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Old 20th October 2020, 03:54 PM   #81
SandorS is offline SandorS
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Default Re: desolder C5 and C6 cathode bypass caps

Hi Ferenc,

May I ask if I understand well that it's okay to simply desolder C5/C6, I.e. should not solder a wire instead the caps or do any further action?
I guess today the signal goes to earth which will change once I desolder C5/6. But I'm not sure.

Thanks a lot as always.

Btw, do you notice any sonic difference for Philips vs. Tungsram E80CC?
I use latter and find great overall, great transparency. Maybe a very little brightness at times, but will see after I desolder C5/6.

Sandor

Quote:
Originally Posted by paczeltf View Post
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 20th October 2020, 07:39 PM   #82
paczeltf is offline paczeltf  Hungary
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Some thoughts on Elekit TU-8200DX
Quote:
Originally Posted by SandorS View Post
May I ask if I understand well that it's okay to simply desolder C5/C6, I.e. should not solder a wire instead the caps or do any further action?
I guess today the signal goes to earth which will change once I desolder C5/6. But I'm not sure.
Hi Sandor,

The cathode resistors should not be bypassed with a wire as that would eliminate the negative DC bias on the grid compared to the cathode. The caps bypass the resistor from a signal perspective and connect it to the ground. If you remove these caps then you will implement some local negative feedback to the cathode. This is also called "cathode degeneration". The expression is weird but it refers to the lower than possible gain in that stage.


Quote:
Btw, do you notice any sonic difference for Philips vs. Tungsram E80CC?
I use latter and find great overall, great transparency. Maybe a very little brightness at times, but will see after I desolder C5/6.
Sandor
The brightness is normal due to the higher gain of the first two stages when these tubes are installed, which leads to higher than designed global negative feedback. I explained this above.

- Ferenc
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Old 21st October 2020, 12:42 PM   #83
SandorS is offline SandorS
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Join Date: May 2020
Many thanks Ferenc.
Look forward to how it will sound once desoldered these bypass caps.

The brightness is not very disturbing really. Actually via loudspeakers it's less disturbing for some reason. But my vintage Stax headphones is on the bright side anyway so hope to get a bit more balanced sound then. I connect these via the loudspeaker terminal and an adapter (a transformer essentially) allowing to switch between the two. But even as is, it's pleasure to listen to.

Cheers
Sandor
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Old 21st October 2020, 02:51 PM   #84
paczeltf is offline paczeltf  Hungary
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Some thoughts on Elekit TU-8200DX
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Originally Posted by SandorS View Post
(...)But my vintage Stax headphones is on the bright side anyway so hope to get a bit more balanced sound then. I connect these via the loudspeaker terminal and an adapter (a transformer essentially) allowing to switch between the two.
Do you know the load impedance of the Stax with these transformers?
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Old 21st October 2020, 05:47 PM   #85
SandorS is offline SandorS
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Hi,
I was not able to find published. However, its predecessor model must be comparable I trust, of which found this:
Input

Input impedance (headphone mode) not more than 30 Ohms anywhere in the audio band. 5 Ohms DC.

Thanks
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Old 21st October 2020, 06:18 PM   #86
SandorS is offline SandorS
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Bias Voltage

The 230V bias voltage is created with an ordinary diode/capacitor voltage doubler connected directly to the power line through a 51k Ohm resistor.

This is fed to the center-tap of the output side of each transformer through a 1 megohm resistor, while the diaphragm of each transducer is connected to this voltage through a 27 megohm resistor.
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Old 22nd October 2020, 06:09 PM   #87
paczeltf is offline paczeltf  Hungary
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Some thoughts on Elekit TU-8200DX
Quote:
Originally Posted by SandorS View Post
(...)
Input impedance (headphone mode) not more than 30 Ohms anywhere in the audio band. 5 Ohms DC.

I suppose that is with the transformer. It is a bit high if you connect it to the speaker outputs, which was designed for load between 4 and 15 Ohms. I would rather bypass the 180 Ohm parallel resistors at each channel on Unit B and use the headphone output that adds a parallel load of 15 Ohms.
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Old 27th October 2020, 08:49 AM   #88
SandorS is offline SandorS
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Thank you.
I asked Stax as was a bit suspicious to me why they would had produced such a device with input impedance rather off the typical loudspeaker values, of at least of nowadays.
I got confirmed input impedance is 8 ohms for the adapter I use SRD7. Cool.

I desoldered C5&6, and got what I hoped for.
Actually, to my ears, it sounds better without these caps even with 12au7. At least, more tangible sound somehow. Maybe a bit robust.

Are C1&2 bypass caps too?
By removing C1&2 (and increasing R3/4 values), can I achieve in theory a better setup? Just by getting rid of caps that I favour, to do, I'd think so. I understand the resistor values will impact the global feedback a lot, just am unsure why the stock model uses C1&2 rather than no caps here and with higher r3/4 values instead. Maybe there is an aspect I'm not getting.

Well thanks for the support.

Kind regards
Sandor
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Old 27th October 2020, 07:48 PM   #89
paczeltf is offline paczeltf  Hungary
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Some thoughts on Elekit TU-8200DX
Quote:
Originally Posted by SandorS View Post
(...)
Actually, to my ears, it sounds better without these caps even with 12au7. At least, more tangible sound somehow. Maybe a bit robust.
That may be because the driver stage distorts less.

Quote:
Are C1&2 bypass caps too?
By removing C1&2 (and increasing R3/4 values), can I achieve in theory a better setup?
Not that much better in theory, but you eliminate a less than perfect (not fully linear) component.

Quote:
(...)just am unsure why the stock model uses C1&2 rather than no caps here and with higher r3/4 values instead. Maybe there is an aspect I'm not getting.
This is the usual approach with 12AU7 as they provide low gain. Elekit uses 12A*7 family tubes because they are easily available new.
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Old 1st November 2020, 12:41 PM   #90
SandorS is offline SandorS
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Thank you!

Kind regards
Sandor
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