I am a tube amp DIYer in Vancouver, BC, Canada. I have been retired for few years. I had much more time for refining the tube amps that I built in the past.
Since I have a Keysight oscilloscope which has FFT spectrum analysis function and the Soundcard Scope that I can measure the distortion of my amps.
During the first two years, I was focused to fine tune the power amps with lowest THD at the output. The method is by adjusting the bias of the driver stage so that it cancels out the distortion at the power stage. After the amps were tuned to lowest THD, the sound quality seems quite OK to me. However, my friends still not quite satisfy with it. A year ago, it came to mind that this idea isn't quite right. The measurement is only good with a single tone. In fact, the tone from the driver stage was distorted that generates so much harmonics contents. Feeding this distorted signal to the power stage would create even more harmonics at the output.
Then, I changed my way fine tuning of my amps. First step is fine tune the driver stages with minimal distortion. For the power stage, I use the online Universal Loadline Calculator to work out the best bias.

The tube amps with low distortion sounds so much better that I enjoy much more with the music. Some music I didn't like the much become so lively and energetic. It is a wonderful feeling when all the tube preamp and power amp fine tuned with minimal distortion at every stage.

Enjoy the music,
Johnny Tang
Facebook : vancouverdiyaudio

最後修改時間: 2023-10-23 22:50:21

歡迎Johnny兄大駕光臨！近年有真技術，又願意主動上來發帖分享的師兄，愈來愈少了。

拜讀Johnny兄的膽差動級間牛推300B單端，十分有創意！有勞師兄解說一下，幸甚幸甚！

希望高手可以建議大家點樣揀膽機.

ECC88 differential interstage drive 300B SE

The driver stage design came from my previous projects of push pull power amps. I used the constant current source for the output stage because it will provide excellent AC balance between the two power tubes. This will reduce the distortion due to unbalance of the power tubes.

The 300B SE requires +/- 70V input signal. It is very difficult to get a tube which can provide such large voltage swing with low distortion. With use of the Universal Loadline Calculator, I found the push pull configuration would provide very low distortion output signal. Using the PP configuration is the best but it needs an inter-stage transformer which has wide frequency response of 10Hz ~ 40KHz. Luckily, I found one made in Belgium which has excellent BW.

I developed a simple "shunt regulator circuit" using a transistor and MOSFET to replace the RC cathode self-bias for the 300B. There is an adjustment pot that I can adjust the bias voltage/current of the 300B. I have a 100mA meter to monitor the cathode current of the 300B. I was surprised with this modification. The sound become much clearer, dynamic and great defined bass. This make me believe the fixed bias may have the same effect too but I am too lazy to try it yet.

Anyway, I am just having fun with these experiments. I can't say this is Good or Bad. It's up to you to try it and see how you like it.

Johnny



最後修改時間: 2023-10-24 18:57:08

感謝師兄的分享！

Hi Johnny, refer to [#4]---＞{ Using the PP configuration is the best but it needs an inter-stage transformer which has wide frequency response of 10Hz ~ 40KHz. Luckily, I found one made in Belgium which has excellent BW. },,,, Could you advise the information about the transformer factory ( Belgium ) ? But I think above inter-stage transformer is very expenvise.

Hello Daniel,

Yes, the interstage transformer is quite expensive. I use the Monolith amorphous core ITA-02 for this project. As I said, I would like to try to drive the 300B with very pure low distortion signal to see how it sounds like. The result is very impressive that I would say it is the very best sounding amplifier that I ever built.

On the other hand, I modified my 845 SE amp having the 5A/152 tube using CCS as plate load. It sound very good too.

Johnny
Facebook : vancouverdiyaudio

Hello Johnny ,
Could you advise the information about the transformer factory ( Belgium ) ?

hi Johnny,

Great day to you.

Glad to know that u have refined ur 300B SE with a lowest THD which could let u enjoy much more better sound than before...

It is vey informative by reading u post ...and achieved better sound enjoyment is very tempting to me..
(only if u don't mind) It however would be even greatful ,if u could share with me the schematic diagram (hand drawing is ok.) and the THD as well as the FFT measurement screen photos etc., so that I can understand in more detail.. to a certain extent , may give it a try to do the same as u did , hopping to get better sound enjoyment as u do as well

Anyway, nice to know u...
James



最後修改時間: 2023-10-26 22:24:31

#2

"""".....膽差動級間牛推300B單端....'''''

請問何處可找到相關文章？

Thomasc2兄，附圖是在樓主Facebook內的，文章....就是#4了。



最後修改時間: 2023-10-26 23:39:14

Thanks, 羅兄， 的確有意思，上 Facebook 睇睇先

有些前輩造膽機，還拿著六/七十年前的電路，搬字過紙，彷彿科技從來沒有進步過，時間停頓了一樣。我就對有創意的電路，更感興趣。



最後修改時間: 2023-10-27 00:23:45

[#9],謝謝!羅師傅分享 Belgium火牛廠資料, Belgium廠的火牛質量,是否比英國Sowter 更好?

Sowter or Monolith transformer?? It's a very good question. I can't tell which one is better because I only tried the Hammond, Plitron, Lunhdal, and Monolith transformer so far.

I would say both Sowter and Monolith are with very good quality.

Johnny

Parallel 6N1P line amp project.
Built the signal section. I will work on the power section later. I am using an external power supply at this moment.

By the way, the schematics are intended for advance DIYer's.

re: [#16] Hi Johnny, Thanks for the audio transformer comment !

#17

thanks Johnny for sharing the parallel 6N1P schematic ...Just curious , is the portion of shunt regulator working fine already ? or it is just a preliminary draft only ?

Anyway, looking forward to ur power section schematic as well...

Have a great weekend.