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Old 9th June 2019, 10:43 PM   #21
paczeltf is offline paczeltf  Hungary
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I saw that. It starts really good, but after a few pages the whole thread should have been renamed to "How to bastardize your TU-8200DX".

That thread also has a very bad message, at least for me. It is fine if you do some experiments but at the end, you should settle, stop tweaking your amp and just listen to music.

There are much better things in life than rolling tubes constantly.
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Old 10th June 2019, 01:42 AM   #22
oface1 is offline oface1
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Oh, I agree with your assessment. it is a tough read, especially if you're like me. I haven't done any "mods" besides what came with the "DX" kit. I have kept the rolling of tubes to a minimum. Mainly am wanting to mod mine to where I can use a less microphonic tube besides the 12au7.....

Was trying to find more info in the thread on the mod to upgrade the driver current so I could use 12by7, since I have a HK Citation II as well. But I can't find it in that thread, since it goes everywhere.....
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Old 10th June 2019, 07:44 PM   #23
paczeltf is offline paczeltf  Hungary
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Quote:
Originally Posted by oface1 View Post
...Mainly am wanting to mod mine to where I can use a less microphonic tube besides the 12au7.....
The 12AU7 should not be microphonic. Power tubes tend to be more microphonic than driver tubes in a power amp anyway.

Quote:
Was trying to find more info in the thread on the mod to upgrade the driver current so I could use 12by7, since I have a HK Citation II as well. But I can't find it in that thread, since it goes everywhere.....
If you apply all the modifications that I recommended in this thread then you will be able to use 12BH7, which is what you presumably meant. 12BY7 is a pentode (beam tetrode actually). On the other hand, 12BH7(A) does not really fit the TU-8200.
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Old 10th June 2019, 07:50 PM   #24
paczeltf is offline paczeltf  Hungary
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Originally Posted by Sandy82579 View Post
Two things. On your spreadsheet updates C34-C35 have the same “original value” and “new value”. Should the “new value be the same as C33? 330-470uf/16v

Also you say about the E80CC “the rectifier upgrade is not enough to support these great tubes correctly”. What would need to be done to correct this.
You also sent these questions in PM, which is what I replied to.

Conclusion is that the spreadsheet is correct, the new C34 and C35 are OS-CONs with the same value like the cathode bypass capacitors in the DX upgrade kit.

Driver tubes with 600mA heater current can be used after these mods as long as you use them with a 6L6GC variant.
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Old 16th June 2019, 04:23 AM   #25
Sandy82579 is offline Sandy82579
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Default Modifications to Driver Section

You mentioned in one of your posts that you were satisfied now that you had completed you updates to the Driver Section. I assume that those changes were above and beyond the changes in the spreadsheet you posted. Could you tell us what mods you made to the Driver Section.

Thanks,
Sandy
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Old 20th June 2019, 07:47 PM   #26
paczeltf is offline paczeltf  Hungary
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Originally Posted by Sandy82579 View Post
You mentioned in one of your posts that you were satisfied now that you had completed you updates to the Driver Section. I assume that those changes were above and beyond the changes in the spreadsheet you posted. Could you tell us what mods you made to the Driver Section.
Yes, the updates in my spreadsheet are recommended even if you do not want to make any other change to the amp except for tube rolling.

Almost 30 years ago I bought 4 Tungsram E80CC tubes based on Jean Hiraga's praising comments in l'Audiophile. These tubes had already been discontinued back then so that he recommended 12BH7A for the driver in his design as an acceptable compromise.

I wanted to design a build a tube amp but getting OPTs was very difficult back then especially here in Hungary, so that I designed and built a solid state amp instead. These 4 tubes, along with some Russian triodes that I stole from spare parts stock or simply pulled out of disassembled units during my military service, had remained in the closet for a few decades unused.

I thought I would give them try after the power supply upgrades and I was deeply impressed. I took 10 seconds to recognize the huge step forward via headphones, which resolved all my complaints about the overly warm and laid back character of the stock amp. The highs cleared up, the stage opened up into multiple layers, and the dynamics finally arrived. Mr. Hiraga was right...

Later, when playing music with higher volumes especially via speakers, I noticed some harshness in the treble. The reason is obvious. Even though the E80CC has similar bias current to the ECC82/12AU7 in the TU-8200, it has about 50% higher gain that affects both stages. This amp has a global negative feedback, and the resulting higher loop gain may cause transient intermodulation distortion and maybe parasitic oscillations.

The solution is simple - desolder C5 and C6 cathode bypass caps, at least if you like the sound otherwise. This will restore the loop gain and fix the harshness problem without sacrificing anything of the benefits of the tube replacement. No cap is the best cap anyway, so if you can live with a slightly less gain, you can get rid of C1 and C2 as well. In that case the negative feedback to the cathodes of the input tube needs to be restored by increasing R3 and R4 from 12 to 16-20 Ohms. (I use 20 Ohms and it works well.) The amount of the GNFB applied has significant impact on the sound, I even tried it without global feedback and it sounded flat and boring so "no feedback" is not always the best option as many believe...

Note that due to the higher heater current requirements of the driver tubes, you can use them with 6L6GC and its variants only. KT66 is on the edge but is probably OK. (I have a pair and have had no issues but preferred the RCA 6L6GC sonic character overall, not to mention the much neater look with this amp compared to the oversized KT66.) No EL34 or KT88 can be used as the total heater current may damage the power transformer.

All brands are great, right now I use Valvo/Philips variants mainly because I am not emotionally connected to them You can still put the 12AU7 back any time if you want to use these big power tubes but as the circuit has changed, it may not sound the same as the stock model. (To my ears, there isn't that much difference though.)
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Old 21st June 2019, 02:19 AM   #27
Sandy82579 is offline Sandy82579
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Thanks for that. I’m almost finished building the tu-8200 with the mods you suggested. I’m still waiting on the DB207 from China for the D7 position. I’m getting very anxious to try it. You’re posts are greatly appreciated. I don’t have the design background for electronics. I chose mechanical engineering.
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Old 21st June 2019, 07:24 PM   #28
IceFyre13th is offline IceFyre13th  United States
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Quote:
Originally Posted by Sandy82579 View Post
I donít have the design background for electronics. I chose mechanical engineering.

Ready for this......


I have a Bachelors in Electronic Engineering, but my job is Mechanical Design Engineer.


I had to step in for a fired ME at an old job to get out projects done on time, found out I am good at it too....and stayed with it.


Its always fun to tell people I am an ME with an EE....lol
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Old 2nd July 2019, 05:00 AM   #29
oface1 is offline oface1
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Quote:
Originally Posted by paczeltf View Post
The 12AU7 should not be microphonic. Power tubes tend to be more microphonic than driver tubes in a power amp anyway.



If you apply all the modifications that I recommended in this thread then you will be able to use 12BH7, which is what you presumably meant. 12BY7 is a pentode (beam tetrode actually). On the other hand, 12BH7(A) does not really fit the TU-8200.


Yes, I meant the 12BH7, I tend to get the nomenclature confused at times....

Thanks for your response!
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Old 25th August 2019, 05:47 PM   #30
Disque is offline Disque  Netherlands
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I have some hesitation about the value of the feedback resistor when the cathode bypass capacitor is removed.
Theoreticaly the gain of the E80CC stage without the cathode capacitors equals the gain of the stage with 12AU7 and bypass capacitors installed. So the total ammount of feedback should be equal..
By removng the bypass capacitors the cathode 1k resistor comes into play for the audio signal and messes up the ammount of feedback set by the original 12 ohm resistor. Raising it to 18 ohm is in my opinion inadequate. Soms basic calculation points at a value of 100- 120 ohm for the feedback resistors.
Am I on the right track here or do I err somewhere?
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