Re: Filter problem

On Fri, 13 Jun 2025 16:46:18 +0100, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:

               r       qweqqaew;ewq john larkin <jl@glen--canyon.com>
wrote:
>

On Fri, 13 Jun 2025 11:30:13 +0100, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
 

john larkin <jl@glen--canyon.com> wrote:
>

On Thu, 12 Jun 2025 17:02:59 +0100, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
 

john larkin <jl@glen--canyon.com> wrote:

>
[..]

>
Heck, 150 MHz is almost DC.

>
Not with valves, it isn't

 
We were discussing building a bandpass fiter. At low power, the parts
can be one per cent of a wavelength long.
 
And at low power, tubes should be small, so plate capacitance will be
small, and can be rolled into the first bp filter capacitance.

>
I was intending to make the characteristic impedance of the filter 50
ohms so I could set it up easily with a VNR.

 
What's the plate capacitance of the tubes you plan to use?

>
ECC91= 2.5pf max with anodes paralleled
EF91 = 2.1pf (3.1pf if shielded)

Xc is over 400 ohms. Basically a pure current source.

There are filter forms that are 50 ohms on one end and Hiz on the
other.