[Central-Electronics] Aligning broadband couplers in 100V or 200V with NanoVNA?

[Laurence M. Szendrei]

Getting a good estimate of the impedance is the tough part. The PA stage 
is the easiest, as we know it's nominally 50 ohms at the output, and 
there is a fudge factor one can look up for a class AB1 stage to 
multiply by the maximum plate current [= 0.175A]/plate voltage [= 600 
V]). That's probably on the order of a few thousand ohms which might be 
a manageable impedance to match with a broadband transformer.

The impedance on the grids of the 6BQ5 driver stages is likely dominated 
be the value of the grid resistor on that stage (4.7K? - schematic is 
though to follow). That approach may not work for the 12BY7 mixer grids, 
because they could well be driven into grid current: mixers aren't/can't 
be linear to begin with. The plate impedance of the 12BY7 mixer stages 
is also tough to get an estimate for. These are probably too high for a 
broadband impedance matching transformer to be a solution.

Thanks for the input!

-Larry/NE1S

On 3/20/26 3:56 PM, Chris Farley via Central-Electronics wrote:
>   What is the necessary impedance?  One could likely make a pair of little ferrite donut matching transformers and cal them out before use.
> RegardsChris
>      On Friday, March 20, 2026 at 01:03:28 PM CDT, Laurence M. Szendrei via Central-Electronics <central-electronics at mailman.qth.net> wrote:
>   
>   Greetings,
>
> Has anyone here tried using a nanoVNA to check the alignment of the
> broadband couplers in a 100V or 200V? The problem to solve is impedance
> matching, since the nanoVNA ports are 50 ohms. Appropriately-sized
> series resistors on the input and output? Those will attenuate the
> signal, which might be acceptable since we are looking for a qualitative
> response. Of course this would be done with no power applied to the
> transmitter.
>
> 73,
> -Larry/NE1S
>
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