Mikael‑Abdellah‑SE‑KT88 Bias & Voltage readings Help?

bigaudioscotto · 22nd April 2012, 01:45 PM

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

rongon · 22nd April 2012, 01:48 PM

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.

--

bigaudioscotto · 22nd April 2012, 02:14 PM

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

rongon · 22nd April 2012, 02:48 PM

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?
--

bigaudioscotto · 22nd April 2012, 04:00 PM

Thanks for the input
I have no schematic for the power supply.

skippyboy · 22nd April 2012, 10:45 PM

Power supply.
DIY Single-Ended KT88 / 6L6 / EL34 / 6CA7 Tube Amplifier

bigaudioscotto · 23rd April 2012, 01:59 AM

Basically that power supply with diode bridge rectifier.

maxro · 23rd April 2012, 02:01 AM

Quote:

Originally Posted by rongon

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?

bigaudioscotto · 01:00 PM

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

Pin 1 of the KT88 is connected to the exposed metal base. Why should it be at cathode potential rather than ground?

scott17 · 23rd April 2012, 01:16 PM

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.

22nd April 2012, 01:45 PM	#1
bigaudioscotto diyAudio Member Join Date: Sep 2004 Location: North Carolina,	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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22nd April 2012, 01:48 PM	#2
rongon diyAudio Member Join Date: Jul 2009 Location: Across the river from Rip's big old tree...	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. --

22nd April 2012, 02:14 PM	#3
bigaudioscotto diyAudio Member Join Date: Sep 2004 Location: North Carolina,	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

22nd April 2012, 02:48 PM	#4
rongon diyAudio Member Join Date: Jul 2009 Location: Across the river from Rip's big old tree...	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? --

22nd April 2012, 04:00 PM	#5
bigaudioscotto diyAudio Member Join Date: Sep 2004 Location: North Carolina,	Thanks for the input I have no schematic for the power supply.

22nd April 2012, 10:45 PM	#6
skippyboy diyAudio Member Join Date: Jun 2008 Location: nsw	Power supply. DIY Single-Ended KT88 / 6L6 / EL34 / 6CA7 Tube Amplifier

23rd April 2012, 01:59 AM	#7
bigaudioscotto diyAudio Member Join Date: Sep 2004 Location: North Carolina,	Basically that power supply with diode bridge rectifier.

23rd April 2012, 01:16 PM	#10
scott17 diyAudio Member Join Date: Aug 2009 Location: JFK	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.