Re: Filter problem

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

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

What's the plate capacitance of the tubes you plan to use? Is your
output differential?

>
>

I am avoiding the use of printed circuits, it is all being built on
tagstrips and standoff pillars - and a lot of the circuit can be
supported off the valveholder tags (but not the filters).

 
Retro look.

>
Not appearance but practicality.  The plan is to avoid semiconductors
altogether; with valves it is much easier to make one-offs on tagstrips
and try out different components and layouts.  At 150 Mc/s there are
unspecified hidden capacitances and inductances waiting to catch you
out.  Some valves for those frequencies were designed with a specific
layout in mind ( ECC91, QQVO 2-6,  QQVO 3-10, QQVO 3-20, QQVO 6-40).
>
I have known laminated printed circuit boards to track across between
the layers.   With transistors this would hardly be noticed but with the
higher voltages and much higher impedances of valves, it can cause all
sorts of strange intermittent faults.

I breadboard with surface mount transistors and passives all the time.
These parts are small and planar, which tubes aren't. And a hunk of
copperclad has a beautiful ground plane on the back side.

A transistor, especially a GaN fet, has transconductance measured in
Siemens. Tubes are mS. Don't need a socket or a heater supply. Really
hard to break.

MMICs are really (and literally) cool.

Come on, try it. It's 2025.