醜女大返身之 -- KT88 (Part II)

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 Cat,

Thanks for info.
We learn something new.

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 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

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,

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

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 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

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,

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

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,

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

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