Some thoughts on Elekit TU-8200DX

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Last year I put together one of these amps mostly for headphone use, based on the good to rave reviews from all forums. I did not exactly got the sound that I had expected. The highs were sweat and nicely rounded but overall it sounded somewhat muddy, laid back, lacked “PRaT” and involvement in general. I used my old trusted Audio Analogue Paganini CD and Sennheiser HD600, and the impression was certainly not made by either of the source or the transducers. I also heard some hum without signal in UL and pentode modes as well as some RF interference occasionally.

What I am looking for is not some AM radio sound from the 50’s but realistic sound, clarity, imaging and the low level details that you expect from a single ended tube amp. Replacing the stock Chinese 12AU7 driver tubes with Tungsram ECC82 led to a significant improvement in clarity, but did not change the overall sound characteristics of the amp.

Being an electrical engineer and an audio enthusiast myself (I designed and built solid state amps over 25 years ago), I analysed the circuit that is available in the manual. The conclusion is that it has great potential especially as a headphone amp but has some area where it could be further improved.

- The best feature is the FET filtered (not stabilised, filtered) power supply but I suggest a few changes in values. The 180 µF (C22) smoothing capacitor is just barely adequate especially with a 50 Hz power line that I have here, because the remaining ripple voltage drops the source-drain voltage on the FET close to the limit where it can operate at all.

- The output power specifications are a bit too optimistic. With acceptable levels of distortion you can get about 7 Watts per channel in pentode mode, 6 W in UL mode and 1.4 W (!) in triode mode from a 6L6GC. Note that it is not entirely impossible to reach to 4 W with these tubes in triode mode, but you need much higher plate voltage, lower bias current, and higher load impedance for that.

- The heating voltages are on a low side because the power transformer has a 6 V winding instead of the standard 6.3 V. This will drop further when using output tubes with high heater current requirements like EL34 or KT88, getting close to the minimum of 5.8 V for these tubes. The DC heater voltage for the driver tubes is also low. The Point# 28 voltage range is 5.4 – 6.0 V, but anything below 5.7 V is way too low. (I measured 5.9 V with 6L6GC, which would become lower with any other tube.)

- The auto bias circuitry is basically a negative feedback from the output to the cathode of the power tubes via a first order low-pass filter formed by R23/C13 and R24/C14. Although the cutoff frequency is low (below 1 Hz), due to the high amplification of a bipolar transistor, the low frequency signal slightly modulates the bias voltage up to about 100 Hz. It’s not a surprise that the later models use more sophisticated approaches.

- The overcurrent protection is based on the same filter hence it is slow. It may protect the power supply from old tubes that gradually lose bias stability but if they have a short then Q3 or Q4 is most likely already dead when the front LED turns into red. This happened to me once as well. The good news is that in most cases all you need to replace is the FET, if you are very unlucky then you can smoke R17 or R18 as well. I strongly suggest you buy a couple of spares especially if you plan to do power tube rolling with old tubes..

- The headphone output is very simple and fits high impedance phones only as the output impedance is almost 100 Ohms. Elekit even recommends taking one pair of resistors out to reduce hum, see Symptom# 6 on page 15. I don’t think this is a good idea as doing this will further increase the output impedance. Instead, just increase the value of C22 and it will be fine.

- The source of RF interference is the cable that connects the power switch on the front to the back of the amp (between Units 2 and 7) where the unfiltered power line voltage is passing through inside the amp. A mechanical connection would be better, but there is a simple workaround. Twist the cable around about four times between the back and front panels and it will be fine. Also, twist the cable around that connects the primary winding of the power transformer to Unit 7. This eliminated the problem.

- The RCA plugs use a thin metal plate and the connection is not reliable. I had to move the cables around in a few cases to get the signal back. If you do not need the jack input on the back ( I don’t), just remove the whole Unit 6, buy some Neutrik or similar RCA plugs, and connect them to Unit 5 with insulated wires. This will also eliminate the soldered connections between Units 5 and 6, which do not seem reliable for long term either.

Finally, the sound…I managed to redesign the driver section without having to mess up the PCB and now it sounds exactly as I prefer. It sounds so well now that I gave up on the idea of buying the TU-8600. I can share the details later if someone is interested.

Regards,
Ferenc
 
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Hi Ferenc

Here are my comments about your feedback.
Yes, the stock 12AU7 is the problem. This is the first component I recommend to replace it.

TU-8200 is designed based on 6L6GC. No matter what output tube you want to use. 6L6GC is always the best match with TU-8200.

Mr Fujita is one of the best Tube amp designers in Japan. Tube amp design is an art work. Sometime I want him to change the design to make it sounds better in theory. He always explains to me why he wants to his way... I always accepts his explanation. As we know there is no prefect setting... i.e. trade-off between quality and price...

Good back to you question
Is TU-8200 better than TU-8600?
TU-8600 is the first amp kit formally reviewed by Stereophile. The review should be available in early Feb/Mar edition. This will tell you a lot....

Is TU-8600 better? Just TU-8600's stock OPT is 2.5 times heaver and bigger than TU-8200's OPT..

There is a big gap between TU-8600 and TU-8200. How big.. there are a lot of unbiased comments available in the web.

Thanks
Victor
 
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Hi Victor,

Thank you for your feedback. I was sure you would send a few comments :)

I did not mean to say TU-8600 is not a great amp or TU-8200 would be better.

What I was saying is that to me the 8200 with modified driver stage sounds good enough as a headphone amp as long as the phones have >200 Ohms impedance. For more sensitive or low impedance phones some reconfiguration of the headphone output may be necessary as it is not user configurable unlike the 8600 that offers a few options. As a speaker amp, the 8600 is almost certainly better at least at relatively high volumes thanks to the bigger OPTs not to mention the Lundahl upgrade option.

I had read most available 8200 reviews before I bought the 8200, which all were very positive with the stock tubes even though we both agree that those driver tubes are not all that great. This confirmed that these reviews need to be taken with a grain of salt just like any equipment review in the magazines or in the net. This is a general rule and has nothing to do with Elekit.

I know the 8600 also got great reviews but I could not find a review that would compare these amps under the same conditions. Those who made some comparisons tended to agree that the 8200 is slower and warmer sounding, which is something that can be fixed.

If I ever buy a speaker amp kit then I will need some more power than either of these SE amps offer, which is another reason why I did not order the 8600.

While building the 8200 I actually contacted Elekit asking if they would ever release a parallel SET amp. (I have some reservations about class A/B push-pull amps in general, which is why I did not order a 8340.) It was in mid 2017 when the 8600 was still in the pipeline and had not been announced yet. They mentioned that an SET amp would be released soon and that they were still working on that one. They said that a parallel SET version depends on the demand from the main markets especially in the US. (This is not a big surprise.) I hope that will be enough demand to justify the release of that model. (Well, I could accept a SET 845 instead but the voltage range these tubes operate with would probably be too risky for a commercially available kit.)

Regards,
Ferenc
 
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Hi,

Give me some time as you will need the part numbers in some cases - the bigger capacitors have to fit the space available.

First of all, let me tell you what I do not recommend you to change.

a) I use the stock metal film resistors. Feel free to order a carbon film resistor package, but please consider that these components went obsolete back in the 60's and for good reasons. Their noise, linearity, long term stability are all inferior to a metal film resistor without any measurable benefit on the other side. I don't know if any contemporary commercial amp is available at all that is built with carbon film resistors.

b) I installed the Amtrans AMCO coupling caps from the DX kit and they are fine. I know the Mundorf EVO Silver Gold are preferred by many builders, but these caps also received mixed reviews. If there is any difference, it must be subtle anyway. Spend your budget on better tubes instead of boutique capacitors.

c) I ordered a TKD 2CP601 as it was recommended as the best potmeter for the 8200. Unfortunately I am disappointed. It is a pain to install, you need to make irreversible changes to Unit 2, find a custom spacer and connect the pot with insulated wires. It thought it would be more logarithmic than the little Alps pot (it is not), it would at least sound slightly better (the difference is negligible if any), and it would have good channel balance. I may have got a lemon, but my sample is way more out of balance at low volumes than the stock pot. I would have returned it if these issues had come out before soldering.
Try Alps RK27 or a good stepped attenuator instead. Or leave it as is.

- Ferenc
 
@paczeltf:

Thank you for your latest (#8) post, all this is really valuable information.

I also considered utilizing an Alps RK27 and I also rejected the idea
for not wanting to cut the UNIT-2 PCB. It would be nice to have a separate
volume control PCB in the kit. I think the TU-8600 has that feature.

Regards,
Peter
 
Wondering About Components for Mod

So I read this thread with interest since I’m about to put together my TU-8200, and I have a few questions:

I found all of the parts and have ordered them except for the Nichicon CS(M) capacitors. Mouser has them listed in their film capicators but they have no stock and you can’t order them. Digikey doesn’t even have them listed. I substituted the Nichicon UCS series electrolytics. Will those work?

I was also wondering why we shouldn’t use “audio grade” caps (+-10%) instead of “general purpose grade” (+-20%), and would it make any difference?

In addition to twisting the power wire that runs from back to front, would it help to shield it somehow to reduce the hum?

Finally, you cannot get the FETs 2N60Z anymore. No one has them. Do you know a good substitute?

Thanks for going through the mods and providing the changes. I just want to build the best sounding TU-8200 I can,and I think these changes will help.
 
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I received this in PM first and replied to that, but now that it has been posted to the thread, here is the summary of what I wrote for those who are interested.

From looking at the respective datasheets, Nichicon CS(M) and UCS are practically identical. You should be fine with either.

I doubt that boutique electrolytic caps would make a big difference in the power supply of this amp, and they also tend to be bigger than regular models. Space is the biggest limitation, unless you want to install these caps to the chassis and connect them to the PCB with wires, which almost certainly will introduce hum.

The biggest reduction in hum was achieved by the bigger C22. Twisting the power wires should be enough, I also recommend twisting the power transformer primary and secondary wires once before connecting them to the respective boards.

I cannot hear any hum with headphones in triode mode (which I do not like), and just a very distant hum in the preferred UL mode, which was not exactly the case with the original setup. The speakers are totally silent in any mode.

If we wanted to reduce the hum even further then we would probably need different PCB layout with separate power transformer secondary windings and grounding for the left and right channels as well as DC heating of the power tubes.

I could not order the FETs online either so I bought 4 pieces from Elekit when a defective "NOS" power tube killed one. Victor may also be able to help. I have not yet managed to find a replacement. The problem here is that Q3 and Q4 are working close to their dissipation limits so we need to be careful here.

Also note that the group of changes that I suggested are kinds of enablers for the driver stage update, which makes the real difference. (This is why I wrote in my post that it was the first part..)

On the other hand, having a more robust power supply with less hum and heater voltages closer to the nominal value should also make a difference.

Regards,
Ferenc
 
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500V caps are bigger and may not fit the board or the space between boards. Be careful as caps in the same product line are usually available with different lengths and diameters. The list that I attached contains caps that you can install without having to modify anything.

350V caps should be fine except maybe for C22, but I tend not to use caps and other components close to their limits in a particular area.
 
I found and ordered all items you have listed in excell document at www.digikey.be

It sounds, that you know what you are talking about, therefore I trust you ;)


It would be interesting to hear any other modifications, if they are necessary. My Elekit TU-8200 will arrive next week and I want to make as you have recommended. Maybe will add some fancy caps later.

Thanks!
 
I Read all the posts with great interest, because I expect a kit amplifier to show many compromisses. Your sugestions for upgrades are to the point and well designed. So I am very interesten in your ideas about modification of the driver stage. Do you think about ecc99 family or ecc88?

With many thanks for your work so far!
 
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These tubes do not fit the TU-8200 for the opposite reasons.

ECC88 was designed for low plate voltage and relatively high cathode current, it is not really a driver tube. You would also need a socket converter.

ECC99 is certainly meant to be a driver, but it draws too much heating current and is an overkill for this kit.

E80CC is the right tube for this kit from many aspects as long as you use it with 6L6GC (or maybe KT66). I know some kit owners have already installed them but the rectifier upgrade is NOT enough to support these great tubes correctly.
 
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.
 
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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.