[Amps] Plate choke and bypassing

[Jim Kearman]

The last time I built an amp I just stuck in an available R-175 choke. This time I'd like to roll my own. The amp will use a pair of 4CX800As to develop 1500 W with Ep about 2500 under load. It will be used primarily on 160-40, although the available bandswitch will permit 20 if I want to add it. So I'm not concerned about self-resonances above 14.5 MHz. 

I have a 1-inch (25.4 mm) ceramic core that will allow a winding length of 4.5 inches (114 mm). If I close-wind #24 wire I can get an inductance of about 255 uH. What interests me is that George Daughters' 4CX1600B amp in my ARRL Handbook uses a much smaller inductance, albeit with a higher Ep. This core is the right length for the physical layout so I'd like to use it. It would give me about 2900 ohms of impedance at 1.8 MHz. If I'm understanding the theory, that means an additional 30 pF of plate-tuning capacitance to compensate. My plate blocking cap will be 3300 pF. 

Another related question: Bypassing the cold end of the choke. RF Parts sells .001-uf, 6-kV disk ceramics at a reasonable price, especially compared to the same value in a doorknob. Would two or three of these be safe to use at the cold end of the plate choke? 

Thanks.

73,

Jim, KR1S