醜女大返身之 -- KT88 (Part II)

Just a few comments and hope you do not mind , I see one of the auricap's connectior had penetrated though several tube's connector , it should be avoid as this will easy pick up noise. The turn pot (VR) should fact towards the bottom plate  so that it is more easy for you to adjust. I am a little bit worry for you to "float" the turnpot as you will find that it will be difficult to adjust the turnpot. I am normally using those normal volume and put the shaft through the top plate so that you can adjust without open the machine. lotherwise, it is a good work.

If you cannot find a 30k pot, , you can better use a 25K + 5K resistor , this will not change the overall value but the adjustment range is smaller.

Based on the data as per attached , when KT88 work in ultralinaear mode at about 450V, the negative bias should be -59V, you should adjust the pot until the grid voltage of the KT88 to -59v before you plug the tube in, otherwise, you will blow the KT88 easily.

http://tdsl.duncanamps.com/show.php?des=kt88

"pak Pak noise" mans some shorting or the tube has problem. Check the capacitor and connection on wire......

Did you swap the transformer again ? if the sound go to right channel this time, then it will be definitely  the transformer problem. Look like that you really need a scope to examine the problem.....

If you change most of the component, and measure the voltage on the key point are the same on both channel, then it will be the transformer problem......

You should measure the voltage across the 25W resistor or the + and - of the 220u 100V, you should got a voltage, Normally between 50V-60V ( 500 ohms x 100- 120 ma for both KT88), if both channel within  5%, it should be ok. It will not affect the filament voltage.

Since you have one channel working now, it is easy, just compare all the major voltage of each point to see there is a big difference. Most likely some connection or component problem. If possible, get a scope and found the sound of "buu" sound. It is possible the 30K resistor had changed value as the machine is already +10 year. It is quite easy to found out by measure the actual value of the working channel and compare to the non-working channel, of course you need to soldier it out first.

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.  

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.

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......

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

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.

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

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

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.

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.

Problem fixed........  

[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.

No worry, can adjust it again.

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.

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.....

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.

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....

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.

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