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Re: [Amps] IM distortion and such

To: "Colin Lamb" <>,"Joe Subich,W4TV" <>
Subject: Re: [Amps] IM distortion and such
From: "Tom W8JI" <>
Date: Fri, 30 Jun 2006 17:48:47 -0400
List-post: <>
> "Can an unloaded tank reach potentials higher than the 
> breakdown
> voltage of its Tune C?"
> Yes.  We are not dealing with dc, but waves.  These waves 
> do not necessarily where you expect them or want them to. 
> The peaks will vary depending upon the frequency and 
> length of components.  It might be easier to imagine 
> standing waves on antennas.  The same thing will apply in 
> tank circuits.

Yes, but not for that reason in HF amplifiers.

You'll find, if you carefully investigate component 
breakdown, voltage flashover point depends heavily on the 
shape of the conductors at different potentials. This is a 
very well known effect.

The same air variable can have a vastly different voltage 
breakdown depending on the micro-shape of edges of the 
plate, the amount of mesh, the humidity or air pressure, and 
so on. I spent a long time looking at switches and other 
components, and while I generally tried to pick an air 
variable that broke down slightly before the switch that 
often is not possible to maintain on all bands and with 
production variations.

This is why a spark gap is so useful, but the gap should 
NEVER be from anode to ground. That would dump the HV. The 
gap should be from the tank input to ground. This is a 
problem people have been fighting since RF transmitters were 
first produced. When the load is removed or changed in 
impedance the wrong way, or when the drive is suddenly 
increased, tank voltages soar until something puts the 
system back into equilibrium.

There isn't any magic involved.

73 Tom 

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