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Old 8th January 2012, 08:02 PM   #1
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Default Direct Coupled Amp

Can I use fixed bias on a direct coupled KT88 single ended amp i'm working on and use an anode load resistor to drop the voltage for the input stage?
What sort of negative voltage am i looking for to bias the KT's?
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Old 8th January 2012, 09:05 PM   #2
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Quote:
Originally Posted by kianbach View Post
Can I use fixed bias on a direct coupled KT88 single ended amp i'm working on and use an anode load resistor to drop the voltage for the input stage?
What sort of negative voltage am i looking for to bias the KT's?
I would be extremely wary of this approach. At least with cathode bias the KT88 is less prone to run away, though the drifting driver still makes this a possibility. With fixed bias the driver will drift as well as the Output - and if they both drift in the wrong way your KT88 will run away and melt.

A solution to this would be to use a CCS as your bias element on the KT88, this way it will never melt and will stay in correct adjustment whatever the driver does. However you need a large cathode bypass cap for this to work.

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