KT88 and push-pull ultra-linear

[rossdt]

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.

 

[gingertube]

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

 

[rossdt]

Really?
Then, how do I do to work in class A?
Which quiescent point?
Thanks.

 

[jane]

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

 

[pengboon]

Since the topic of cathode resistors is brought up, why do some designs share the resistor between 2, sometimes even 4 tubes?