Hi Ferenc,
Many thanks for the great thread and support.
I finally made the referred update for C13/14. It appears it is the biggest step forward in sound I hear, considering also the bypass caps' removal.
The change is almost comparable in scale to the driver stage update to E80cc, to my ears.
My solid state tuner sounds after this change a bit like how my tube tuner sounded before this update made.
Kind regards,
Sandor
Many thanks for the great thread and support.
I finally made the referred update for C13/14. It appears it is the biggest step forward in sound I hear, considering also the bypass caps' removal.
The change is almost comparable in scale to the driver stage update to E80cc, to my ears.
My solid state tuner sounds after this change a bit like how my tube tuner sounded before this update made.
Kind regards,
Sandor
Great post and read with interest as I have my 8200 for quite a few years now. Been in and out of this hobby as well.
Specifically use the 8200 for headphones both low and high Z - both planars and dynamics since build completion. I have always find for higher sensitivity phones not adequately utilizing the volume pot due to the amp having very high gain - most of the time volume control at 7-8 o'clock!
What is the consensus for headphone output attenuation? Attenuate source, modify driver stage for lower gain, attenuate headphone output circuit with more attenuation, or etc?
Specifically use the 8200 for headphones both low and high Z - both planars and dynamics since build completion. I have always find for higher sensitivity phones not adequately utilizing the volume pot due to the amp having very high gain - most of the time volume control at 7-8 o'clock!
What is the consensus for headphone output attenuation? Attenuate source, modify driver stage for lower gain, attenuate headphone output circuit with more attenuation, or etc?
The TU-8200 uses low gain tubes and further significant gain reduction would lead to distortions in the input stage. This leaves you with two choices: Source or output attenuation. The "R" version gives you different jumper settings for output level attenuation but they should be used with care. I would use the setting that gives the highest acceptable background noise and implement fixed resistors at the input to lower the input volume if sensitivity was an issue.
Also note that the headphone output impedance of the original model is almost 100 Ohms, which already attenuates the output level with low impedance phones.
Suspected oscillation or distortion on one channel
First of all, big thanks for the modification Ferenc. I do have a few questions though.
I implemented both the PSU mods and the driver mods (R3/R4=18ohm) and then hooked everything up to a resistive 8ohm load and a signal generator for verification. Measuring with a 1kHz square wave input, the output voltages looks like this:
First of all, is this ringing ok in general? Seems a bit underdamped to me. There is also a difference in the ringing of the channels, seems to be a difference in harmonic content judging by the attenuation of the even peaks in the right channel ringing.
There is also a significant difference in "reverse microphonics". I can hear the 1kHz tone in the amplifier itself on both channels, but it's significantly stronger on the right channel. Hard to say which component is vibrating.
I tried swapping the driver tubes, but no difference. Also tried without load on the output, no difference. Checked the output of the signal generator itself, no ringing whatsoever.
I've measured all the test point voltages of both channels and they all show almost exactly the same value. Obviously the heater voltages are higher than the schematic specifies (6.15V). Points 1 through 8 are a bit off from schematic specs, but that's to be expected with the change of tubes (1/2=2.65V, 3/4=2.6V, 5/6=131V, 7/8=131V). Resistance values are also correct and identical as far as I can see.
I don't want to hook the amp up to my speakers until I figure out where these differences between the channels come from. Any ideas? Points I should measure? Many thanks in advance.
First of all, big thanks for the modification Ferenc. I do have a few questions though.
I implemented both the PSU mods and the driver mods (R3/R4=18ohm) and then hooked everything up to a resistive 8ohm load and a signal generator for verification. Measuring with a 1kHz square wave input, the output voltages looks like this:
First of all, is this ringing ok in general? Seems a bit underdamped to me. There is also a difference in the ringing of the channels, seems to be a difference in harmonic content judging by the attenuation of the even peaks in the right channel ringing.
There is also a significant difference in "reverse microphonics". I can hear the 1kHz tone in the amplifier itself on both channels, but it's significantly stronger on the right channel. Hard to say which component is vibrating.
I tried swapping the driver tubes, but no difference. Also tried without load on the output, no difference. Checked the output of the signal generator itself, no ringing whatsoever.
I've measured all the test point voltages of both channels and they all show almost exactly the same value. Obviously the heater voltages are higher than the schematic specifies (6.15V). Points 1 through 8 are a bit off from schematic specs, but that's to be expected with the change of tubes (1/2=2.65V, 3/4=2.6V, 5/6=131V, 7/8=131V). Resistance values are also correct and identical as far as I can see.
I don't want to hook the amp up to my speakers until I figure out where these differences between the channels come from. Any ideas? Points I should measure? Many thanks in advance.
Did you make similar measurements with the stock configuration? If yes, what was the result?
You said you had swapped the driver tubes, but did you also swap the power tubes? Do you hear noise when switching between inputs or turning off the amp?
Seems ok
Thanks for the reply. Seems my worry might have been a bit exaggerated. But to answer your questions; I've tried switching power tubes, input, turned the amp off/on again. Sound remains the same. But, I realized that it could come from the output transformers. A difference in vibrations from them shouldn't be unexpected. Now it's really hard to locate the exact source of that sound, but I'm fairly confident that it is the OPT. So I'm not too worried about that anymore.
Now about the step response. Unfortunately I didn't measure before the mods, but I reverted one channel and measured, and the step response is the same in the "original" configuration. I'm not too worried about that either anymore. So in short, thanks, and sorry for wasting your time
. That said, I'm still very curious about the source of that "wiggle". It's only visible for square waves, not triangular waves. So I will keep measuring, just to satisfy my curiosity.
Again, thanks for the help.
Thanks for the reply. Seems my worry might have been a bit exaggerated. But to answer your questions; I've tried switching power tubes, input, turned the amp off/on again. Sound remains the same. But, I realized that it could come from the output transformers. A difference in vibrations from them shouldn't be unexpected. Now it's really hard to locate the exact source of that sound, but I'm fairly confident that it is the OPT. So I'm not too worried about that anymore.
Now about the step response. Unfortunately I didn't measure before the mods, but I reverted one channel and measured, and the step response is the same in the "original" configuration. I'm not too worried about that either anymore. So in short, thanks, and sorry for wasting your time
. That said, I'm still very curious about the source of that "wiggle". It's only visible for square waves, not triangular waves. So I will keep measuring, just to satisfy my curiosity. Again, thanks for the help.
If you temporarily bypass R3/R4 then you will eliminate global NFB completely. If you still can see ringing then it comes from the resonance of the OPT+power tube combination. The extent is not that big from a single-ended tube amp though, if a solid state amp produced that, it would be a different story.
Also try triode mode.
Also try triode mode.
I just finished building an 8200R kit with Lundahl transformers and the mods from this thread (save R64, which is the original 22k ohm value.) The test point voltages all seem within spec. The amp sounds pretty dead, though. It just doesn't have the vibrance and holograpy that I expect from a high quality tube amplifier. Granted, my normal listening system is a pair of 300B ultralinear amps with Electraprint transformers. I swapped the stock JJ 6L6GC power tubes for a pair of KT88 Gold Lions and it is much more open and airy, but still sounds one dimensional. I suspect the JJ ECC82's are the issue, but wanted to get others opinions before pulling the trigger on tubes. I don't have any experience with JJ tubes other than reading decent reviews. Should I upgrade the ECC82's with some other variant?
PS: it doesn't sound bad by any measure. It just sounds undetailed and boring. Speakers are Kipsch Heresy I with modified crossovers to run the squawkers with bandpass rather than high pass. The speakers sound magical with my 300B amps.
PS: it doesn't sound bad by any measure. It just sounds undetailed and boring. Speakers are Kipsch Heresy I with modified crossovers to run the squawkers with bandpass rather than high pass. The speakers sound magical with my 300B amps.
To be honest with you. I do not know why you short R64 and R63.
I have just talked to Mr Fujita. He said it is a crazy moddification. This modification will damage Q8, ZD24 and Q4.
Also 0.47ohm is not equal to short the circuit..
Please insert back 22K to R63 and R64. I hope your Q8, ZD24 and Q4 still ok..
I sold 150 pairs of LL2770B for TU-8200. Everyone loves this OPT..
I have just talked to Mr Fujita. He said it is a crazy moddification. This modification will damage Q8, ZD24 and Q4.
Also 0.47ohm is not equal to short the circuit..
Please insert back 22K to R63 and R64. I hope your Q8, ZD24 and Q4 still ok..
I sold 150 pairs of LL2770B for TU-8200. Everyone loves this OPT..
This is the reply from Mr Fujita..
This location is the protective circuit.
The user might not notice the difference until there is a problem.
With a jumper wire on R64,
If big current runs to R64, the protective circuit does not work.
2 consequences can be possible.
If ZD2 is shorted, the voltage is wrong.
If ZD2 is disconnected, the FETs of Q8 and maybe Q4 are damaged and the amp might get oscillated.
Check point 24.
He thinks if it is just a jumper on R64, maybe a hum occurs but the sound should be OK.
PLEASE install everything according to the manual.. Once your amp is working properly..You can modify your amp
This location is the protective circuit.
The user might not notice the difference until there is a problem.
With a jumper wire on R64,
If big current runs to R64, the protective circuit does not work.
2 consequences can be possible.
If ZD2 is shorted, the voltage is wrong.
If ZD2 is disconnected, the FETs of Q8 and maybe Q4 are damaged and the amp might get oscillated.
Check point 24.
He thinks if it is just a jumper on R64, maybe a hum occurs but the sound should be OK.
PLEASE install everything according to the manual.. Once your amp is working properly..You can modify your amp
Sorry for the confusion. My R63 and R64 are both at the original value of 22k ohm. I was referring to the spreadsheet on the first page of this thread. It states that the original value is 0.47ohm and recommends shorting it. Given the major difference in value, I did not change mine from the original values.
I have the schematics of both the original TU-8200 and TU-8200R in front of me. R64 is a 0.47 Ohm resistor between C31 and C32. If you found a 22k value in that position then there must be something seriously wrong there.
My unit came with Chinese 12AU7's, and also sounded flat and boring despite the mostly favorable reviews I had seen. It improved a lot with Tungsrams, but I still found room for improvement, which is why I designed the mods and opened this thread.
I am very sorry that Victor deleted the attached table which summarized all the changes suggested by Ferenc ....
I personally found them very useful.
I also believe that Ferenc knows exactly the fact of him .... after all he has always been very helpful with everyone in explaining the theoretical assumptions behind the suggested changes.
I own a "modified" TU-8200 as suggested by Ferenc with 2x E80cc as driver + 2x 7581A as power tubes and 2 OPT Lundahls ..... to me, now, it sounds "divine".
It would be nice if all the people who found the suggested changes useful, testify their satisfaction here !!!
I personally found them very useful.
I also believe that Ferenc knows exactly the fact of him .... after all he has always been very helpful with everyone in explaining the theoretical assumptions behind the suggested changes.
I own a "modified" TU-8200 as suggested by Ferenc with 2x E80cc as driver + 2x 7581A as power tubes and 2 OPT Lundahls ..... to me, now, it sounds "divine".
It would be nice if all the people who found the suggested changes useful, testify their satisfaction here !!!