6922 / E88CC tube pre-amp

After finished my simple 12AX7 tube preamp, I decided to try other medium mu tubes, including the 12AU7 and also this 6922. The design is almost the same as the 12AX7 preamp, it is a single gain stage triode tube with cathode follower. However there is an important difference, a 1k "stopper" resistor is added at each grid input. This 1k resistor is required to avoid parasitic oscillation. Without it, the 6922 would oscillate easily. Please note that you should NOT use 6DJ8 in this design, as the heater-cathode maximum voltage limitation is different with 6922. Besides 6922, the E88CC or E188CC tubes CAN be used. Schematics :

Gain stage loadline :

Cathode follower loadline :

When design the loadline for the cathode follower, the 6922's maximum cathode to heater voltage limit of 150V has to be taken into consideration. The cathode voltage is chosen at 90V so that the 150V limit is not exceeded. If the cathode voltage exceed 150V, the heater is needed to be elevated. However, if the heater is elevated then its ground is not grounded, the heater supply wire can pick up noise or EMI more easily. My measurement showed that when the heater is elevated the amp's noise floor is increased by more than 10dB. So I would avoid heater elevation if possible. For 90V cathode that means the tube has to drop 160V. I also want the tube to have a high output current at 9mA, so the operation point (the red dot) is chosen at 9mA and 160V. The load line can then be drawn by connecting the red dot and B+ at the voltage axis. I am not going into the details that how the resistor values are calculated, please reference to my 12AX7 preamp page if you want more information. The gain for this amp can be calculated by this formula :Gain = (Tube Gain x Plate load resistor)/(plate load resistor + plate resistance + ((tube gain +1) * cathode resistor)). With 6922, the plate resistance = 3k, tube gain = 33. Using these value in the above formula, the calculated gain is 22. The output impedance of the 1st gain stage is equal to the plate load resistor paralleled with the plate resistance, so in my case it is about 2.7k, which is a bit high. So a cathode follower is needed to reduce the output impedance. With the cathode follower, the output impedance would roughly equal to 1/(tube's transconductance). For 6922, its transconductance is about 12000 micromhos. So the cathode follower's output impedance would be at about 85 ohms, which is a good low figure. It is very important that the regulated power supply can provide a clean B+ and heater voltage supply. For this design, any ripple in the B+ line will appear on the output signal. Noise or ripple in the heater supply will also be coupled to the output.

With the use of the copper component stands, I can change components easily. This is especially useful for trying out different loadlines.

DACT switched volume control, it has very smooth switching action

DACT 4 poles 5 positions input selector. Both signal and ground are switched.

The 6922/E88CC has low distortion, high output current, and has high transconductance which make a cathode follower with low output impedance of 85 ohms. All these properties make the 6922 a good choice as a preamp tube.

In fact, among the 3 tube preamps I created, this 6922 preamp sounds the best, it has the most resolution details and having excellent dynamics. I like it so much that I searched for the best 6922/E88CC tubes to use in it. Page 1 of 2 >> NEXT Page : Measurement with Siemens and Telefunken's E88CC Tubes

在完成了一个简单的12AX7电子管前级后，我决定尝试其他的电子管，包括12AU7，也就是这个6922。

设计基本与12AX7前级相同，它是一个单增益级三极管阴极跟随器。然而，有一个重要的区别，是我在每格输入增加了一个1k电阻。这1K的电阻是必要的，可以避免寄生振荡。没有它，6922很容易产生振荡。

需要注意，在这个设计中不能使用6DJ8，因为作为阴极加热器，最大电压限制与6922不同。不用6922的话，E88CC或E188CC管都可以作为替换。

原理图

当为阴极跟随器设计最大负载时，必须考虑6922在150V时能承受的最大电压极限。我选择90V，如果阴极电压大于150V，加热器需要升高。然而，升高后不接地，线路容易产生噪声或受到电磁干扰。测试结果也表明，升高后，放大器的噪声增加了10dB以上。

对于90V的阴极，这意味着该管得降到160V。我也希望在9毫安时管子具有较高的输出电流，所以工作点（红点）选择在9mA 160V。

本篇我不再详细介绍电阻值的计算方法。增益计算公式：（电子管增益×板负载电阻）/(屏极负载电阻+板阻+ ((电子管增益 +1) ×阴极电阻))。6922的板电阻=3K，管增益= 33。套用公式，计算出的增益为22。

第一增益级的输出阻抗大约是2.7K，有点高。所以阴极跟随器需要降低输出阻抗，输出阻抗大致等于1 /（管的跨导）。

6922的跨导是12000毫欧。所以阴极跟随器的输出阻抗会在85欧姆这样一个较低的数值。

板子上设计的铜柱方便了我可以随时更换组件，这对于尝试不同的载重线特别有用。

DACT切换音量控制，切换时非常平滑。

位置输入选择，信号、接地切换。

6922/E88CC具有低失真，高输出电流，高跨导的特性，使得阴极跟随器具有85欧姆的低输出阻抗。所有这些特性使得6922成为一个不错的前置放大器管选择。

事实上，在我做过的三个电子管全集中，这个6922前级的声音表现是最好的，细节表现、动态方面都很优秀。

西门子E88CC银盾

这对西门子E88CC管是西门子公司后来生产的，E88CC印在管内金属板上的日期。

在底部，有一个数字标签。

2V峰值(满负载10K欧姆)测量，em-u 1212m PCI声卡+ RMAA 6.1：

Siemens E88CC的THD测量，THD = 0.026%

西门子E88CC灰盾

1960年份的Siemens & Halske E88CC，这是早期西门子的产品。

左：管身上的AΦ1≠5F日期代码标注。右：另一例标注：AΦ1≠3C。

西门子E88CC灰盾THD测量，THD = 0.032%：

德律风根E88CC管

德律风根管底部有菱形标记

德律风根E88CC管THD测量，THD = 0.035%：

总谐波失真的测试结果表明，西门子比德律风根失真低，这可能解释了两个管之间的声音速度的差别：“Telefunken E88CC，相比6922寿命长、音乐性好、声音细节表现力强的特点，有其温暖、准确再现的特点。”（以上评论引述https://www.tubeworld.com/6922.htm）

在听了这些管子后，我的建议是，如果你看重的最大的细节分辨性，可以选择新的银盾西门子。如果想要好的温暖的声音细节，选择德律风根吧。老款的灰盾西门子管表现则介于两者中间，能平衡地表现出声音细节和音乐性。（原文选自shine7.com）

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