醜女大返身之 -- KT88 (Part II)

看過了 Part I, 是否好得人驚呢?

再看看這部份 .... 醜女是否變了靚女呢. 我這隻入行一年的貓手功如何呢?

還要多多向 Modi Sir 和其他人多多學習....

Post 住幾張相先, 部機未完全做完.  :

CAT

Use capacitor meter to test rather than inspection. Hard to tell.

Dear Accphoto,

Is this cap already dried? The paper seems to be a bit dry though I can still tell it has oil on it.

The yellowish thing is the solidified oil. If I use a cigarette lighter to heat it up for 5 - 10 seconds, the oil will melt back into liquid form. Then it becomes solid again.

So, this cap is no good, right?

CAT

Dear Accphoto,

I have drawn the diagram here. The original layout is the blue lines. The Red and Pencil markings are my thoughts for the Jensen caps layout.

Please point out my mistakes if they exist.

What should I do with the Original Cap (0.1nf / 600V round one)  + Transistor (27K 2W for draining voltage after power down)?

CAT

Dear Accphoto,

Do you know where I can get these sheet metal? They need to be at least 3" wide as the holes were quite large.

I was hoping to go over to Yau Ma Tei yesterday but it was raining terribly. This morning it was even worse and I was almost caught in the car as the water surrounding the road is well over 2 feet in certain areas.

I tried something but I am not sure whether I did the right thing. If the cap and the rectification process is good, I should have a almost FLAT wave shown from the scope, right?

What I did was I switched the scope to AC. I put the probe to the +ve and -ve terminals of the 1200uf cap. Then I turned on the power of the amp, let the cap charge up for 2 - 3 mins. Finally I input a CD signal to pull the current from the cap. The scope showed very UNEVEN waves; like that of Hang Seng Index. This is NOT good, am I correct?

CAT

Simple, get two piece of aluminum sheet that larger than the hole,  Just open the appropriate size for Jensen caps, then spay them with black paint and cover the opening from underneath. It should look ok from outside. Alternatively, I have two big ENLA 200u +200U caps which is almost the same size, you should able to fit them it. I think Bro Yuen should have this capacitor too.

Dear Accphoto,

I am glad I waited for your response first.

I won 2 GE 5U4 just now. The price is okay. I ALMOST bid on a pair of 5AR4 ... ha.ha.. Now, I should go for 5R4 instead.

There are TWO holes at the rear of the Amp Chasis to hold the 1200uf capacitors. If I use Jensen 200+200uf, the size won't be the same as the Chemi-con 1200uf caps ... Therefore, I need to find some way to stuff the space in the two holes.

CAT

5R4 is not 5AR4, The output voltage of 5AR4 is higher than 5U4 and is not suitable for this type of tube.  5R4 is another type of rectifiying tube which is close to 5U4 and can be used.

What holes you are talking about........????

Yes, certainly the filtering caps play important role for sound quality....

Dear Accphoto,

I will call Ah Yuen to see what he has to offer this afternoon or tomorrow. My Physics colleague said the amp should sound much better if those filtering caps are of better quality. He said with good caps, even if there is a high current draw, the output is still stable and you can see on the scope the line is relatively smooth.

Just ONE question. How to fill up the HOLES? Ha..ha..

I am searching for RCA & GE 5U4 already. Just no luck as bidding ends at either 3am or 5am ... You typed 5R4 GE. Can I use 5AR4 as well?

CAT

Yes, you better change the caps, First of all, they are computer caps, not for audio cap, Secondly, you do not really need those big caps. Thirdly, I think they are really old.

I suggest you to change to Jensen 200U + 200U 500V or ENLA Cerefine 200u+200U 500V if you can still find some.

I do have some RCA  5U4 and  GE 5R4, You can plug in and test the sound first. IF the sound is ok, then you can buy them in ebay with the same brand. The rectifiying tube normally has the greatest effect on sound, so you better chose something that is ok. Otherwise it is a waste of money.

Dear Accphoto,

This KT88 indeed brings a lot of new experiences to me.

I totally agree with you, it is very important to test out the performance of the amp by means of a scope after you finished it.

Today, I also test out the rectifying stage using the scope, to see if it is a straight line or got lots of ripples. Fortunately, after the capacitors fully charged up and the tubes warmed up as well, the scope showed a more or less straight line with no jagging.

The 6SL7 I am using right now are mine. I put them in before I do the adjustments. However, I haven't get hold of the 5u4 yet. Still trying to get some from eBay.

Since you can't come today and one of my colleagues suddenly got a meeting to attend, we don't have enough "feet" for the game. We ended up playing continously for two hours with just 5 people ... totally exhausted, ha..ha...

My colleague teaching Physics suggested me to change the 1200uf caps as well ... as they tend to deteriorate after a few years. The two on board are well over 15 years ... What do you think?

Anyways, thanks again for all the help. I am now enjoying the amp as the new KT88 runs in and the caps as well.

CAT

This mainly related to the design of the amp. 使用"屏陰分相" All push pull amp needs to separate the signal into upper and lower wave and let one Kt88 to do the power output. The phase splitting is based  drawing output from  Anode and Cathode of  the 2nd triod of  6SL7,. Since  the voltage difference from Anode and Cathode is different from each others, so a 30K pot was used to balance the Signal strength output from both  the Anode and Cathode. The orignal KT88 is not match, so the setting will be different if the KT88 is matched. The DC offsetting is not balance so the transformer is push by two different power, there will be all sort of strange sound from it.  Actually, you should adjust it again if you change the 6 SL7 in future.

The reason that i like Single End Amp is that you will never got this problem on single end amp.

Therefore, people can't really do a good HiFi if they do not put their equipment on the scope.....

Dear Cat,

Thanks for info.
We learn something new.

Dear WINDWSS,

I can't exactly tell what is wrong. I think Accphoto can give more details and amend my sayings if I am wrong.

It seems to me that the tubes are NOT MATCHED (the original Chinese KT88) and the Bias not set right. It is not surprising as the Cary uses other KT88 tubes in the first place. My friend swapped the Chinese tubes in for me to do the test run. He kept the expensive KT88 at his place.

An incorrect BIAS setting makes the wrong voltage flowing into the tubes. If you hook the amp onto a scope, the wave from the 1KHz CD source and the Amp output do NOT match.

Accphoto told me to adjust the BIAS (30K resistor) until the wave form is clean and sharp and the two can approximately overlap with each other.
I posted the photos here for your reference.

Accphoto said many forgot to perform this step after they finished constructing their amp. I do agree with him that this is a very important stage. I recall some time ago, another Westerner also told me the same thing --- check the amp using a scope before the actual run in.

Hope this helps.

CAT

Dear Accphoto,

As it is already late, I dare not call you anymore.

Spent an hour listening to the KT88 and then let it idle for another 30 mins or so after our last conversation.

PROBLEM FIXED!

This is one interesting exercise for me instead. The curve for the Right Channel is now smooth. I haven't test the left channel yet. Will do that tomorrow. The two output transformers are of more or less the same temp again.

MANY THANKS!!

CAT

Dear Accphoto,

I swapped the 6SL7. Problem still there. I discovered something new.

NO problem exists if I don't switch on the Pre-Amp. I therefore swapped the Connecting cables from the Pre-amp. Problem still there. That suggests whenever the KT88 receives anything from the pre-amp, the RIGHT Channel will have problem; no matter the source is from the LEFT or the RIGHT channel of the PRE-AMP.

I haven't turn on the CD player. There is no signal into the Pre-amp. What could have come out from the pre-amp and cause the woofer to vibrate?

CAT

That sort of distortion is not caused by capacitor, it will be either the 6SL7 or the transformer. Try to swarp the 6SL7 to see the problem still exist. If yes, then need to use the scope to check which stage has the distortion come out. When you use the scope to check the distortion, mainly put the scope on the signal path of each tube input and ouput, you should able to see whether the wave was distorted. Unfortunately, I cannot come on Thursday, otherwise, I can give a quick look with you.

Of course, the capacitor need to be replace soon but we should fix this distortion problem. I aussme that you check all component value and sometimes, it will be possible for some component and connection problem.

Dear Accphoto,

I went up to the lab yesterday and tried out that signal generator. It seems to be it is erratic. Can generate a wave but can't display properly at the oscilloscope.

Today, I use the scope with a CD player, playing that 1khz 0 db CD. The wave can be shown without trouble.

The Re-issue KT88 should be matched QUAD as stated by the label attached to them. Now, the Right Channel got this woofer problem whereas the LEFT one is perfectly okay. Do this suggest the problem of the Capacitor? Should I change them as well? They had been used for over 15 years already. If I indeed need to change them, what should I use instead?

CAT

No worry, can adjust it again.

Dear Accphoto,

I got a QUAD of new issue Gold Lion KT-88 from Ah Yuen this afternoon. Really understand why you use the word "HARSH" to describe the Chinese KT88 tubes. The Gold Lion give much better sound BUT the problem comes back again ..... pumping of the Right Channel Woofer.

If I switch back to the Chinese KT88, no problem at all. I think I have to adjust the 30K thing again for this pair of new Gold Lion.

The LEFT channel is perfectly okay

CAT

[quote:c4ceefea32="alfcat"]Dear Accphoto,

I think I will go to Ah Yuen's place to get some good KT88. These Chinese tubes got lots of limitation, I suppose.

CAT[/quote]

Yes, they sound harsh to my ears. Get a Gold Lion Replica KT88 , they are good but a little bit expensive.  For EH, forget about it , you should just stick to CHina KT88.

Dear Cat,

So finally what's the main cause of the "sss" & "bu bu bu" sound on your KT88? How you both solve it? is it because of faulty part or connection problem or grounding problem?

Dear Accphoto,

I think I will go to Ah Yuen's place to get some good KT88. These Chinese tubes got lots of limitation, I suppose.

CAT

Problem fixed........  

Dear Accphoto,

I read your re-posted comments already.

The problem won't exists if I just turn off the MAIN switch. At most (once or twice) the woofer moves a little bit.

I swapped the rectifier tubes, looks better. Seldom got any trouble. The two output transformers' temperature are very closely matched now.

At stated in the last posting, no more "distortion", "sss...sss..." or "buu..buu" now.

THANK YOU very much indeed!!!!

8)

CAT

40-42C is very normal, Because the tube's heat will also heat up the transformer. I have made some changes in my last comment, please check.

Dear Accphoto,

Yes, that is exactly what Ah Yuen suggested; no more than 200uf. He said no need to go for this 1200uf high capacitance.

I will try your suggestion for a few days. The heat is now even across the two output transformer ... but is it a bit hot? Around 40 to 42C.

CAT

If this  happens only  when switch on the machine from standby mode? did you wait for a while after turning on the main power ? It is quite common as the tube were not stable in the first 2-3 minutes when turn on.
The reason why the speakers move may due to some DC current goes into the signal path. This may be resulted from either instable tubes or excessive current high voltage from the main power.

This should be avoided as the current is harmful to the the power supply.
You should try the followings:
1. You should swap the rectifier tube to see whether it will goes to the other channel if one rectifiier tube responsble for the each channel. If it happens again on the other channel, then the rectifier tube is likely to be faulty soon; If it repeats on the same channel, then it will be the capacitor problem.
2. You should also try not to use the standby button, which means that turn on the main power supply when the standby button on. The problem will not happen as the rectifiying tube will heat up slowly to avoid the high voltage flush when using the standby button. If the problem happen again, then likely to be the instable  rectifier tube.

Really need to thank Accphoto. He spent the whole afternoon checking the CARY SLA-70 Signature for me !

The problem of "sss...sss" noise in the Right channel is now gone. No more "buuuu.....buuu..." noise as well. The temperature for BOTH output transformer is more or less the same (but still at 107F without air conditioner turned on).

There is NO funny noise when I switch to STANDBY mode.

BUT still got one problem left. When I switch it back on from Standby mofe, the RIGHT woofer is bouncing mildly like a heart beat. No sound, just see the RIGHT woofer moving back and forth gently.... Really funny.

Do you think it is the 1200uf capacitor?

CAT

I am thinking of taping it down if Accphoto doesn't mind.

As we don't do this often, it is easy for us to forget about it totally after a month or so.

CAT

Dear Cat & accphoto,

Kindly share with us the finding after u had done the troubleshooting..

TQ

The most important is the 1KHz signal, right?

I will have the equipment ready

CAT

A signal generator and some multi-meter should do the job.

Dear Accphoto,

I am glad you can come. Just tell me what is required except the scope.
Many thanks.

CAT

It shall be fine. Call you when I back to Hong Kong.

Dear Accphoto,

You are right. There is something fishy about that setting.

Last night, I swapped the Output Transformers again. This time, NO obvious trouble. I played the thing for > 3 hours and everything seemed to be normal. It is only when I turn it off, the right woofer bounced mildly once (like lost of power).

I will be free on Sat > 2:00pm. You don't need to bring the scope (I think it is heavy) as we have plenty of them here in the Physices Lab.

What time will you be okay?

CAT

I think it is the 30K resistor. By guess a value of 8k may not solvethe problem as we cannot see the problem. I am still thinking of checking the whole thing with a scope to confirm the problem. Buying a Tango is too expensive if the transformer has some problem. I   suggest that you can buy a PO shan which is really of good quality and should cost you about $ 1,400. Let meet this weekend , I bring it the scope and let the machine had a full check.

Dear Accphoto,

I swapped the 1200uf capacitors again just now. The problem still exists.

I think there were a couple of mistakes I made during the previous testings

I focused too much on the "buu...buuu" sound in the LEFT channel. I did not connect the CD signal to the Right. So I presume the RIGHT channel is okay. Eventually it is not.

The funny thing is, if I don't input a signal, there is NO problem with the Right Channel. It is silent and quiet. But if I feed a signal in, very soon, music become distorted and the woofer moved abnormally. It is like the output from the amp is not stable / irregular.

My last try to confirm ... swap the Output transformer again later. If the problem now goes to the LEFT channel with CD signal in ... then it MUST be the Output Transformer.

If that is so ... a Tango pair cost > HK$ 3,700 ...... should I change them....


CAT

Dear Accphoto,

I think we have to meet somehow. I was happy for just a while.

The 1200uf Huge Capacitors were swapped before I tuned down the 30K resistor. The left channel now is perfect.

However, the bumping of the speaker woofer comes back with the RIGHT channel after I played 3 or 4 songs. I thought it was the Right 30K problem so I tuned it down to 8K as well but it was not. After turning the amp on, the bumping of the woofer comes back right away.

As I swapped the 1200uf capacitors, now I wonder whether it is the capacitor problem. The output transformers remained cool.

CAT

Sometimes, next week, we can meet together and see what happen.

Dear Accphoto,

I dis-assembled everything on the LEFT channel and re-assembled it again. This should get rid of any mis-match of resistors, cold soldering, etc.

The problem is still there with the LEFT channel. Very depressed .... :cry:

CAT

Still no good.

This time, even if I pull away the cable, it still gives off buuu ... buuuu noise.

It is strange. I am 100% sure about this. I have tried just connect the speaker terminals to the KT88. Turn it on for 2 mins ... no noise. The second time I turn it on, the noise comes back. If I connect the cable, noise comes back rightaway.

Sigh .... looks like I need to spend a day to dis-assemble the whole LEFT channel and try re-constructing it again.

CAT

I think the problem occurs because I did the LEFT channel at different periods of time. For the Right channel, I completed the whole thing in ONE past (2 hours or so).

The worst case, I dis-assemble everything on the LEFT channel and do it again in one pass... .ha..ha..

Just wish I can spare the time this weekend.

CAT

Connect to the speaker terminal, red and black, this one is loading. Look like that it is really either cold soldering and connection problem as the blu sound has disappeared, you should check what you should have done on after last resistor. Properly something is shorted.

Don't be too upsetting , this is DIY but you do enjoy when you complete the project and solve the problem......

BTW, how should I connect the 8 Ohm resistor to the Output transformer? Except the Speaker Terminal RED & BLACK, there are 5 wires; Brown, White, Grey, Blue, Black. Which TWO should I connect?

Dear Accphoto,

Sigh ... I have double, triple checked .... I think it is better for me to leave it for a while. Perhaps really 當局者迷 ... maybe I overlooked something.

Tonight, I changed the LAST used resistor on the left channel. For a while, the buuuu noise is gone. I was more than pleased. Then I turned it off and connect the other wires back .... when I turn it on again, the noise came back in 10 seconds.

Very disappointing.

CAT

CD-ROM means read only........

For the bu bu sound,  I talk to 棠師傅,I believe that there must be some wrong connection, missing resistor or wrong resisitor value, Since you have already got one channel working, you should able compare two channel  to find the difference.

For measurement of voltage, it must be DC not AC, just compare the voltage point by point ( get the circuit and mark it). you can also find the difference. You can also connect one resistor 8 ohms to the output transformer, if the resistor getting hot for a while, then the wire for the tranformer is not correctly wired.

Oh, I am glad that you can find a good owner for the SP-10. And generous of you to have the proceeds donated to the victims of the disaster. Our school raised over HK$ 22x,000 just from the boys and their parents as well. I am glad that many of our boys are willing to donate as well.

I don't understand what you mean by "there are lots of CD ROMS in this forum" ....

CAT

Dear Accphoto,

Where should I take the voltage? AC or DC? One probe to the ground or ..
I have tried with CD Player but not an amp.

I will have to borrow a oscilloscope from the lab to do that. And I have to have a CD player to play that 1KHz sound. Will try that later.

For the time being, I just hope to cure that "buuu..." noise.

CAT

For measurement of voltage,  no need to feed in signal, just measure it at idle.  For the 30K volume, basically, it is adjusting the bias for the upper and lower wave, if your 11K work in the left channel, it should also work in the right channel. If you got the scope, it is better you should feed in 1Khz sine wave, and adjust the pot until the upper wave as close as to the lower wave.

Just talk to a buyer of my Sp-10 per-amp, I just found out that there are  lots of CD-ROM in this forum.  

I just sold my SP-10 per-amp and all proceeds had gone to Charity  for China Earthquake.