[HomeBrew] Plate Choke

[N2EY at aol.com]

In a message dated 5/16/07 12:13:18 PM Eastern Daylight Time, 
km1h at jeremy.mv.com writes:


> Alpha used the B&W in the 77 series and B&W used the 802 in their 
> own 1500W amp. Heath used 50uh on their SB series of 80-10M amps.

I think what they're doing is making the choke part of the tuned circuit.

Let's say we have a 50 uH plate choke and a pi-net that calls for the tuning 
capacitor to be 200 uuF on 80 meters (3.5 MHz) *IF* the choke reactance is 
infinite.

If my math is right:

The reactance of the plate tuning cap is 227 ohms (capacitive)

The reactance of the RF choke is 1099 ohms (inductive)

The resultant reactance of the two in parallel is 283 ohms (capacitive).

So all you need to do to make it work is to add enough additional C to get 
back to 227 ohms, and you're all set.

Of course there's a limit to how far this trick can be taken. If the choke 
reactance is such that it significantly affects the tank circuit, that means a 
considerable amount of RF current will flow through the choke itself, and 
through the bypass C at the cold end. That means choke and cap have to be able to 
handle the RF without burning up or ruining the Q.

73 de Jim, N2EY


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