CARY KT-88 Final / 醜女大返身 -- 終極篇

Finally, after TWO months of HARD work, trial & error, short term joy and all kinds of frustration, the overhaul of the Cary KT88 is now complete.

I am glad I learned so much from Accphoto. I can definitely say that I can't finish the job without his help. He spent more than 10 hours struggling with this thing by my side.

Now, I am enjoying the music from this baby, like it very much. Perhaps it is like the first wiener for every BBQ .... always good tasting.  :

玩了兩個月, 終於整好部 CARY KT-88.

成功一大部份是有 Accphoto 兄之幫助. 同我一起努力了最少十小時.

終於可以坐下來真正的聽吓歌 ... 聲都唔錯 ... 可能好似燒烤時第一條腸一樣, 永遠都是好味的 ...  :

CAT

Dear Accphoto,

The LEFT channel Jensen is abnormally HOT (around 55C) whereas the RIGHT channel Jensen is okay (30C).

I removed the original 0.1nf / 1.2KV small capacitor. Will this cause the trouble?

CAT

Which one is left ? Which one is right ? You need to tell me the one with 27K or the one without 27K ? I just check the circuit, I think you a make mistake, the second capacitor after the power supply, should have a 200K resistor that connected to the +Ve and -Ve , not 27K. If it is a 27K, the resistor is quite hot and will burn if run for  a while.

Sorry Accphoto, my mistake.

It should be 270K (Red-Purple-Yellow). This is the RIGHT Channel one, no problem at all.

The one with problem is the LEFT channel. The one with just the 10K? 2W you recommended to act as the choke. There should be another 0.1nf / 12KV small Red Round capacitor there (around the size of 3 $10 coins) but I removed this Red Capacitor. Could it be the source of the trouble?

ALF

See attached photo please

Dear Accphoto,

I turned on the Amp and let the tubes warmed up for around 2 mins and then I took the reading.

For the LEFT one (which gets hot), the reading is 324V DC. The one on the Right is around 310V DC. Both below the rating of the Jensen stated 500V.

I will keep an eye on the LEFT one today.

Thank you.

CAT

For this voltage, i do not see there will be any heat generated frim the capacitor, check the metal resistor, it may affected the temperature of the cap.

Dear Accphoto,

What should I check with the resistor? The temperature of both resistor seems be to quite normal ~ 40C when used for > 15 mins. The temp did not go up afterwards.

I double checked the wiring, no problem found. I put back the RED capacitor, problem still exists. Capacitor (LEFT) channel getting hot after 5 mins of use ...

BTW, I got a Tektronix OSC ... ha..ha

CAT

Dear Accphoto,

One important discovery. May not be too good a piece of news.

I realised that the LEFT Channel RMG resistor is sort of "burned". The middle part of the resistor had turned brownish.

Could this be the cause of the overheating of the LEFT channel Jensen Cap? I didn't realise this yesterday as there was not sufficient light. Tonight, I put the amp back on my bench and I can spot this right away.

Is it possible that the current there exceeds 2W? As the resistors (2 x 1K) are both 5W.

CAT

Dear Accphoto,

I de-soldered the transistor out. There is indeed a crack and some powder like substance sprayed onto the Jensen as well.

After re-soldering everything, I put the amp back to the test. The "noise" (ng... ng...) seems to be a bit higher than before. The Jensen now remains cool as normal.

Do you think I should go for a 100Ohm 7W or so?

CAT

May be you can do that : Use 2 x 100ohms and in parallel, it will become 50 ohms 4 W , it will not so hot but the filtering effect will not as good as 100 ohms. Alternative you can try 100W 7W RS resistor or 2 X 200 ohms in parallel.

I don't have time to go over to Ah Yuen's place today. Instead I got a 100Ohm 10W clay from Wo Hing in Wan Chai.

I removed the connecting wire and then solder the resistor in. I don't understand why the Jensen will get HOT whenever I put in the resistor... I have thought of the possibility of heat dispersion ....

Still got the "ng ... ng..." noise. Much more serious than when I use the big 1200uf caps. I will hock it up to the scope later and see if the power supply is in a smooth wave form.

CAT

Dear Accphoto,

No wonder the Jensen gets hot.

The 10W 100Ohm resistor was SO hot that it almost (well, almost) burn my hand. I think the heat dispensed to the Jensen. It is not the Jensen that gets hot.

I think I will get two 100Ohm 25W metal one and mount them on the case instead of using the clay one. Jensen won't live long under HOT temperature.

CAT

Based on my calculation, Say 100 ohms and current is 280ma ( Max 70ma per KT88). The voltage drops will be 100 x .28 = 28V. Given W=VI So the watt require is only 7.84W, so a 10Watt should be good enough. Can you measure the voltage before the resistor and after the resistor to see what is the problem. You mention that there is some "Ng Ng" with the resistor,  Should it be "Wu Wu" sound? If that was the case, it means the filtering without the resistor is not eoungh. There is another way you shoud try is to increase the filtering of the first stage. At the moment , we use 100u+100ohms+100h  fitlering, you should try to use 200U+100ohms+100u, In this case, you short two + on capcaitor and connect the 100 ohms between two jenesen and move the 1K to the second jenesen caps. Three is no need to put on the 100 ohms for the second caps.
I think you better use 50 ohms , the filtering effect is ok but the voltage will not be that great.
The quality of the power transformer is not so good as it indicates that its internal resistance is very great.

Dear Accphoto,

The voltage difference before and after the 100Ohm is 20V DC. So a 4W should be okay, right?

Well, I think it is what we called the "sound of electricity" that exists in all tube amps. With or Without the resistor modification, the sound is larger than my ASL 6L6. But my other CARY KT88 has much lower noise of this. I think you can call it the "wu .. wu ... " sound. The noise is serious when I turn on the amp (COLD). But after playing for an hour or so, the noise seems to be much much lower. Will try it out again tonight.

I will try to draw out what you proposed to me later for you to check check first.

The output transformer is okay, just warm (Both).

Attached are the photos I took from the OSC this afternoon.

CAT

This is what we called 交流聲，mostly like ly come from the rectifiying tube and the construction the wire. Make sure all AC ( filament) wire is close to the case and make sure the grounding is properly, this will reduce 交流聲.
For the reduction of the value of the capacitor, it should not be that matter as the capacitor value for rectifying tube is normally 100-200u at most.

Dear Accphoto,

Too busy these days; setting exam papers, marking, etc.

Have to put aside Hi Fi for a week ... :cry:

CAT

Dear Accphoto,

Managed to squeeze some time today.

I have the layout attached. You mean I should arrange the resistors and the caps like this?

The sound is more like "ng ...ng..." to me rather than "wu ... wu...". Unfortunately I have nothing to do recording.

The 100Ohm 10W resistor is SO HOT that I think the temp exceeded 60C. I can't even touch it. Is it normal?

CAT

I can't attach a .tif file here. I will send to your email instead.

CAT

Dear Accphoto,

Finally got back from China and start my whole busy week.

I pulled out all the KT88 today and removed the 100Ohm 10W cement resistor. The DC Voltage after the rectifying tube is 418V.

Replaced the 10W cement resistor with a DALE 25W wire wound resistor. I think the resistor won't get HOT if I don't put the KT88s in. Now, I mount the DALE on the chasis next to the other two DALE metal case resistors. The Jensen no longer gets hot and the 3 resistors are all at their acceptable temperature.

I hope this is the last modification I need to do with the KT88, ha..ha..

CAT

Hope this is indeed the FINAL ....

If the voltage after rectifying tube before loading is 418V, I think you need to pay attention to the position of the 100ohms resistor. You must connect the capacictor  first before the resistor otherwise the voltage with loading should not drop to 320V, It means that the voltage drop is almost 100V which is too high for a 100 ohms resistor.

Make sure the voltage from the rectifying tubes goesto the capacitor first and then the 100 ohms resistor.

Dear Accphoto,

I just replaced the 10W cement one with this one. The connection method is identical.

1. WIRE OUT from rectifying tube to ONE + terminal of Jensen.
2. WIRE OUT from the SAME + terminal to DALE
3. WIRE OUT from the other end of DALE to SECOND + side of Jensen
4. WIRE OUT from Jensen Second + to Right Channel Jensen FIRST + pole

Is that okay?

CAT

ok, should try this first......

Dear Accphoto,

Tried it out already. NO major problem encountered. Jensen just warm. Transformers at 4x C after using for 2 hours. Chasis a bit hot at the rear, not surprising as there are a total of THREE metal resistors.

I think the 100uf + 100Ohm + 100uf is already pretty good in doing the rectification as the power wave form is quite smooth.

CAT

Nice to hear that.... enjoy your music.....

Dear Accphoto,

You have all the credit for this. Without you, I won't be able to use this CARY KT88 at all. It would have ended up in the Garbage already.

Many thanks.

CAT

放電電阻都用RMG?!

當時買開 RMG 電阻, 唔想又走去第二度買過 ....

CAT Sir,

你是否拿部 VT60 去玩?

Dear Accphoto,

嘩, 部 VT-60 都幾毒 .... 正想方法去毒其他人 ...

CAT

[quote:112ce4c488="alfcat"]Hope this is indeed the FINAL ....[/quote]

管子好拥挤，怕对管子寿命不利

"管子好拥挤，怕对管子寿命不利"

對. 每一次聽歌我都會開空調. 溫度在25c 左右. 有風從管子中間吹過 ....

部 VT60 效果如何？

MO DUCK DING