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.
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
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
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
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.
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.
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.
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
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
That may be because the driver stage distorts less.
Not that much better in theory, but you eliminate a less than perfect (not fully linear) component.
This is the usual approach with 12AU7 as they provide low gain. Elekit uses 12A*7 family tubes because they are easily available new.
Hi guys.
I have a TU-8200r. I made all the mods that mr. Ferenc advised, plus:
R43, R44 220ohm (old) 20ohm (new)
C5, C6 desolder
C1, C2 desolder
R3, R4 12ohm (old) 20ohm (new)
1. Using e80cc + 6l6gc: after 5-10 minutes of playing, the on/off light goes off and the driver tubes goes off. The power tubes are still glowing. It looks like it happens when the amp heats up. When I turn it off manually after that and then turn it on, same happens, but more quickly, after a minute or so.
2. Using 12au7 family + 6l6gc: there are no similar problems. But, I found a sound a little, how could I say, raw, loosen, not as smooth and too sharp on upper freqs, and basses are a not as compact as before. The bass is not linear, because I hear some bass freqs more (boosted) and some less. That was not so before or at least not so obvious.
What do you think?
Best, Gasper
I have a TU-8200r. I made all the mods that mr. Ferenc advised, plus:
R43, R44 220ohm (old) 20ohm (new)
C5, C6 desolder
C1, C2 desolder
R3, R4 12ohm (old) 20ohm (new)
1. Using e80cc + 6l6gc: after 5-10 minutes of playing, the on/off light goes off and the driver tubes goes off. The power tubes are still glowing. It looks like it happens when the amp heats up. When I turn it off manually after that and then turn it on, same happens, but more quickly, after a minute or so.
2. Using 12au7 family + 6l6gc: there are no similar problems. But, I found a sound a little, how could I say, raw, loosen, not as smooth and too sharp on upper freqs, and basses are a not as compact as before. The bass is not linear, because I hear some bass freqs more (boosted) and some less. That was not so before or at least not so obvious.
What do you think?
Best, Gasper
As I explained in more details in PM, it is PTC6 that is causing these problems. This component did not exist in TU-8200.
That may be because you did not hear it with E80CC so there was no base for comparison.
- Ferenc
Hi Paczelt,
I ordered all the parts and I have just one question regarding R64.
You say regarding that resistor "Cut off a pin of a resistor, bend it twice". You mean bending the cut pin to touch the uncut one? Or only bend the cut pin in order it to not touch the original circuit path?
I know this questions shows my low technical knowledge,,, lets say that I have recapped amps before following closely the recipe!
Thanks in advance and happy new year,
Alejandro
I ordered all the parts and I have just one question regarding R64.
You say regarding that resistor "Cut off a pin of a resistor, bend it twice". You mean bending the cut pin to touch the uncut one? Or only bend the cut pin in order it to not touch the original circuit path?
I know this questions shows my low technical knowledge,,, lets say that I have recapped amps before following closely the recipe!
Thanks in advance and happy new year,
Alejandro
Hi,
Just create an U-shaped jumper and replace the resistor with that. You can also leave the resistor in place and connect the two pins on the PCB side.
NP, same to you!
- Ferenc
Last edited:
Hi Ferenc,
So well; it's a success!
I performed all the changes except the bridge rectifier as I ordered the incorrect one...
What has changed for me soundwise; greater detail which shows itself more in the decay of the instrument notes. Perhaps a more defined bass but I am only listening through headphones so it is not so easy for me to analize it.
Many thanks
Alejandro
So well; it's a success!
I performed all the changes except the bridge rectifier as I ordered the incorrect one...
What has changed for me soundwise; greater detail which shows itself more in the decay of the instrument notes. Perhaps a more defined bass but I am only listening through headphones so it is not so easy for me to analize it.
Many thanks
Alejandro
Attachments
You do not need to short R64 if you want to continue using 12AU7. Doing that is required for E80CC only.
Hi all, just some afterthought after more listening.
The improvements are all over the place. Highs seem for me less edgy, mids have more snap, but for me the best improvement was in the low end. I can follow now more easily the bass guitar in many tracks where it was not that easy before. For me that is very important as I am a bass player. Additionally, I did some tube rolling. My favorite has been a CBS 5814, but I had a couple of RCA 5963 that sounded a shy in the highs, veiled, but now, they sound just right, with great midrange and a nice low character, which is the sound I am after.
So again, these modifications are very recommended.
Bought a pair of Tungsram E80CC. I'll post my impresions in the coming days
The improvements are all over the place. Highs seem for me less edgy, mids have more snap, but for me the best improvement was in the low end. I can follow now more easily the bass guitar in many tracks where it was not that easy before. For me that is very important as I am a bass player. Additionally, I did some tube rolling. My favorite has been a CBS 5814, but I had a couple of RCA 5963 that sounded a shy in the highs, veiled, but now, they sound just right, with great midrange and a nice low character, which is the sound I am after.
So again, these modifications are very recommended.
Bought a pair of Tungsram E80CC. I'll post my impresions in the coming days