我的馬歇爾JCM2000日前送修了近日, 維修人告訴我後級有2隻管子也有問題使偏壓上到-35V(據他說正常應該在-38~-40V)後來他說幫我換了FENDER的管子偏壓也回來了 請問後級管子有問題時是否真的會使偏壓跑掉甚至於造成高壓電路故障? 因為第一次通知我修好了去試AMP操到一半高壓迴路就聽到燒毀的聲音了
我也很好奇這個問題，看了一些討論換管子後都需要重新BIAS，也一直沒搞懂到底啥是BIAS，找到一篇文章，大家一起研究一下 <BLOCKQUOTE>quote:</font><HR>What is Biasing? General When people talk about "biasing" an amplifier, they are referring to setting the "idle", or quiescent, current in the power output tubes. All tubes must be biased, both preamp and output tubes, but it is not always clear whether or not the bias needs to be adjusted when changing tubes. Why do you need to bias a tube? Tubes have to be properly biased in order to function as amplification stages. A tube is biased by setting the amount of DC current that flows in the tube when there is no signal present at the tube's grid with respect to it's cathode. This DC bias current can be set in a number of ways. The bias point determines several things about a tube amplification stage. It determines the power output, amount of distortion, headroom (the size of input signal that can be applied before the output signal clips), efficiency of the stage (the amount of output signal power vs. DC input power), gain of the stage (the magnitude of the output signal for a given input signal), noise of the stage, and class of operation (class A, AB, etc.). The proper bias point is a tradeoff between all of these factors, and selecting the optimum bias point can sometimes be difficult, and it will vary depending on the amplification stage requirements. Biasing methods There are two main types of biasing: fixed biasing and cathode biasing. Fixed biasing does not mean the bias is not adjustable, in fact, it usually means the opposite. Cathode biasing is usually fixed, and not adjustable, and fixed biasing is usually adjustable with a small trimmer potentiometer, or "trimpot". It is no wonder the subject is confusing to people! Fixed biasing means the tube is biased by means of a DC voltage, which is usually a negative voltage applied to the grid of the tube with respect to the cathode. As the negative grid voltage is adjusted, the bias current will increase or decrease, depending upon the direction the bias voltage is going. In general, as the bias voltage becomes more negative, the bias current becomes smaller, and the tube is biased "colder". As the bias voltage is adjusted less negative, towards zero volts DC, the bias current becomes larger, and the tube is biased "hotter". This is because a tube is a "normally on" device; that is, it allows current to flow from the cathode to the plate when the grid is at zero volts with respect to the cathode. The tube can be turned off, and the current flow stopped, by making the grid voltage negative with respect to the cathode. The tube can also be biased by referencing the grid to ground, or zero volts DC, and applying a positive DC voltage to the cathode. This is the same as keeping the cathode at ground and applying a negative DC voltage to the grid, because it is the grid voltage with respect to the cathode that determines the amount of bias current in the tube. Since vacuum tubes are "normally on" devices, a trick can be used to bias them without having to supply a negative DC voltage source to the grid. If a resistor is placed between the cathode and ground, and the grid of the tube is referenced to ground (usually by connecting a large value resistor, such as a 1Meg, from grid to ground), the tube will try to conduct a large current from cathode to plate, since the grid and cathode are initially at ground potential. However, this cathode current flow will cause a voltage drop across the cathode resistor, making the cathode voltage positive with respect to the grid. Since the cathode voltage is now positive with respect to the grid, the current flow will decrease, and the tube will head back towards cutoff. A point of equilibrium will quickly be reached where the increase in current is offset exactly by the increase in cathode voltage, and the bias current will stabilize at some particular value. It will remain at this value unless the resistor value is changed, or a different tube with different characteristics is plugged in. This allows the desired bias point to be set by varying the value of the cathode resistor. When do you use fixed biasing instead of cathode biasing? Since cathode biasing eliminates the need for a special negative DC bias supply, why don't all amplifiers use cathode biasing? Well, cathode biasing is not without its faults. It turns out that in order to keep the DC bias supply voltage at the cathode constant while the input signal is changing, the cathode resistor must be bypassed with a large capacitor. This capacitor effectively "shorts" the AC signal component to ground, while allowing the DC voltage to remain relatively constant. If the capacitor is removed, the cathode DC voltage will have a signal voltage superimposed on it, which will subtract from the grid-to-cathode signal voltage, and reduce the gain of the stage. The problem comes in when there are large signal level changes, and the average DC level of the cathode voltage changes. This causes a bias shift, usually in the direction of a colder bias point. This bias shift can be audible, but is sometimes desirable for guitar amp use, as it adds varying harmonic overtones to the sound. If the bias shift is severe, the tube will go into cutoff, and large amounts of "crossover" distortion will occur. In addition, the current flow through the cathode resistor generates a necessarily large bias voltage on the cathode for proper tube operation (typically 30-50 volts for most higher power output tubes). This voltage subtracts from the total plate voltage, which decreases the available output power. Between this voltage decrease and the bias shift, the output power in cathode biased operation is reduced when compared to fixed bias operation. Therefore, fixed bias is usually used for higher power amplifiers (50W and higher), and cathode bias is usually used for lower power amplifiers. Preamp tubes are almost universally cathode biased, because they are used for signal amplification, not power amplification, and the side effects of cathode biasing are not as important. Also, cathode biasing makes the circuit less dependent on tube parameters, and more forgiving with respect to bias point. This allows replacement of tubes without having to rebias the amplifier stage. Output tubes that are cathode-biased should always be checked when replacing tubes, however, because they vary widely in terms of idle current for a given cathode resistor value, and it may be necessary to change the cathode resistor value to return the output stage to it's proper bias current. <HR></BLOCKQUOTE> PS.不過有些朋友換管子後沒有BIAS也沒聽說有問題？真怪！
以下是原留言之備份 (其實是替 Blash 插嘴啦！) ================================ Blash 版主 會員號碼# 47 post 11-28-2002 03:02 PM 原來想自己寫的，後來實在沒時間，這裡簡單說一下。任何主動電子零件（真空管、半導體零件）都需要偏壓。我們可以把bias當成是主動電子零件工作狀態的configure。又主動電子零件就算是同一 型號（例如都是12AX7，或都是2N7000），其下每一件的放大特性可能還是有些差異，所以舊的換新的時bias就需要重設，特別是真空管跟場效電晶體。通常是測量一下換些電阻之類。當然若新、舊零件剛 好特性相仿時，則也許只到“測量一下“就完成了這檔麻煩事。 有興趣的話找討論真空管或半導體理論的書來看。在台灣的書店裡，大概前者已不容易找，後者則通常是大專“電子學“的課程在討論，故書名也通常是“電子學“。最好找大學教授去翻譯原文的書（ 原文書名也會寫在封面），他們通常寫得比較合理、有體系跟符合教育目的。不要買那種編參考書考卷的建Ο書局，不知打哪找來的半仙所寫不知所云的書。
回到問題的原點藉此機會feedback我amp修回後的經驗供大家參考: 記得以前在此有人討論過其馬歇爾的AMP PLAY時粉燙我那時也有同感, 當時覺得這大概是它的特色或設計上就是這樣吧! 不料一陣子後居然又壞掉了!!(半年之前已修過一次) 這次送修(當然送不同人修)因他手上無原廠的管子改用FENDER的管子不知道是管子的關析還是修的人確實有把偏壓調整好(我比較相信後者的猜想)現在我的馬歇爾運作時溫度已不會那麼高了, 記得以前連前面的面板摸起來都燙燙的而且破音變得很粗糙(第一次修回來時) VOL 跟GAIN怎麼調都怪怪的(好像變成是VOL在控制破音的程度) 現在這些困擾都沒了!! 只是有點遺憾現在改成FENDER管子音色跟以前有些類別上的差別(不是便差啦)但想想如果又換回馬歇爾原廠配的管子原先的問題又跑出來了那代價可慘痛咧....
阿...這個ｐｏｓｔ好像很久了... 可是那個JCM2000應該都是EL34的管子... 一般的FENDER都是用6L6\5881系的管子... 是都可以互插啦...也是會有聲音...可是是很令人懷疑聲音會變怎樣... 想到以前去南部做場也看過這種...JCM900內有5881也有EL34的.... 像是MESA的都還有可以讓你選TUBE種類的開關好像比較好... 真空管的按譜當然是會很熱...而且就像燈泡那樣是消耗品.... 所以... 有看過管子在那第二個開關開啟時的樣子嗎？？ 很像被雷打到唷...
有個好奇的問題，因為之前看過掌門人有介紹過THD的Univalve，後來也真的有人從國外進了這類的AMP，我看的結果是，如同THD所介紹的，他似乎都不需要調整偏壓，前後級管子可以任意更換，不曉得有 H知道相關的技術嗎？