Sssseee noise in KT88 Amp

Hi,

朋友部機開得3-4個鍾就會有 sssss 聲. 請問各位會是什麼問題所在? 已換了電容, 但沒有用.

如果 3-4小時才有，9成是膽的問題。

Dear Accphoto,

已試過換晒新胆, 但都無用. Output 牛也十分熱 ....

Cat,
is the shs sound hear from 1 speaker or both speaker?

check your soider point (connection point) of the component u had chnage. Sometimes if not soldering good (dry solder) when temperature increase then the bad solder point may have bad contact (expansion of metal) n can cause the noise too....

ss

Dear WINDWSS,

The KT88 Amp has been used for more than 3 years. All the way, it has NO problem at all. Then starting last year, it starts to give off this annoying "sssssseee" noise after using for > 3 hours. The noise is coming out from BOTH speakers. I asked some people in Hi Fi store and they said it could be the Caps.

Therefore, we changed the Caps to Jensens with the same value. However, the problem still exists.

The Amp has NOT been modified until we replace the Jensens Caps.

CAT

oh i see... then i also don't know already...

But i will check the grounding point, for screw type sometimes they may oxidize...
or the old solder is internally crack but outside can't see....
too many possibility....

sorry my knowledge in this is too little can't really helps : (

normally film cap seldom give problem.....

Dear WINDWSS,

I will ask my friend to take it to me later when we have more spare time. These three months (Sept to Nov) are the worst months we have each and every year 'cos there are overwhelming amount of work to be done and no long holiday.

I still haven't quite look into my CD 33 case as well. Just let it run for 1 hour or so each day; helping me to relax. As long as it can still play and produce reasonably good music, I just leave it as is.

Thanks for the advice anyway.

Cat,

Can you take a few pictures of this amplifier. When was the last time you changes all the tubes? Also, give the amp a few hours after shut down, and make sure all caps are drained, and then check your resistors for both channels with your meter to make sure they are still within tolerance. Typically if both channels share a sound, it is in your power supply. You should have a particular point at thich the PS separates into two legs (one for each channel). This is where you should stop looking because after this point each channel is independant. Check all resistor, and caps in the PS before that point.

Not to sound critical, but you mentioned replacing caps with Jensens. If you replaced the PS caps with Jensens, I need your job

Typically they are well above 50uf, and very high voltage. Please take your time and be careful while under your amp. These voltages can easily send you to the hospital if not the grave :shock:

I hope this info helps in your conquest.

Blair

Dear Blair,

Sorry that you missed the Chinese part at the very top. I should have typed in English as well.

The KT88 belongs to my colleague who lent me the Matisee Pre-Amp, the Musical Fidelity X-Pre, the Chord Power Amp and many other stuffs I can't mention here... ha..ha..

I will see if I can pick it up from his place later as we teachers tend to be VERY busy these few months. Will try to get the Amp from him and take some photos. Can't recall what brand it is but should be a good brand as well.

He told me he changed all the tubes (he got plenty of every model) and the Jensens as well. Not sure if he is talking about the PS ones or something else. He said the Output Transformer is HOT as well ... no idea why he told me that.

Thanks for the time being. Will pass your message onto him tomorrow and see if I can do the arrangement.

CAT

I know you are not a techie or anything, but do you mean output transformer(s) instead of transformer? If it is stereo, then it has two ouput transformers and typically one Power transformer.

It is very typical for a power transformer to get extremely warm. Almost ot the point where you do not want to touch it at all. Do not worry about that, but if the output transformers are getting hot, there is an issue. My 60wpc amplifier can run at moderate / high volumes for about 5 hours and still not get the OPTs hot. The PT on the other hand will get pretty warm. Get that amplifier and lets take a look at it. Then we will make a plan of action.

Blair

Dear Blair,

I need to do some clarifications. I double checked with my colleague today and he said he has been using the KT88 amp for 10 years ... not three.

Second, the sssseee noise comes from ONE channel only, not both. There is a 70% chance to hear this sound after playing > 3 hours.

Third, it is the OUTPUT transformer of that channel which gets really hot, so hot that he would consider abnormal when compared with the other channel.

I will see if I can drive to his place to pick it up. Should weight over 10 kgs and he hasn't got a car. If I ask him to take it back to the office for me it will be a terrible thing. Even driving to his place takes 40 mins at 100km/ hr (i.e. 60 miles / hour).

CAT

Cat,

When you get the amp, let me know and we will do our best to troubleshoot it from across the world

Blair

Right Blair, that will be one interesting project across the globe .. ha.ha.. :wink:

Cat Sir,

You may consider the following:-

First of all, test all the power tubes(KT88) if you have a tube tester.  Or move the power tubes (Push Pull?) in Channel L to Channel R and those in R to L.  Yes, I mean "swop".  If the OPT Transformer still gets hot, the problem should not be with power tubes.  OPT transformer gets hot because too much current passing through.  Does the plate of the power tube turn to red (burn) gradually?  Should this  be the case, you have to check the value of the Cathode Resistor(s) if it is self-biasing designed or the negative voltage supplied.  Another factor causing the plate current increased would be with the coupling-cap, the one between driver tube and power tube.  The main function of a coupling-cap is to block d.c.  When leakage occurs, B+ at the driver stage would go through it into the power tube making the current increased.  So, check that capacitor by using a capacitor checker.  If not, "swop" again with the one in another channel.  In case the plate of power tube does not turn red and the plate voltage of all the power tubes is normal, then swop the OPT transformers.  By then, you should know if the problem is with the transformer.

Dear cheaplow,

My colleague had tried swapping the tubes already. He even replaced them with NEW ones.

With my limited knowledge, I agree with you that the OPT getting hot could be a problem arised from a current issue.

Second, I think my colleague also changed the coupling caps to new Jensens as well. This should be the next step he took after changing the KT88 tubes.

I haven't got a chance to look at his amp yet as we are all terribly busy in these two months. We won't be able to get our breath back until we have half yearly exam in mid Dec. By then, we will open the whole thing up for photos and alike and see what ideas you guys will come up with.

CAT