[AMPS] amps

Tom Rauch W8JI@contesting.com
Fri, 9 Feb 2001 19:55:52 -0500

Hi carl
> in the use of a shorting or non-shorting switch in the pi-network
> output of an amp?? Are there any real advantages one way or the
> other?? always wondered about it!!                   carl / kz5ca

These switches are called the wrong names all the time. 

A "shorting switch" picks up the next contact before releasing the 
one it was just on. It has less contact-to-contact holdoff voltage, 
and is a bad choice for this application.

A shorting switch is normally used when switching isolated loads, 
where the moving contact (rotor) always has to be connected to 
something while rotating to prevent a failure of some type.   


A "progressively-shorting" switch, or "pick-up-and-hold" switch 
progressively picks up contacts when rotated, and is a good switch 
to have. 

They can be ordered with either shorting or non-shorting type 

A "non-shorting" switch rotor releases the stator contact fully 
before hitting the next stator contact. They generally have much 
higher contact-to-contact voltage ratings.  

They are used where the rotor has to fully drop a contact before 
connecting to the next one. They are often used to switch multiple 
power sources that can not be cross-connected.


The best switch for amplifier tanks is a non-shorting contact 
configuration using a pick-up-and-hold rotor.

The reason you need the pick-up-and-hold rotor is because on 
higher bands the tank can have a self-resonance where it looks like 
two or four back-to-back L networks. If the voltage at the center of 
the L-networks formed by the stray capacitance and unshorted 
areas of inductor becomes high enough, the switch or other 
components will arc over and fail.

The same problem occurs with *some* roller inductors, when a 
wide frequency range is covered, unless they have some form of 
additional shorting other than a single moving tap.


73, Tom W8JI

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