Right Channel Noise in KT88
Dear friends,My friend got a KT88 power amp that gives off "ssss....sss" noise on the right channel. The funny thing is, the noise is random. Sometimes you hear it when you start it up, sometimes it appears when the machine is turned on for 1 hour or more.
What could have caused the problem? Swapped the tubes already, still no good.
AND the right channel OUTPUT transformer is really HOT compared with the LEFT one.
CAT 1. Output Tube problem....
if not
2. Check the Bias of the tube, if one of the transformer is not, then the bias is too high and the tube may end up have problem.The high bias may be one of the resistor that give bias to the Grid is not good.
Do you know how to measure the bias of the output tube ? Dear Accphoto,
My colleague used different tubes already (e.g. swap left right) and the problem is still consistent in the Right Channel. So probably it is not the tube.
I know how to measure BIAS for my 6L6 as there is a hole in the chasis. For this CARY KT-88, I have no idea how it can be done.
Should I post some photos here so that you can give me more advice?
Thanks in advance.
CAT Dear Accphoto,
Additional information. The KT-88 is a CARY SLA-70 Signature Model. My friend said it is AUTO Biasing.
CAT For auto bais , KT88 第八腳會接一支 數百 ohms 的電阻落地，流請check 是否這支電阻有事。亦有可能是接第四腳落地的電阻有問題。 Dear Accphoto,
I managed to shoot some photos with the KT88 before I dis-assembled it. Hope this can help.
The owner took the Amp to Apliu Street for repair but was unable to cure the problem.
CAT Look like that this is not auto bias, from the photo, it had a trimer which provide the grid with a negative voltage. It is better to draw the circuit and measure the voltage on the grid of the kt88 Dear Accphoto,
Is it because of the potentiometer you see in the photo?
I am re-building the whole thing. After that, I will consult you on what to measure at which point.
Thanks:)
CAT Yes, the pot should adjusting the negative votlage to the grip and also I see there is 1 6.8 ohms resistor for both KT88, if they are using auto bias, this resistor should be a few hundred ohms. This resistors are connected to Ground so it should be fixed bias. Dear Accphoto,
You are indeed an expert to these stuffs. Yes, you are right, there is ONE 6.8Ohm for both KT 88.
I am now re-painting the chasis. After that, I will put everything back together.
Please teach me HOW to adjust the BIAS and tune this thing back to normal. Thanks in advance.
CAT
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