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Svetlana 26th July 2006 07:41 AM

Help with a cathode coupled pentode driver?
2 Attachment(s)
Will this work?
Will I see ~-90 V at the grid of the 211?

How will I bias the 5881? By varing the G#2 voltage?

SY 26th July 2006 11:50 AM

What's the point of the plate choke? How do you intend to decouple the screen to the cathode? There are a lot of ways of getting the bias right- a CCS in the cathode instead of the choke might be the easiest. And a CCS will cost about 5% of a good choke.

Tubelab_com 26th July 2006 01:06 PM


Will I see ~-90 V at the grid of the 211?
No you won't. The DC resistance of your cathode choke is low. You will see -230 volts + the IR drop across the choke which depends on the tube current and the DC resistance of the choke. With the tube bias fixed at -90 volts the tube current will be very high, and something will have a short unhappy life. Either the 5881 or the choke.

Substitute a CCS or a resistor for the cathode choke. You need the ability to drop about 130 volts. The plate choke is not needed (but might not hurt anything). The screen should be decoupled to the cathode as SY suggested. You need a capacitor from the screen to the cathode.

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