Re: Filter problem

On Fri, 13 Jun 2025 21:18:21 +0100, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:

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

>
[..]

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Heck, 150 MHz is almost DC.

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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?

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

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

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There is an inductor in the anode circuit, which resonates with the
valve capacitance (and a trimmer) at the output frequency.

But then you want a bandpsss filter, I think.

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At lower frequencies, pentodes are generally considered to be current
sources and triodes are nearer to voltage sources.  As they approach
their highest operating frequencies, electron transit time messes that
up.  The triode is a mixer, so Miller capacitance doesn't apply because
the two input frequencies are not the same as the output frequency.

There's no Miller effect only if the grids are zero impedance to
ground at 150 MHz.

>

>

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

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What form do they take?  Are they band-pass, band-stop or low-pass?

Generally one starts with a lowpass filter and translates that into a
bandpass or bandstop.

This is the classic reference:

https://www.amazon.com/s?i=garden&srs=12653393011&bbn=1063498&rh=n%3A1055398%2Cn%3A284507

It has tables for all sorts of LC filters, and explains the bandpass
transformation. My Elsie program helps with the arithmetic.

Some lowpass filters are terminated on both ends and some just one.
You can also use a transformer on one end.