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Old 15th January 2009, 11:22 AM   #1
OaG_sGm is offline OaG_sGm  Australia
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Default Operating Temperature of KT88?

Hi Guys,

The tubes get very hot even with very low Volume?

I wonder how hot do they get?



I looked up in some data sheet it says the glass tube can get to 250 degrees!?



My tubes is around 80 degrees now with only 1/6 of the full volume. Is this normal£¿
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Old 15th January 2009, 12:10 PM   #2
SY is offline SY  United States
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80 degrees is quite cool. Good work in setting up the ventilation!
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Old 15th January 2009, 02:54 PM   #3
OaG_sGm is offline OaG_sGm  Australia
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So I can turn up the volume and it will not blow up?...
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Old 15th January 2009, 06:19 PM   #4
kmaier is offline kmaier  United States
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If you examine the spec sheet carefully, you'll find specs for the heater and typical operating modes. Depending on how old your spec sheet, filament current could be 1.8amps or 1.6amps... at 6.3VAC, so you have ~11 watts of heater dissipation right away. You've not given a single detail on your amplifier so it's almost impossible to give you any meaningful input. Is it a SE class-A or push-pull, if push-pull is it cathode bias of fixed bias.. and in either case, what is idle current? Looking at an very old example, the Dyna MK-III ran 140ma of current thru a pair of KT88 tubes... pretty much max operating spec and they ran really hot but that was common back in the 50's and 60's. At max ratings you can dissipate over 50 watts between the heater and plate per tube... and that's with "real" KT-88 tubes, not the new bottles they label as such. Again, without knowing a single detail of your amplifier any input would be pure speculation. As Sy pointed out, ventilation is key here... how about some pics of the amp to see how well the ventilation is managed?

Regards, KM
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Old 16th January 2009, 01:56 PM   #5
OaG_sGm is offline OaG_sGm  Australia
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Thanks a lot mate.

My amp is a pretty cheap Yaqin 100b...bought from the ebay.

I am loving the sound now. Much better than I thought.

It push and pull Class AB.

I think the idle current should not be small. The tube getting pretty hot even with no INPUT and minimum volume.

The circuit diagram is in one of the post in this forum.



Chinese KT88 tube, I dunt know how good they are. Some say the tubes have no filament?...is this possible?

Never mind, i feel better to turn up the volume now.
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Old 16th January 2009, 02:40 PM   #6
kmaier is offline kmaier  United States
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They won't work without filaments.... no worries, crank it up.

Regards, KM
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