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[AMPS] Inductively Coupled VHF Parasitic Suppressor?

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Subject: [AMPS] Inductively Coupled VHF Parasitic Suppressor?
From: (Tom Rauch)
Date: Tue, 24 Jul 2001 15:56:30 -0400
Hi Todd,

>  I would like to run this by the AMPS group. Has anyone ever experimented
> with a VHF parasitic suppressor like the ones used in large-cavity
> enclosure High-Power HF amplifiers where the size of the enclosure could
> be self-resonant in the VHF range and the cure was to position a resistive
> loop in the enclosure to dampen the VHF resonance?

There are dozens of ways to solve a problem, so that might work in 
the few cases where anode impedance is controlled by the enclose 
and where that cavity resonance just happens to be at the same 
frequency where the grid structure inside the tube is not well 

In general the best solution is the most simple and most direct 
approach possible. It really isn't any big deal to stabilize an HF  
amplifier, unless the tube is of a type that oscillates near the 
operating frequency. 

Problems aren't anywhere near as big or as common as some 
people would have you think.

> As Rich described in his amplifier web page a grid-dip meter can be easily
> coupled into the anode circuit to check for a VHF resonant condition, 

So what? Resonance on a dip meter tells you absolutely nothing at 
all about the anode impedance, and that is what you need to know.

> One could check the effectiveness of the resistive loop by again placing a
> grid-dip meter near the circuit and checking for strength of dip at the
> VHF resonant frequency.

Please tell me how to measure anode impedance or suppressor 
impedance with a grid dip meter.

For example, can I tell if the anode impedance is 100 ohms j0 or 
tell if anode system Q is two or twenty at a given frequency?  

What about the grid. How do I measure the impedance or 
effectiveness of grid grounding with a dip meter? 

73, Tom W8JI 

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