我相信呢度好多高手第一眼睇到你post個圗就覺得怪怪地九成將X7同U7掉亂咗

if you had actually shown the plate load of hte first stage in the beginning, it should have been clear from the start that you had the tubes swapped. anyway, i still don't think this design is quite right. ax7 has a mu of 100, with the load, you are looking at a stage with gain in order 50. why on earth do you want to do that? it is very likely that you are reducing the input signal significantly in order have meaningful output for you power amp.

膽互換後最新電壓值。

喺哩度得蒙兩位膽機高手Thomasc2同Not-So-GoodSamaritan指教啟蒙下，對個線路作了深入探討，哩兩日思前想後確係覺得兩支膽的偏壓數值唔妥，個設計者係好有名氣的，無理由咁差。尋晚直情瞓唔著，的起心肝行動去驗證我的懷疑 -- 兩支膽係裝錯左，雖然
位置係跟足Manual。因為各方面的參數都非常完美的配合兩支膽互換位置。
附圖係互換膽之後量度的電壓，可看出偏壓，電流，屏耗等等，都完全符合兩支膽的特性。最重要嘅係，一試音---嘩一聲---之前的不滿意完全掃走，音色動態力度細節，完全出乎意料的大躍進。印證了原設計者係得嘅，不過Document就認真大問題。部機係十幾廿年前的初版，二手買回來。聽講上手係香港代理用來出Show。我估出廠時應該冇插錯，之後被代理拆來拆去試膽，然後唔知邊條茂利唔記得位置就摷本manual嚟跟住插。於是造就左我上了一堂寶貴的膽機設計基本知識課。
而家唯一要顧慮的是Cathode follower支陰極電阻功耗超過3W，佢係由兩支Dale RN65D 24.9K並聯而成，網上資料顯示Dale RN65D power rating 係0.5W. 但淘寶的Dale RN65D又話係2W，搞得我很糊塗了。
唔理三七二十一，已經定左4支Riken RMG 2W電阻行串聯，實冇甩拖。

再次多謝兩位師兄指教，小弟得益不淺。





最後修改時間: 2014-08-21 23:16:42

#38,

i don't think 12ay7 is as good as 12au7 as a cathode follower. you need to look at the Gm for a cathode follower.

#42
why not? the max current for 12ax7 is 8ma. of course the conditions are different. but the fact remains that the cathode can emit enough electrons to produce that current.

最後修改時間: 2014-08-21 10:29:59

I have to acknowledge that this is beyond my ability to understand why the 12ax7 has been designed to work and how it would/could work at that poing.

I have to agree #30 “those who design this preamp are suckers”
If all figures are true, how above max swing of input & output?

12AU7 plate is 97V. 12AX7 cathode is 98V.

""""支12AX7兩邊triode合共流通8ma, work到無問題...""""

U7的屏極對地電壓是幾多V

量清楚兼睇清楚了, 我無量度錯，電阻係Dale RN65D, 數值很容易讀。
支12AX7兩邊triode合共流通8ma, work到無問題，只不過屏耗攞到盡1W, 支膽壽命會短D咁啫。

不過睇完晒D膽datasheet及個線路，我有一個新想法。
因為前面12AU7的屏壓及後面Cathode follower的12.5K陰極電阻已經決定左Cathode follower的屏流約為4ma及屏耗約為1W, 這些數值非常符合12AY7的特性，我打算淘一對返來試下。
唔支Thomasc2同Not-So-GoodSamaritan兩位大哥有何高見？



最後修改時間: 2014-08-20 20:59:26

12ax7 can get to 4ma. at 250v, the bias will be like -0.65. this no doubt a quick way to kill the tube but then it will work.

好，今晚量清楚再交流。

問題是12ax7根本是上唔到4mA！

Thomas,

有可能我量度錯。
但我有個疑問，支12AX7係Cathod Follower, Cathode輸出係跟隨Grid輸入，就算偏離線性區域是否都work?

最後修改時間: 2014-08-20 12:12:49

照個chart的走勢來睇，要上到正偏壓至有機會谷到4mA，對12ax7而言，這當然是不可能的。設計線路點可能唔睇Ia/Va chart來定 operating points，估計是你自己量錯多。

""""Max Ia of 12AX7 is 8ma. Now it is 4ma for one triode, Pa is 256V x 4ma = 1W, well within the limit..."""""

並不是within the Pa limit就可以亂定 operating points. Pa limit祇不過是設計線路時你要顧及的""其中""一個指標， 你有冇望一望我link給你的chart？ 4mA是完全超出12ax7的線性工作範圍，在4mA時，就算well within the Pa limit枝膽根本就唔work。

最後修改時間: 2014-08-20 12:11:58

Whatever, the preamp has been serving for a long time with good sound. There is still a chance I was not measuring the values in the right way. I will do it again by desoldering the resistors to measure.

Finally I found that the sound quality problem could be improved a lot by changing the 12AX7 tubes to another one. Most likely the 12AX7s are approaching their end of cycle life.

最後修改時間: 2014-08-20 11:47:52

i must say those who design this preamp are suckers^-^

Not-So-GoodSamaritan,

Thank you for your detailed explanation.
You're right. At first I didn't pay attention to plate dissipation, just thought about the operating current & biasing voltage. Considering the plate dissipation limit, the values have to be reconsidered.
For the 12AX7 I mistakenly thought the Pp max was 2W. However the value is true per my measurement on the existing design. In fact the preamp's manual did mention the tubes were specially tested to fit the design, replacing them with tubes from other source will have reliability problem. Probably the problem is this Pp max issue.

:) You see I am not good at tube circuit design.

最後修改時間: 2014-08-20 11:33:47