More armchair theory...
Rich Measures (SK) wrote extensively about this last century for "tubes
with handles". The "trick" is to use a roller coaster coil in the grid
circuit to resonate with the Cin of the valve. Strays in the un-un and 450
ohm resistor are absorbed when the coil is tuned for minimum input SWR.
See the section on "Tuned Input Circuits for Neutralized, Class AB1
Grid-Driven Operation" here
http://www.somis.org/D-amplifiers4.html
and the outline circuit diagram here
http://www.somis.org/D_a_05.GIF
I would be inclined to use a biggish core (FT240, maybe even two stacked?)
of type 43 ferrite and a tightly twisted trifilar winding with "just
enough" turns to satisfy the Bmax flux density limit on 80m. It will
probably be necessary to add compensation capacitance across the 50 ohm
winding as well to mitigate the effects of leakage inductance. 80-10m
operation is quite a challenging wide frequency range.
73, Alan G3XAQ
> Date: Mon, 25 Oct 2021 08:31:46 +0100
> From: Steve <g8gsq72@gmail.com>
> To: amps@contesting.com
> Subject: Re: [Amps] 1:9 un-un for grid driven tetrode
>
>
> Doesn't a coil in parallel with the 450ohms to cancel the tube's
> capacitance mess things up at lower frequencies? What's the Cin?
>
> Steve G8GSQ
>
> > A buddy is trying to get a 1:9 un-un to work on his grid driven tetrode.
> > Terminated in a 450 ohm globar, SWR is 1:1 but only up to aprx 20
> mhz.
> > Above 20 mhz, it rapidly all goes to hell....reaching 2.5:1 at 29.00
> > mhz. ( this on the test bench)
> >
> > He needs the 1:9 un-un to work from 80-10m. What's the trick here ?
> > What is the ideal material to wind the trifilar onto ?
>
>
--
Alan Ibbetson
alan@g3xaq.net
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