Genalex KT88 Plate Dissipation Question - diyAudio
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Old 27th April 2011, 09:40 PM   #1
Loren42 is offline Loren42  United States
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Default Genalex KT88 Plate Dissipation Question

I just received a new set of Genalex Gold Lion KT88s.

I am looking for the recommended plate dissipation for these tubes. Current B+ is about 505 to 510 VDC.

The cathode has a 10.16Ω*resistor and I measure the voltage across that resistor. That voltage is now set to 0.567 VDC.

I calculate the plate current as follows:

P (watts) = (505 VDC * .567 VDC) / 10.16Ω = 28 Watts

So I think that seems like a good target, but wanted to hear form others using this tube where they recommend to set them.
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Old 27th April 2011, 10:28 PM   #2
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Your amps output configuration will play a part in the idle current setting, so if you tell us, I'm sure you'll get a recommendation.

Appears to be fixed bias, class AB1 maybe? Tetrode or Ultra Linear?

jeff
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Old 27th April 2011, 10:30 PM   #3
DF96 is offline DF96  England
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What does the amplifier designer recommend? Too high and too low current can both cause distortion, although too high also shortens valve life.
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Old 27th April 2011, 10:34 PM   #4
Loren42 is offline Loren42  United States
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The amp is a home brew Williamson design using Edcore output transformers in ultralinear configuration.

Bias is fixed bias. I'll see if I can post a schematic.

I was using Ruby tubes, but one of them malfunctioned and its current dropped to zero. So, I had an excuse to order some Gold Lions and they arrived this afternoon and they are now burning in with the plate dissipation set to 28 Watts. ;-)

Last edited by Loren42; 27th April 2011 at 10:44 PM.
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Old 27th April 2011, 10:40 PM   #5
Loren42 is offline Loren42  United States
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Here is the schematic:

Click the image to open in full size.
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Old 28th April 2011, 12:50 AM   #6
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Your operating plate voltage would be a little down from 505V due to OT primary resistive drop. Your operating anode current would be a little down on cathode current due to idle screen current. They may reduce your actual plate dissipation by about 5-10% from what you initially calculated.
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Old 28th April 2011, 01:00 AM   #7
Loren42 is offline Loren42  United States
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Quote:
Originally Posted by trobbins View Post
Your operating plate voltage would be a little down from 505V due to OT primary resistive drop. Your operating anode current would be a little down on cathode current due to idle screen current. They may reduce your actual plate dissipation by about 5-10% from what you initially calculated.
Thanks! Yeah, that makes sense. I just wanted to get a feel if others are in agreement with my target dissipation.

I should measure the actual voltage on the plates when I get a chance, but I think it is very close to the 505 VDC figure.

Genalex rates these at 35 Watts maximum. I don't know what the new version of these tubes really are rated for, but 28 should be a safe bet, I would think.
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Old 28th April 2011, 10:05 AM   #8
DF96 is offline DF96  England
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With fixed bias I think it is generally best to run valves cooler than when using cathode resistor bias. Two reasons for this:
1. there is no automatic protection against thermal runaway, so best to be cautious
2. there is no bias shift with signal so you don't have to allow for this by running hot when quiescent
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