guitar amps are class AB1 or A1 (it takes a DC-coupled cathode-follower or interstage transformer to be able to drive the power tubes into the positive grid region), the phase inverter does not have to supply any current to the grids (it can't really source current anyway - it only sinks current through the plate load resistor, which is quite large, and inherently limits the plate current to a few mA).既然功放输出级都是AB1型或者A1型,那么显然前一级的驱动管或者倒相管就不需要输出电流给末级电子管的栅极  The AC coupling (capacitor coupling) between the phase inverter and the output tubes precludes any flow of grid current anyway.这种交流耦合的方式排除了出现栅流的可能  The output tube grids will merely act as a grid clamp, shifting the bias downward as the output is driven harder.输出级电子管只受电压而不是电流的驱动  This in no way stresses the phase inverter tube. 这样根本不会对倒相电子管带来任何压力  In addition, the plate load resistors and the bias current in a typical phase inverter are identical to the preamp stage values, indicating that they are operating in the same area, dissipation-wise, so they cannot be "wearing out" any faster. 在前置级的其他电子管显然也和倒相电子管工作状态相仿,根本不会那么容易就老损 In fact, the reverb driver tube on a typical amplifier is dissipating more power than the phase inverter, and should be replaced more often, if anything.事实上倒是回声反馈级的电子管更容易老损,需要定期更换  Don't be misled by the higher plate voltage on the phase inverter either, because the cathode is usually sitting somewhere around 30V - 100V above ground, which lowers the plate-to-cathode voltage by that amount.  The plate-to-cathode voltage is what determines the power dissipation of the tube, not the absolute plate voltage. 也不要因为倒相管屏压比较高而被误导以为它容易老损,事实上他的阴极电位通常比地高30 – 100 V,因此屏极相对阴极的电位并不比别的管子更高。


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Q: Why do you use point-to-point instead of PCB?  Because it sounds better?

你为什么偏好搭棚焊,而不怎么使用PCB板?是不是这样出来的音色更好?
A: No. There is no difference in tone between a properly designed PC board and a properly layed out turret board or other point-to-point construction technique.不是的,对于一个设计合理的排列来说,采用哪种焊接方式对音色并无太大的影响。  Note that I used the term "properly designed". 注意到我是说设计一定要合理。 There is a world of difference in the tone of an improper PCB layout and a proper one, just as there is between a proper PTP layout and an improper one.如果PCB设计得不合理的话,做出来的音色千差万别,搭棚焊也是一样的。 If high-impedance traces are run too close to other parts of the circuit, unwanted coupling can occur, leading to oscillations in the worst case, or just odd tone caused by frequency cancellations.例如某一支高阻抗信号线离别的信号线太近,相互之间就会产生耦合,带来同步振荡,或者由于差拍频产生奇怪的声音 This is not a problem that is unique to PC boards, as some "gurus" would have you believe.这种现象并不是PCB所特有的  If you run a wire on a PTP layout too close to another wire or component, you will have the same problems. 在搭棚焊机子里,用一根信号线靠近另一根也会产生同样的现象。 Don't believe the hype about "tiny capacitances" and "co-planar" traces robbing high end.不要听信某些人所说的PCB有微电容效应之类的胡说八道  PC boards are used in the gigahertz range, and are perfectly adequate for the audio frequency range. 在G赫兹的领域PCB一样发挥出色,更不要说是音频放大了 The problem comes in when an amp manufacturer uses a layout guy who doesn't know anything about proper layout of high-impedance circuitry, and when cheap, single-sided PC boards with non-plated-through holes are used.问题来自于AMP制造商雇佣了一个不懂什么是高阻抗电路的人来绘制PCB,并且这个人只会在单面PCB板上绘图,从来不知道使用过孔技巧。  Now, why don't I use PC boards? 好吧现在我来回答你我为什么不用PCB而偏爱搭棚焊了。
Simple .  A turret board provides better mechanical reliability and ease of maintenance or modification. 原因其实很简单,一个搭棚焊支架就可以提供足够机械稳定的支点 I have seen far too many PC boards with pads and traces ripped up after one or two component changes.我见过无数次,在更换几次元件之后,PCB板的铜皮就会起皱甚至掉落  A turret board allows both a good mechanical connection and a good solder connection, and components are easy to remove and replace many times without any damage to the board.而一个搭棚焊片则可以随便你折腾,就是不坏。  Turret boards can even incorporate ground planes or circuit traces, without the downside of  worrying about traces and pads lifting up during repeated removal and replacement of parts. 搭棚焊片还可以同时连接底盘层元件和电路层元件,在重复多次更换元件后,依然不必担心脱焊碰极 I don't use eyelet boards, because they don't allow a good mechanical connection; instead, they depend solely on the solder for mechanical strength at the connection.我不喜欢使用多孔板,因为他们的机械稳定性非常差。实际上,他们的机械连接其实是取决于上面电路部分的焊锡连接  In addition, solder tends to blob up on the underside of the board, potentially leading to loose solder balls inside the amp, or accidental short circuits.焊锡经常会自行卷曲或溅落,万一掉落进机箱内部,还会引起短路  Will I ever use PC boards? 我会使用PCB板吗? Probably at some point,  especially on the more complex amplifier designs, mainly for consistency of manufacture or when the order volume outruns our small manufacturing capability, but you can bet that they will be better and more serviceable than any PC board you've ever seen in a guitar amplifier. 也许有些情况下会使用PCB,例如很复杂的功放电路,或者要求加工制造的一致性很高的时候,或者音量量级已经超出了我们DIY的制作能力的时候。不过对于吉他专箱来说,我敢打赌,搭棚焊接总是比使用PCB更耐用。  I have been laying out PC boards for many, many years in my previous audio/video/medical engineering jobs, some of them being very complex multi-layer boards with traces/spaces as fine as 5 mils, some even using surface-mount components on both sides of the board, operating at frequencies from DC to the high MHz region, so I know all the ins and outs of proper PC board design and manufacture, and won't be at the mercy of an outside contract layout guy.我曾经早年长期从事电子工程和医疗工程工作,绘制PCB无数,知道里面的门道,不管是细致到5个mil的多层板还是全部贴片元件的双面焊接,还是从直流放大一直到射频领域的设计和加工,我都门清得很,所以看到那些外行人画的PCB我就来气,对他们我是深恶痛绝毫不留情的。

Q: Can I change the preamp cathode or plate resistors to change the gain or tone of my amp?

通过改变前置放大级的阴极电阻或者屏极电阻,是不是就可以改变增益或者音色?
A: Yes, but you should change the plate resistor value when you change the cathode resistor value, in order to compensate the quiescent plate voltage shift, to rebias the tube near the center of the plate voltage swing, or to the place it was originally biased.是可以这样做,但是阴极和屏极电阻要同时改变,因为你要补偿屏压改变带来的静态工作点偏移,必须给屏极重新设定偏置电压,或者使之达到原来的偏压才行。 If you don't change the plate resistor, the change in the static DC bias point may cause the tube to clip very asymmetrically, and the headroom will be lower, which may or may not be a good thing, depending on your needs.如果你不这样做,静态工作点会偏移太多,动态范围就会不对称,那你也得不到好的音色。 The two go hand-in-hand.阴极电阻和屏极电阻要一起改变。 If you change the cathode resistor, the plate resistor should (ideally) be changed, and if you change the plate resistor, the cathode resistor should be changed, unless you are designing for an asymmetric bias point.除非你刻意追求一个不对称的动态范围,那么改变其中一个,另外一个也会随之而变, At any rate, it is always a good idea to check the operating conditions of the tube circuit on a scope to make sure things aren't getting out of hand.用示波器直接监测电子管的动态范围是个好办法 People tend to view preamp tubes differently from power tubes, but in reality, they are the same animal, just on a different scale.人们总是把前级管和后级管分开来对待,其实本质上他们是一样的,只是功率有大小的区别而已。 Preamp tubes need to be biased, just as power amp tubes do, and there are ranges that are more optimum than others.前级管和后级管一样都需要偏置,并且都有一个各自最佳的偏置范围, This is why it is not a fair test to just swap different tube types and compare tones.这就是为什么不能直接把电子管替换,然后来评判对比电子管之间音色的不同。 A 12AX7 will require different plate and cathode resistors than a 12AT7, for instance, for optimum bias point and tone.例如12AX7 和 12AT7 的最佳工作点显然是不一样的,音色也是不一样的。

Q: Can I substitute a 12AT7/12AU7/etc. for a 12AX7?

我是不是能用 12AT7,12AU7等等电子管来替换12AX7 ?
A: Yes and no.可以也不可以。  While they are the same pinout, the tubes are very different with regards to internal plate resistance, bias current, gain, etc. 尽管这些型号的管子引脚定义都类似,但是他们的内阻、偏流、增益等等特性都相差甚远。 It won't usually hurt anything to substitute them, but it is not a fair tonal comparison if you don't also change the cathode and plate resistors to optimally bias the tube.直接替换电子管虽说不会造成什么损坏,但是也不会因此得到好的音色。你必须针对新的电子管重新设置阴极和屏极电阻 A straight swap without regard for circuit operating conditions will lead to incorrect conclusions regarding the characteristic tone of the tube. 直接替换就对电子管的音色下定论,只会误导你得到一个错误的结论。

Q: How can I find out if the hum in my amp is caused by the filament wiring?

我怎样判断我的吉他音箱的哼声是来自灯丝的交流声?
A: Disconnect the 6.3V AC wires from the power transformer, and temporarily connect in a 6V lantern battery.把给灯丝供电的交流变压器用6V电池替换一下  If the hum is caused by filament wiring, or by induction from the filament inside the tube, it will go away with the battery connected. 如果真的是灯丝的交流声,那么在电池供电是会消失 If the hum is caused by something else, such as a ground loop, it will not go away when the filaments are run on DC. 如果还是有哼声,那么就不是灯丝的问题,而是其他的问题,比如地线干扰之类的。

Q: How can I get rid of the hum if it is caused by the filaments?

怎样去除灯丝带来的交流声?
A: The best way to get rid of the hum is to generate a DC filament supply, but this isn't always practical, because of the high dissipation caused by the large current draw of the filaments.最直接的办法是采用直流灯丝供电,当然很多时候这样做并不实际。  First, make sure the filament wires are run as twisted pairs, then make sure they aren't routed too close to sensitive grid wires or next to coupling capacitors. 你可以把灯丝供电线路做成双绞线,并且远离敏感的栅极信号线以及耦合电容。 If that doesn't do it, try "elevating" the center tap of the filament winding (or junction of the two filament reference resistors) to a potential above the cathode voltage of the tubes, typically between 10V - 50V or so.如果这样做还不奏效,那么你可以试着把灯丝电位提高到比阴极高10 – 50v 的水平  You can generate this voltage with a couple of resistors arranged as a voltage divider from the plate supply.从屏压电源采用分压电阻的办法很容易做到这一点。  Be sure to bypass the junction of the resistors to ground  with a suitable filter capacitor, or you may inject some buzz or noise into the amplifier from the power supply. 在分压电阻的节点处要采用一个滤波电容,要不然反而会把阳极干扰传进来。

Q: What if the hum is not caused by the filaments?

如果哼声不是来自灯丝,那我该咋整啊?
A: If the amp still hums when a battery is substituted for the filament supply, the cause is most likely one of four things:如果用电池给灯丝供电还是有哼声,那么噪音很可能来自以下几种情况: either the AC wiring (plate supply, filament supply, or mains input) is running too close to a sensitive stage, the transformers themselves are inducing hum into sensitive stages, there is too much residual ripple on the main power or bias supplies, or there is ground loop hum caused by a poor grounding scheme in the amplifier.交流线路离敏感线路太近,变压器离敏感线路太近,主电源或者偏压电源寄生纹波太大,以及地线设计连接的不合理。  The true cause has to be determined by a process of elimination.应该逐一排查以上原因才能找出真正元凶  Sometimes it is advantageous to pull preamp tubes, one by one, starting at the input, to determine if the hum is getting in to a particular stage. 有的时候可以逐一地从前级开始拔电子管,以便判断是哪一级引入的噪音。

Q: Why do some amps connect the center tap of the filament winding, or the junction of the two resistors off the filament string, to the cathodes of the output tubes?

有时候把灯丝中点接到输出电子管的阴极是为啥啊?
A: In a cathode-biased amplifier, the cathode is at a positive voltage, somewhere around 10-40V with respect to ground.在阴极偏压的放大器里,电子管阴极电位一般会比地线高出10 – 40V  If you elevate the filament "reference" above the potential of the cathode by connecting the center tap to this point, you can effectively reduce the amount of hum coupled into the tube.把灯丝中点与阴极电位相连接可以有效减小交流声的感染机会。   This is because the filament is now positive with respect to the cathode, so the cathode doesn't attract electrons (i.e. hum) from the filament.这使因为灯丝与阴极等势,阴极就不会从灯丝吸收热电子。  This is a very inexpensive and easy method of reducing the hum in an amplifier without having to go to a DC filament supply.如果灯丝不能采用直流供电的话,那么这样做法是一个简单有效抑制交流声的好办法。  In a fixed bias amplifier, the output tube cathodes are usually at ground potential, so you have to add a voltage divider from the plate supply to generate the elevated filament reference. 如果栅极是固定负偏压,那只能还是从屏极分压的办法,来得到灯丝参考电压。 You can experiment with the voltage level to determine the value that best minimizes the hum. 你可以改变调整分压来把噪音降到最小  Be sure to bypass the junction of the resistors to ground  with a suitable filter capacitor, or you may inject some buzz or noise into the amplifier from the power supply. 还是要注意在分压电阻的节点处要采用一个滤波电容,要不然反而会把阳极干扰传进来


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好文章,学习了!!!

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Q: Are cathode followers gain stages?

阴极跟随电路也是增益级吗?
A: Yes.  是。However, the problem is that people associate "gain stages" with, well...gain. 在人们印象里,也许增益这个概念并不明确 The cathode follower has a maximum theoretical gain of unity, and typically a gain of around 0.5 to 0.7 or so.  阴极跟随器在理论上电压增益小于1,只有0.5 – 0.7 的电压增益。To us engineering types, a gain of less than unity is still called a gain (that is, unless it's called a loss or an attenuation, in which case the attenuation is the reciprocal of the gain), so a stage with a gain of unity still has a gain - a gain of 1.对于一个工程技术人员来说,增益小于一也是增益,只不过是负增益而已,或者称之为损耗、衰减等等。 The cathode follower *is* an amplifier stage, but not a voltage amplifier in the typical sense.阴极跟随是一种放大电路,只不过不是我们普通概念上的电压增益级  It is used as a "buffer" amplifier, which means it has a high input impedance and a low output impedance.这是一种缓冲电路,具有高输入阻抗和低输出阻抗。  This means it does not appreciably load the previous stage it is connected to, and the very low output impedance allows it to drive low impedance loads without much signal voltage loss.它可以不需要精确计算和调整前后级的阻抗
,就可以自动进行匹配和驱动,而不损失电压增益。  For example, if the previous stage had an output impedance of 100K, and you tried to connect a stage with a 10K input impedance to it, you would only get 9% of your original signal when you connected the second stage, because of the voltage divider formed by the 100K output impedance and the 10K input impedance. 举例来说,一个100K的输出源要带动一个10K输入阻抗的放大级,将会损失90%的有效信号电压,因为阻抗不匹配。  If you insert a cathode follower with a 1K output impedance and a 1Meg input impedance, the 10K stage can be driven with 90% of the original signal, because there is now effectively a 1K:10K voltage divider instead of a 100K:10K voltage divider. 如果你在这两级之间插入输入阻抗1M输出阻抗1K的阴极跟随电路作为缓冲, 那么只需要90%的源信号就足以驱动后级放大器。The cathode follower is basically an active "impedance transformer", in this sense.从这种意义上说,阴极跟随电路是一种有源的阻抗匹配变压器。 The reason the cathode follower is used in driving a tone stack is not only because the tone control network impedance is relatively low in comparison to the output impedance of the previous stage, so it would cause a loss of gain, but more importantly, the tone stack is a filter network that is designed to ideally be driven from a zero source impedance to achieve it's proper frequency response.在吉他音箱的音调调整电路里使用阴极跟随器,不仅仅是用于阻抗匹配缓冲,更重要的理由是,音调调整本来目的就是要在没有输入信号损失的情况下,改变电路的频率响应特性。  The cathode follower provides a very low source impedance that allows the tone stack to work as designed. 使用阴极跟随器可以使之在损耗很小一部分信号功率的情况下达到这个目的。If the tone stack is driven from too large a source impedance, not only will there be a loss of gain, but there will be a different frequency response to the network, typically quite a few dB loss of the highs.如果音调调整电路都将吸收很多信号功率,那么损失的不仅仅是整机增益,还可能会使整体的高频部分响应特性变糟。  The cathode follower prevents this loss, allowing the tone stack to retain more of it's theoretical frequency response. 阴极跟随器则保证了这部分电路的正常工作。

Q: Does cathode biasing mean class A?

阴极偏压是不是意味着A类放大?
A: Absolutely not.  You can have a cathode-biased class AB or class A amplifier, just as you can have a fixed-bias class A or class AB amplifier. 当然不是了,你可以同时拥有一台阴极偏压的A或AB类音箱,或者固定栅偏压的A或AB类音箱。

Q: Is it true that the only true class A amplifier is a single-ended amplifier?

是不是只有真正的A类放大器才是单端放大器?
A: No.  You can design a true class A single-ended or push-pull amplifier. 不是的。你可以设计一个A类的SEPP放大器。 The presence of a phase inverter tube does not automatically mean the amplifier is class AB, just as the presence of a cathode resistor doesn't automatically mean the amplifier is class A.具有倒相电子管并不说明这是AB类放大器,就像使用阴极偏压电阻并不意味着这就是A类放大器一样。  It is all a function of where the output tubes are biased, and under what voltage/impedance conditions they are operating. 这只取决于输出级电子管的工作状态及其偏压情况而定。 In fact, unless you know the plate voltage, plate bias current, and output transformer reflected impedance, you can't tell the class of an amplifier just by looking at the schematic.事实上只看电路图是看不出来什么的,除非具体知道屏压、屏流以及输出变压器阻抗。   A push-pull class A amplifier differs from a single-ended class A amplifier tonally in that it cancels even-order harmonics generated in the output stage (but passes through even harmonics generated in the preamp stages, of course).一个A类推挽的音色明显有别于A类单端的音色,因为它能够减少偶次谐波  It also has inherent power supply rejection for lower hum and noise levels than a single-ended class A amplifier. 还对高压电源有更小的干扰,也因此噪音更低 Typically, a push-pull class A amplifier will clip rather symmetrically, while a single-ended class A amplifier usually clips asymmetrically, most often rounded on one side while hard-clipped on the other. 一个A类推挽动态范围也比A类单端更对称,因此不易发生消波失真。 While both amps are indeed true class A amplifiers, their tones are dramatically different.  尽管他们都属于A类放大器,但是音色上却差别很大。 This further illustrates the fact that there is really no such thing as a "class A" tone. 所以也不听某些人所说的“A类放大”音色,那是错误的概念。

Q: Are class AB amplifiers actually running in class A at lower volumes, as is commonly claimed?

是不是AB类放大器在小音量运行的情况下就成了A类放大器?
A: No. 怎么可能。 They are operating in conditions similar to class A, but they are not actually "class A at lower volumes". AB类在
小音量下确实接近A类放大器,但是绝不等同于A类小音量。 Now, what are the differences, you might ask?也许你现在要为区别在哪
Well, for one, the Class AB amplifier is biased in a more non-linear portion of the characteristic curves, which means it has more harmonic distortion than a true class A amplifier, even when running "clean". 首先一点,AB类放大电路的偏压更接近于伏安特性曲线的非线性部分,因而比A类放大器会有更多的谐波失真,即使是 CLEAN 档也会有失真。 Also, the efficiency will be greater than is theoretically possible with a class A amplifier at these levels. 另外,AB类的效率也比A类放大器高出许多。 There is a very real difference in tone and operating conditions between a true class A 10W amplifier running at say, 1W, and a 10W class AB amplifier running at 1W. 10瓦A类在1瓦运行跟10瓦AB类1瓦运行时的音色真的是完全不一样的 Same output level, same overall power level,but a different class of operation,  different amount of distortion, different efficiency, and a different tone, even though neither one of them is in cutoff for any portion of the output cycle at that low level. 同样的输出功率,同样的总功率,但是由于电路不同,失真度就不一样,效率不一样,音色也不一样,甚至他们都在未发生消波失真之前就是这样了。 This is due to the bias point differences and load line differences.这是由于偏置点的不同以及负载线的不同而引起的。 The differences become even more apparent when the amplifiers are run at their full undistorted output power.在满功率不指失真输出时,这种差别将更明显。  The true class A amplifier will have no crossover distortion, while the class AB amplifier will. AB类放大器有交越失真而A类放大器没有。  The average plate current for the true class A amplifier will not change, or will change very little, from idle to full output power, while the average plate current in a class AB amplifier will increase dramatically. A类放大器的平均屏流基本不因输出功率而改变,AB类放大器的输出管屏流在不同的输出功率下差别巨大。 This will lead to "sag" in the power supply that doesn't exist in the true class A amplifier, which again results in a tonal change.因此A类不会发生电源电压因输出功率而变化的情况,因而导致音色的变化。

Q: Take a single EL34. The RCA manual says the maximum plate dissipation is 25 watts. Put the plate voltage at 400V. Set the bias so that the current is 60mA (24 watts dissipation). Put the plate voltage at 300V. Set the bias so that the current is 80mA.  Both give you a dissipation of 24 watts. Both seem to be within operating parameters for this tube, but I'm guessing the tube is going to have a different effect on the signal.  Is there a basic, overview explanation on how the tube is going to operate under these two conditions?

就拿EL34 来说吧,RCA手册说它的最大屏极热耗散为25瓦,那么在屏压400V屏流设置为60mA时屏极热耗散为24瓦,屏压300V屏流设置80mA是热耗散也是24瓦。这两种情况下看起来电子管工作状态相同,但是实际的使用效果却很不一样。你对此如何解释?
A: You cannot bias an amp at any arbitrary voltage for maximum dissipation, and expect to end up with a properly operating class A amp.你不能以最大屏极热耗散作为基准,随便取一个屏压就设置电子管到最佳的A类放大状态。  While the amplifier is biased at idle to a point not exceeding the plate dissipation, which seems okay on the surface, this is only half the story. 虽然电子管的静态屏极热耗散并没有超过极限值,看起来完全没有问题,但是这只是故事的一半而已。

When you put a signal into the amp, the plate dissipation changes from the idle value.  当吉他功放接收到输入信号以后,屏极热耗散将会偏离静态工作点。It can either go up, down, or stay the same, depending on the idle bias point. 它可能大,可能小,也可能不变,这取决于栅极偏置电压。 This is because the plate dissipation is dependent on the average signal at the plate, along with the plate voltage, idle bias current, and plate load impedance. 这是因为决定屏极热耗散的因素很多,包括屏压,静态屏流以及屏极负载阻抗。

If the tube is biased so that it just hits plate current cutoff/saturation on top and bottom at the same time, the tube is said to be operating at the limit of class A operation.电子管偏压保证它的静态工作点正好在截止/饱和居中位置的时候,我们称它工作在A类放大状态。  The average current draw at full signal will be the same as the average current draw at idle with no signal applied.这个时候,屏极的静态工作平均电流应该等于最大输入信号时候的平均电流
。 This is because the average value of the unclipped, full power output sine wave is zero - there is an equal area above the center as there is below the center of the waveform. 这一点很好理解,一列理想正弦波的输出平均值是0,因为上下半波的绝对值相等而符号相反而抵消了.  This "ideal" symmetrical bias point can only be achieved at one plate voltage, if the constraint of biasing to exactly max dissipation at idle is applied.这种理想状态下,你当然可以选取静态工作点在屏极最大热耗散状态。  It is interesting to note that the plate dissipation will actually drop at full power, compared to idle, because the plate dissipation is equal to the DC input power minus the output signal power, which can be at most equal to 1/2 the idle DC input power.注意屏极的静态热耗散将在施加信号以后降低,因为在理想状态下,屏极动态热功率是总输入功率减掉输出负载功率的。负载功率平均值一般是总输入功率的一半。  Essentially, the part of the DC input power that is not passed on to the load must be dissipated as heat in the plate of the tube. 实际上,所有输入功率如果没有转化成输出负载功率的话,都将成为电子管屏极的热损耗功率。 In a true class A amplifier at idle, no power is developed in the load, so all the power is dissipated in the tube.在A类放大的静态工作点,由于没有输出功率,所有的输入功率全部转化成热量耗散在屏极。  At full output power (unclipped sine wave), half the DC input power is used to produce the AC output signal, the other half is dissipated as heat in the tube. 在最大不失真输出的情况下,一半输出,一半转变成热量耗散在屏极。

Now, if you bias that same tube to max dissipation at a significantly higher voltage/lower current point, it will again be okay at idle, because it isn't exceeding the max dissipation rating for the tube.迄今为止,你把静态工作点设置为电子管最大屏极热耗散点还是没有错。  However, when a signal is applied, the tube will reach saturation before it reaches cutoff on the other side, so the average area above the zero bias line will be larger than the average area below the zero bias line, even when the tube is not clipped on top or bottom.但是加上输入信号以后,电子管通常都是先达到饱和点,后达到截止点的。也就是说,交流输入信号的高电平部分电压一般是要比低电平电压来得多一些,尽管这时候电子管还没有发生截止或者饱和。  This results in a net average increase in plate dissipation over the no-signal bias point. 这就会造成动态屏极热损耗高于静态。The tube, which was perfectly happy at idle, may now exceeding the max plate dissipation with a signal applied, and the plates may start to glow red when you start playing, and go back to normal when you stop playing.这样设置屏极偏流的电子管会在你开始弹奏吉他以后,屏极慢慢发红,当你停止弹奏后再慢慢复原。  If you were to use cathode bias instead of fixed bias, the increase in average plate current will cause an increase in cathode voltage, which will act to reduce the plate current and counteract the increase in dissipation, at the expense of a shift in operating point.如果你是使用阴极电阻偏压的方式,增加的屏流也就是阴极电流会导致阴极电压升高,这是一个负反馈过程,反过来降低屏极电流以及热耗散,但是原来的静态工作点会发生很大的偏移。

If you were to bias the tube to a lower idle dissipation, say 70% of the max dissipation, the increase in plate dissipation that occurs at max signal is now offset by the lower average idle dissipation, so the tube is again operating in a safe area at all times. 如果你降额使用,比方说屏极热耗散只有极限值的70%,就不会有任何问题了。 In this case, however, you no longer have a true class A amplifier, because the plate current will go into cutoff at some point before it hits saturation on the other side, because the bias is offset relative to the available grid swing.在这种情况下,理论上放大器不再是一个A类放大器,因为静态工作点不是正中心点,而是偏下一些,屏流会先达到截止后达到饱和。  The tube has now been biased to class AB operation.理论上说这是一种AB类放大状态。  This may be fine for a single-ended amp, provided that you don't mind a bit of asymmetrical clipping when you drive the amp hard, which creates a predominant even-order harmonic structure.在单端放大电路里,这根本对你没有什么影响,只有在你最使劲弹奏的时候会由于削波带来一些偶次谐波失真。  In the case of a push-pull amp, the other tube "takes over" when the first one goes into cutoff, so the output is not clipped, even though the plate current of the first tube has completely shut off for a portion of time. 在推挽电路里,由于对面电子管的补偿,则根本不会产生消波。即使其中一个电子管在完全截止的状态下也不会发生消波。

As mentioned earlier, there is a bias shift that can occur when the grid signal is AC-coupled into the tube through the typical coupling caps present in most guitar amplifiers.以前曾经提到过,大多数吉他功放都会因为栅极信号是通过电容交流耦合进来的,而发生偏置点漂移。 When the grid drive voltage exceeds the cathode voltage, the grid voltage is "clamped" at the peak to a point near the cathode voltage. 当栅极信号驱动电压超出了阴极电压的时候,信号会被阴极电位钳位而发生削波。 As the signal is increased, the peak stays at the same point, but the "center" of the grid drive signal is shifted downward.当信号输入电位继续升高,但是输出依然停留在原来位置,不过栅极驱动信号的中心点却发生了向下的漂移。  This changes the duty-cycle of the plate waveform, and therefore changes the average dissipation in the output tube, in a manner dependent on the magnitude of the applied signal. 这将会改变屏极波形的负载,继而改变屏极热耗散功率,而这又取决于输入信号的振幅。

You can use a load-line superimposed on the characteristics of the tube to determine the max plate voltage, idle current, and load impedance that will allow you to bias the tube to the optimum class A operating point.你可以把电子管的负载线、伏安特性曲线,静态屏流以及负载阻抗等等曲线叠加在一起,来决定如何把A类放大静态工作点偏置到最佳状态。  The aforementioned EL34 typically cannot be run at a plate voltage much higher than 250V without exceeding the "true" limiting class A operating range at reasonable distortion levels. 前面提及的EL34,如果在A类放大下,不产生难以忍受的失真,那么屏压就不应该超过250V。 You can run a "flatter" load line,  i.e. higher impedance load, and colder bias up to a certain point to try and eke out as much symmetry as you can if you are using higher plate voltages. 你可以在更容易产生噪音的负载下提升屏压,比方说提高负载阻抗,降低栅极偏置。 The downside to this higher impedance load is increased harmonic distortion and nonlinearity at "clean" outputs, and higher plate voltage swings that may exceed tube maximum ratings or output transformer insulation ratings.提高变压器初级阻抗会带来谐波失真以及非线性失真,而更高的屏压也会增加高压击穿变压器的可能性。  For a better explanation of this, see this paper on biasing - The Last Word On Biasing.如果想知道得更多,你可以参考这篇文章中得另一篇专门关于栅极偏置的问答 —— 关于偏置的最后一句话。


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兄,辛苦拉。
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陕西蒜农 在 2011-11-22 20:11 说:
有人看就好

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Q: How far can the load line in a class AB amplifier go above the maximum dissipation limit?

在最大屏极热耗散限制下,AB类放大器静态工作点的负载线可以最高达到多少?
A: You can allow excursions into the "no-man's land" above the max dissipation curve *if* (a) you don't stay there very long and (b) the average dissipation is less than the max plate dissipation.在下面两种情况时,你可以尝试一下超过最大屏极热耗散限制:1、功放不会在这种状态下工作多长时间2、平均下来屏极发热远小于最大热耗散功率。 In the real world, a guitar signal generally runs up and down the load line all over the place and doesn't stay at any one dissipation for any particular amount of time, except at idle or full power if you are getting feedback.在真实的演奏中,吉他信号总是在负载线的全部范围内变化着的,而不是总停留在一个水平,除非你什么也不弹或者在做反馈啸叫声。 If the load line crosses over into the max dissipation area at idle, you're screwed. 如果在静态时负载线就已经进入了最大耗散区,那你就完蛋啦。If the load line crosses over in the middle somewhere, there will be a period of time where the signal goes above the max dissipation, and a corresponding period of time where it goes below the max dissipation如果负载线从中间某区域穿过,那么将会有一部分时间超过最大热耗散,而另一部分时间段内低于该极限值。 In class AB, the time above the max dissipation curve will always be less than the time below the curve, because the tube is in cutoff for some portion of the cycle (the load line goes below the zero current axis at higher plate swings). 在AB类放大器里,由于有截止区域的存在,电子管的屏极负载总是趋向于超出极限的部分小于低于极限的部分。This allows you to use a load line that goes above the max dissipation curve for some amount of time. 因此允许你把负载线适当的超过最大屏极热耗散极限的一定水平。Calculating the max distance you can go above the limit is not straightforward, because the voltage and current functions are no longer sinusoidal, they are clipped sinusoids, or clipped square waves.很难精确计算出你能够超出这个极限值多高,因为此时的电压电流响应曲线不再是一个正弦曲线,而是削顶正弦曲线,或者甚至是方波。

There are three main variables that control static and active dissipation: (1) plate voltage (2) plate current and (3) reflected load impedance.
有三个主要因素决定了阳极的静态和动态热耗散功率:屏压、屏流以及输出变压器的初级阻抗。 Once the amp is built, the only one you usually have control over is plate current, so if the tube plates are glowing red at any point in the operation, you have no choice but to lower the plate current if the average dissipation if too high.但是一旦机器做好了,你唯一能够调节的就只有屏流。所以当你发现屏极发红的情况发生时,你除了降低屏流别无选择。

In an "ideal" 11W dissipation tube, for example, if you are running 250V and an 875 ohm (3.5K p-p) load, you can't run a bias of more than 32mA without exceeding the dissipation at any point on the load line with either sine wave or square wave excitation.举例来说,一个理想的屏极热耗散11W的电子管,如果屏压设定为250V,负载阻抗875ohm(输出变压器为初级阻抗3k5的推挽型)不论你用何种波形激励,偏流都不可能超过32mA,否则肯定会超出热耗散极限。 The idle dissipation at this point will be 8W.该静态工作点的热耗散已经达到了8W。 The worst-case squarewave dissipation of 11W will occur at around 110V peak above the idle point of 250V, or a peak-to-peak swing of 220V around the idle 250V point.最糟糕的情况是,峰值比静态250V还高110V的时候,或者峰-谷电压围绕静态工作点达到220V的时候。

Q: Any time one tube or set of tubes is said to be "pushing" and the other "pulling" inherently, the amp is not operating Class A, right?

是不是说任何时候,只要一个管在推另一个管在挽,放大器就不是工作在A类状态?
Not true.  In a true class A push-pull amplifier, one tube is indeed "pushing" and the other tube is "pulling", but neither reaches cutoff at any point up to the unclipped full output level.

不是这样的,一个A类推挽放大器里就是一个管子在推的同时另一个在挽,但是每一个管子都不发生截止或者饱和。

The difference between a class A push-pull output stage and a class B push-pull output stage is primarily the bias point.  The class A output stage is biased in the center of the linear portion of the transfer characteristics, and the class B output stage is biased at cutoff.  

A类推挽与B类推挽真正的区别在于输出电子管的偏置状态不同。A类管偏置静态工作点在特性曲线的最中间线性部分,而B类则偏置在截止的状态。

In a class A amp at idle, with no signal present, each tube is drawing (ideally) an amount of current halfway between cutoff and saturation. 一个A类放大器在没有输入信号的静态工作点,每个电子管屏流居于截止和饱和的中间状态。  For example purposes, let's assume this is 100mA, and assume the plate voltage is 250V.  The tube is therefore idling at 250V*100mA = 25W. 假设这个静态屏流100mA,屏压为250V,那么屏极静态热耗散功率为25W  Now, if a signal is applied to each of the two output tubes from the phase inverter, one tube's current will increase and the other tube's current will decrease by the same amount (because the phase inverter generates two drive signals that are 180 degrees out of phase).现在假设有一个输入信号从倒相级进入功放管,那么一个管子屏流增加,另一个管子屏流就会减小同样的电流。Let's assume that the input signal  amplitude is enough to drive the first tube to 150mA - in this case the second tube is now at 50mA, because it decreased by the same amount as the increase of the other tube. 假设一个管子屏流增加到了150mA,那么另一个管子屏流这时候就应该为50mA Taken to the limit, if the input voltage is enough to drive the first tube to 199mA, the second tube will be driven to 1mA, which is right at the limit of cutoff.   If this is the maximum clean output of the amp, and the saturation point occurs at 200mA,  the amp is operating in perfect class A, because neither tube has ever hit cutoff or saturation up to the maximum clean output of the amp. 极限的情况是,一个管子屏流增加到了199mA,而另一个管子屏流降低到了1mA,此时功放仍然在A类放大区运行良好,两个管子都没有发生削波失真。 The output transformer sums these two complete, unclipped, out-of-phase sine waves to generate a sine wave of twice the equivalent level of one side (this is how you get twice the output power of a single tube running class A single-ended).输出变压器初级线圈会把这两个管子输出的信号自动复合成为一个完整的但是电压却是两倍的正弦波形

If you average the current draw over the full sine wave, the increases and decreases cancel each other out and the average current is the same as the idle value, or 100mA.  把一列正弦变化的电流取平均值就可以看出,峰和谷互相抵消,总的平均电流等于静态电流 100mA Therefore, there is no change in the current drawn from the supply. 也就是说,动态和静态从电源那里取得的总电流没有变。 The plate dissipation in a class A amplifier output tube goes down with applied signal, because as the current goes up, the voltage goes down, and as the voltage goes up, the current goes down (inverting) and the power dissipated in the tube is the product of the voltage multiplied by the current. A类放大电路里面,屏极耗散在动态下是要降低的,因为屏极耗散是屏压乘以屏流。At the extreme, when the current is zero and the voltage is at max, the instantaneous dissipation in the tube is zero, and at the other extreme, when the voltage is zero and the current is at max, the instantaneous dissipation  is also zero. 极限的情况下,如果屏流为零屏压达到最大,或者屏压为零屏流达到最大,总的瞬时热耗散都是零。  Since the average plate dissipation in the tube decreases, the tube can be biased at the max rated power at idle.由于动态屏极热耗散总是低于静态,所以可以把静态偏置取到屏极耗散的最大值。

In a class B amp, since the tubes are biased at 0mA, one tube will increase to a maximum as a signal is applied, but the other tube will stay in cutoff until the second half of the input cycle. B类放大电路里面,电子管总是偏置于0mA,而动态时总是一个管子工作,另外一个就截止。  Then that tube will increase in current while the other tube stays in cutoff.一个管子的屏流增加,但是另一个屏流还是截止。  These two "halves" will sum together in the output transformer to make a full cycle. 这两个半波的总和在变压器那里相加形成完整的波形。 Since the tubes aren't perfectly linear in the region near cutoff, there is a type of distortion introduced called "crossover distortion", caused when the two tubes "hand off" operation to each other.由于两个管子的性能不可能完全对称和线性,因此在两个管子工作交替之间会产生交越失真。 Because the tubes are in cutoff for half of each cycle, the average plate dissipation is much lower than it is for class A, so more output power can be obtained from the class B configuration.因为总有一只管子工作在截止状态下,屏极热耗散为零,所以B类放大器的电子管屏极热耗散要比A类低得多,因此B类可以输出的功率也要大得多。

Class AB operation is obtained by biasing to a point in between class B and class A. AB类放大器的偏置状态处于A类和B类之间。 for a portion of both half cycles of the input sine wave, both tubes are conducting current in opposition as in class A, but one tube hits cutoff before the other tube hits saturation.  在输入信号不大的情况下,两个管子几乎像A类状态下都处于导通状态,但是一个管子会再另一个饱和之前就发生截止。 The other tube's current keeps increasing up to a maximum, and back down to the "changeover" point, which is above the zero current point. 这个时候另一个管子会继续增大屏极电流,然后回落到交越工作点,这个工作点通常会比截止稍微高一点点。 The maximum output power is less than in class B, but more than in class A. 最大输出功率比A类高但是比B类低一些。 The distortion is less than in class B, but more than in class A.   失真度比B类小但是比A类要来得大一些。Most push-pull guitar amps run in this class of operation. 大多数吉他音箱都是AB类工作的。

In a class AB or class B amp, the power dissipation will increase or decrease depending on operating conditions and applied signal level. AB类和B类工作时,屏极耗散会随着工作状态和输入信号而起伏。 This is why they have to be biased at a lower dissipation than the max rating of the tube, typically 70% max. 这就是为什么我们把管子的静态工作点偏置到屏极热耗散只有极限值的70% If you bias them at the rated dissipation, they will be fine at idle, but they will run too hot when you start playing.如果你偏置点已经达到极限值,那么他们静态没事,但是一旦你开始弹奏,屏极就会烧成灯笼。

Q: You reccomend biasing the Intruder 30 watt head to 45mV - 50mV on the dual external pots which corresponds to 45 - 50 mA per side.  This is loaded with a pair of EL34's.  Is this specific to his head and others like this?
你曾经建议把Intruder 30 瓦音箱头偏置在45 – 50 mV推动一对EL34,是这个箱子的特例还是别的箱子也必须这样?
A: Yes, that is specific to the Intruder (and Invader) 30W head, which runs very close to true class A push-pull, at around 300-320VDC on the plates. 是的,只有 Intruder 以及 Invader 30瓦音箱头是这样,因为他们的电路几乎是A类推挽,屏压大约是直流300 320 伏。

You can't just set the bias of a particular tube to "xx mA", without taking into account the plate voltage the amp is running at, and in some cases, the design of the phase inverter. 在不知道屏压的情况下,谁也不知道栅极应该偏置到多少合适,对于倒相级也是一样。

Higher plate voltages generally require lower bias currents to keep the tube running in the safe dissipation area.  屏压高的时候一般屏流就要偏置得低,以免屏极热耗散超标。This usually means the amp is running in class AB or, if the plate voltage is extremely high, class B, with the tubes biased at or near cutoff because of the high plate voltage involved.这也就通常意味着,高屏压的功放工作在AB类甚至是B类状态。  If the amp is running class A, or very near, you can bias it much hotter,  as is the case in the 30W Intruder.如果功放工作在A类状态,或者接近A类,你就必须偏置得低一些以免屏极过热,就像 Intruder 30 的情况。

The other thing to take into consideration is the phase inverter output voltage swing.  As you adjust the DC bias voltage, you also are adjusting the baseline  negative DC voltage level of the drive signal to the grids of the output tubes.   If you set the bias for less current, what you are actually doing is raising this negative DC baseline voltage up towards ground.  另一个要考虑的因素是倒相级输出电压。当你调整偏置电压的同时,也会改变输出管栅极的基准电平。如果你的偏置导致屏流降低,也就会使栅极电压向地电压升高。

As you raise this voltage, you are effectively limiting the headroom of the signal, because it will clip when the peak voltage hits zero volts.  可是你升高这个栅压,实际上也就限制了信号的动态空间。因为一旦信号电平达到0V就会发生削波。 If the phase inverter is designed for a lot of gain/output voltage swing, it will distort very early and possibly go into "blocking" distortion because of the clamping action at the grids of the output tubes when the signal hits zero volts on the peak. 如果倒相级本身设计的输出电压要就是动态范围很大,那么就会因此而发生严重的失真。 This can sound "mushy", so you must bias the amp colder to compensate.  In the case of the 30W Intruder, the phase inverter output voltage swing is designed to be lower than a 50W amplifier of similar design.这种声音听起来很浑浊。你必须降低偏压以补偿。在Intruder 30 的情况时,倒相级输出的动态空间要比同样类型的50瓦机子要低。


The bottom line is this: you can't just arbitrarily say that any tube should be biased at some particular range of currents, because it depends on the circuit design of the amplifier.  所以你必须保守这样一个原则:在不知道电路是如何设计之前,不能武断地确定管子应该有多高的偏置电压。The manufacturer should provide the specified biasing procedure for the tubes suitable for use in each particular amplifier. 制造商应该对每一种音箱设计给出特定的偏置方案。

点评
蝎之狂舞 在 2012-4-1 14:52 说:
辛苦了

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好贴留名啊
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蝎之狂舞 在 2012-4-1 14:51 说:
辛苦了

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前辈,功德无量啊!

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花了不少功夫啊!谢谢分享!

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