Thoughts on YAQIN MS-300C
- Thread starter hugo454
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mroboto
Super Member
The main problem I encountered with this amp, is the first cap too big, when a 5U4GB calls for 40uf first cap, it was 165uf (2x330uf in series)
With 165uf the 5U4GB rectifier dies too fast.
So I did replace with 2x100uf in series for 50uf , works fine for now.
There is a picture of the beginning of the mods on page 2 https://audiokarma.org/forums/index.php?threads/yaqin-ms-300c-upgrade.322511/
The two 100uf 450V Kendeil caps are the new ones for the concern.
With 165uf the 5U4GB rectifier dies too fast.
So I did replace with 2x100uf in series for 50uf , works fine for now.
There is a picture of the beginning of the mods on page 2 https://audiokarma.org/forums/index.php?threads/yaqin-ms-300c-upgrade.322511/
The two 100uf 450V Kendeil caps are the new ones for the concern.
thanks, I have a new JJ5U4gb and will replace those caps should I buy one.Aside from that, what do you think about the amp overall?
mroboto
Super Member
You should replace the two caps on top left of image on the link on page 2 with about 100uf or less capacitors, rated at 450V for the best results, maybe you can go lower than 100uf, I did not test less.
The amp sound very great, but I must use it with quite high sensitivity speakers, because a few 300B give up the ghost, not absolutely sure why, maybe I overdriven 300B with my 90dB sensitivity speakers.
Now I have Klipsch Cornwall speakers, sound wonderful, lot of bass in the 15 inches speakers, but not boomy or harsh with the mods I done, and a good source.
Source and speakers are very important in audio.
The amp sound very great, but I must use it with quite high sensitivity speakers, because a few 300B give up the ghost, not absolutely sure why, maybe I overdriven 300B with my 90dB sensitivity speakers.
Now I have Klipsch Cornwall speakers, sound wonderful, lot of bass in the 15 inches speakers, but not boomy or harsh with the mods I done, and a good source.
Source and speakers are very important in audio.
I have an MS-300C that I have been using daily since 2016 as the tweeter amp in a bi-amp system.
The suggestion about the first capacitor is a good guideline but it is not the whole picture. You have to consider plate impedance, average and peak current demand, to determine the maximum allowable size.
The 5Z3 has higher plate impedance than a 5U4 and hence a larger voltage drop. I thought about changing the capacitors but decided to leave them as is and see what happens to the rectifier. 6 years later, and maybe 4 thousand hours of use, I had no problems and have not replaced the rectifier. Maybe that is because the low power class A circuit used as my tweeter amp is makes minimal demands upon the rectifier. Perhaps if I used it for a bass amp I would have a different experience.
MROBOTO's suggestion is a good design concept. It is easy to do. If you are using the amp full range I would do it.
I did have problems with the 10k 2 watt B+ dropping resistors. They were underrated and eventually failed. I replaced both of them with 5 watt value resistors.
Last year one of the original 300B tubes failed. I only adjusted the bias when I acquired the amp and when I replaced the 2 resistors. Perhaps if I had been checking it regularly and adjusting it as the tubes aged I might not have lost the one 300B.
I have not had to make any other changes or repairs.
I have acquired and tried many other amps for the tweeters. I tried solid state, push pull, single ended pentode with schade and global NFB(6V6, 6L6, EL34) and none of them bested the Yaqin.
For the price point it is hard to beat. I have no intentions of replacing it.
Steve
I am very happy with the results
The suggestion about the first capacitor is a good guideline but it is not the whole picture. You have to consider plate impedance, average and peak current demand, to determine the maximum allowable size.
The 5Z3 has higher plate impedance than a 5U4 and hence a larger voltage drop. I thought about changing the capacitors but decided to leave them as is and see what happens to the rectifier. 6 years later, and maybe 4 thousand hours of use, I had no problems and have not replaced the rectifier. Maybe that is because the low power class A circuit used as my tweeter amp is makes minimal demands upon the rectifier. Perhaps if I used it for a bass amp I would have a different experience.
MROBOTO's suggestion is a good design concept. It is easy to do. If you are using the amp full range I would do it.
I did have problems with the 10k 2 watt B+ dropping resistors. They were underrated and eventually failed. I replaced both of them with 5 watt value resistors.
Last year one of the original 300B tubes failed. I only adjusted the bias when I acquired the amp and when I replaced the 2 resistors. Perhaps if I had been checking it regularly and adjusting it as the tubes aged I might not have lost the one 300B.
I have not had to make any other changes or repairs.
I have acquired and tried many other amps for the tweeters. I tried solid state, push pull, single ended pentode with schade and global NFB(6V6, 6L6, EL34) and none of them bested the Yaqin.
For the price point it is hard to beat. I have no intentions of replacing it.
Steve
I am very happy with the results
My choice would be Bottlehead or Elekit in that order.
Jack
Jack
