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Old 2nd April 2011, 09:59 PM   #1
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Default KT88 doubt

I'm designing a circlotron amplifier with output transformer using a pair of KT88 valves.

I need a signal + bias current of 300mA in each valve to produce 50W (RMS).

The Genalex datasheet (http://www.drtube.com/datasheets/kt88-mov74.pdf) states a maximum value for cathode current of 230mA which i believe to be the maximum quiescent current for this valve. Is this correct? Is it possible to have a 300mA signal current in a KT88 without damaging the device?

The Svetlana datasheet (http://www.drtube.com/datasheets/kt88-sed2001.pdf) states that "Cathode current, DC, at idle" is 235mA.

Thank you for your time.
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Old 2nd April 2011, 10:17 PM   #2
SY is offline SY  United States
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To get 300mA, you'll have to parallel two to four KT88 per side.
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Old 2nd April 2011, 10:49 PM   #3
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Quote:
Originally Posted by tidalweapon View Post
I need a signal + bias current of 300mA in each valve to produce 50W (RMS).
Not quite sure how bias current through a transformer primary is going to transfer power to a load on the secondary. Signal current is all that matters here.
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Old 2nd April 2011, 10:52 PM   #4
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Quote:
Originally Posted by SpreadSpectrum View Post
Not quite sure how bias current through a transformer primary is going to transfer power to a load on the secondary. Signal current is all that matters here.
That's not the idea. What I wanted to say with that is that bias current also accounts in the maximum possible current from that valve. To that signal current is summed.
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Old 3rd April 2011, 03:03 PM   #5
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I am assuming you are going for class AB. KT88 should be fine with 400+mA on peaks. Just keep the average below 230mA. (you would have to have a pretty low plate voltage to run at this current)

If the maximum total current were limited to that, almost every class AB KT88 amp would be in violation of this limit.
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Old 4th April 2011, 03:00 AM   #6
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Quote:
Originally Posted by SpreadSpectrum View Post
I am assuming you are going for class AB. KT88 should be fine with 400+mA on peaks. Just keep the average below 230mA. (you would have to have a pretty low plate voltage to run at this current)

If the maximum total current were limited to that, almost every class AB KT88 amp would be in violation of this limit.
Yes. Class AB. I was hoping for that answer. Thanks for your help.
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Old 17th April 2011, 03:09 PM   #7
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Are the absolute maximum ratings stated on the datasheet for anode power dissipation really the maximum value or may I assume they are RMS values?
I used this datasheet: http://www.drtube.com/datasheets/kt88-mov74.pdf
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Old 17th April 2011, 03:49 PM   #8
DF96 is offline DF96  England
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No such thing as RMS power. The maximum power rating will be for average power, averaged over a short time period. How long is short? Depends on the size of the anode, but I would guess a second or two at the most. Brief music peaks can exceed the power rating, provided the valve can cool down between them. However, pushing a valve to its limits will shorten its life anyway - they don't go bang if you abuse them, they just fade away more quickly.
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