KT88 and push-pull ultra-linear
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 22nd July 2006, 08:51 PM #1 rossdt   diyAudio Member   Join Date: Jul 2006 KT88 and push-pull ultra-linear What do you think about this quiescent point for KT88 in push-pull ultra-linear operation? Values for TWO tubes - Class A DC Plate Voltage = 395 V DC Screen Voltage = 395 V Cathode Bias Resistor = 200 ohms Peak AF Grid-to-Grid Voltage = 70 V Zero-Signal DC Plate Current = 170 mA Zero-Signal DC Screen Current = 12,5 mA Effective Load Resistance, Plate-to-Plate = 5600 ohms Screen Tapped at 40% I use Cathode Bias without capacitor Better ideas? Thanks.
 23rd July 2006, 11:39 PM #2 gingertube   diyAudio Member   Join Date: Oct 2004 Location: Adelaide South Oz What I think is that with the values you describe you will get about 30W in Class AB (not Class A) and that you should use separate 400 Ohm resistors on the cathode of each KT88, rather than a common 200 Ohm cathode resistor. All the data sheets and application notes are quite clear on this point. Cheers, Ian
 24th July 2006, 09:28 AM #3 rossdt   diyAudio Member   Join Date: Jul 2006 Really? Then, how do I do to work in class A? Which quiescent point? Thanks.
 24th July 2006, 11:27 AM #4 jane   diyAudio Member     Join Date: Jul 2004 Location: N-6600 Norway 2 x 100mA into a 5600 ohms p-p load will give you ~30W class A. Check the plate dissipation; I'll suppose 100mA will bring you close to the limit. PmaxA = [(Iq x 2)^2 x Rpp/4] / 2 __________________ Life is hard - Then you die.
 25th July 2006, 04:54 AM #5 pengboon   diyAudio Member   Join Date: Dec 2005 Location: Singapore Since the topic of cathode resistors is brought up, why do some designs share the resistor between 2, sometimes even 4 tubes?

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