Mikael‑Abdellah‑SE‑KT88 Bias & Voltage readings Help? - diyAudio
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Old 22nd April 2012, 01:45 PM   #1
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Default Mikael‑Abdellah‑SE‑KT88 Bias & Voltage readings Help?

I just finished wiring up this SE KT88 amp
the only difference I used a 560 ohm cathode resistor
across pin 1 & 8 of KT88 socket.
was getting ready to try her out for the first time
and need to know where to check voltages and
what voltage readings should I be getting at critical spots.

Scott
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Old 22nd April 2012, 01:48 PM   #2
rongon is offline rongon  United States
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Is there a schematic you were using to work from? Could you post it here so others can see it? That way it would be much easier to see what you are referring to.

It's kind of hard to understand why a resistor would be placed from pin 1 to pin 8 on a KT88. Usually pin 1 and pin 8 are shorted together, and then 560 ohms might be the cathode load resistor to ground. A schematic should make all that easier to understand.

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Old 22nd April 2012, 02:14 PM   #3
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You are correct 560 ohm resistor goes to pin 1 & 8 and feeds ground.
here is the schematic.

http://diyaudioprojects.com/Tubes/KT...-Amplifier.png
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Old 22nd April 2012, 02:48 PM   #4
rongon is offline rongon  United States
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First check the voltages in red in the schematic. Do yours match the schematic's? If they do, then you're probably in business. It looks like the 6N1P should be drawing 4mA per triode, and each KT88 should be drawing 70mA.

You could also measure the voltages at the plates of the 6N1P. That would be pins 1 and 6 (if I'm not mistaken). The voltages there should be 188V lower than the voltage measured at the other side of the 47k resistor, coming off the 10uF polypro cap.

Do you have a schematic for the power supply?
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Old 22nd April 2012, 04:00 PM   #5
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Thanks for the input
I have no schematic for the power supply.
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Old 22nd April 2012, 10:45 PM   #6
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Power supply.
DIY Single-Ended KT88 / 6L6 / EL34 / 6CA7 Tube Amplifier
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Old 23rd April 2012, 01:59 AM   #7
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Basically that power supply with diode bridge rectifier.
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Old 23rd April 2012, 02:01 AM   #8
maxro is offline maxro  Canada
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Quote:
Originally Posted by rongon View Post
Usually pin 1 and pin 8 are shorted together, and then 560 ohms might be the cathode load resistor to ground.
Pin 1 of the KT88 is connected to the exposed metal base. Why should it be at cathode potential rather than ground?
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Old 23rd April 2012, 12:37 PM   #9
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Are you saying pin 1 would just go to ground and not
connect to pin 8.
I thought 1 & 8 should always tie together.

Quote:
Originally Posted by maxro View Post
Pin 1 of the KT88 is connected to the exposed metal base. Why should it be at cathode potential rather than ground?

Last edited by bigaudioscotto; 23rd April 2012 at 01:00 PM.
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Old 23rd April 2012, 01:16 PM   #10
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Pin 1 is unused on the KT88 & the 6550. It does not need to be connected. If you want to run EL34/6CA7 in your amp though, pin 1 & 8 need to be connected. Take a look at the data sheets for all of the tubes and you will see.
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