[Amps] Pi Net Switch

[Bill, W6WRT]

ORIGINAL MESSAGE:

On Fri, 15 Oct 2010 09:26:41 -0700 (PDT), Lee Buller <k0wa at swbell.net>
wrote:

>
>Some builders say a "shorting" band switch is better that an "non-shorting" band 
>switch.  My question is why? 

REPLY:

If you have a single coil with taps for each band, the shorting type of
switch will reduce or prevent the Tesla coil effect by shorting out
unused sections of the coil. 

If you have separate coils for each band, and they have little or no
magnetic coupling to each other, the non-shorting switch will work fine.
This is the approach I used in my homebrew amp. 

Use caution with the shorting type if you have an iron-core toroid as
part of the tank circuit. Shorting out turns on an air-core coil causes
little or no loss, but shorting turns on an iron-core toroid can lead to
severe overheating of the core. It can be made to work but it requires
some careful design and testing. 

IMO, if you have the room, an all air-core design is more reliable. I
have personally had two iron-core toroids in a commercial amp burn up
during extended RTTY contesting. Air-core coils with properly sized wire
are nearly indestructible. 

73, Bill W6WRT