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

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bigaudioscotto

Member
Joined 2004
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

rongon

Member
Joined 2009
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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rongon

rongon

Member
Joined 2009
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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rongon

rongon

Member
Joined 2009
scitizen17 said:
http://frank.pocnet.net/sheets/049/6/6L6.pdf

I don't think I would run a 6L6 in that amp. The max plate voltage is 275V for the 6L6.
Click to expand...

6L6 is the old metal tube.

6L6G is the coke bottle glass version.

6L6GB is the straight glass tube with higher ratings. 19W max plate dissipation, something like 360V max plate volts.

6L6GC is the bigger straight glass tube with 30W max plate dissipation and higher max voltage ratings. You can probably use that in your amp with no harm done. The GE datasheet states max plate voltage is 450V for triode operation, 500V for pentode.

Since your heater supply is designed for KT88, which draws 1.6A heater current, you might want to check your heater voltage when you have a 6L6GC in the amp. 6L6GC draws on 900mA heater current. If the heater voltage is much higher than about 6.6V, that is likely to decrease the life of the tube. Ideally the heater voltage should be between 6.0 and 6.3V AC.
 
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