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From: liz@poppyrecords.invalid.invalid (Liz Tuddenham)
Newsgroups: sci.electronics.design
Subject: Re: Valve frequency multipliers (followup)
Date: Mon, 7 Apr 2025 18:22:57 +0100
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Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

> On 2025-04-06 12:35, john larkin wrote:
> > On Sat, 5 Apr 2025 18:33:39 +0100, liz@poppyrecords.invalid.invalid
> > (Liz Tuddenham) wrote:
> > 
> >> You may remember a couple of month ago I was trying to derive 150 Mc/s
> >>from a 16.667 Mc/s crystal with two triode triplers.  I now think I have
> >> found the cause of my problems:
> >>
> >> The first tripler circuit didn't seem to tune up correctly and all sorts
> >> of spurious harmonics were coming out of it.  There was never enough 50
> >> Mc/s signal to drive the second tripler far enough into non-linearity,
> >> so the 150 Mc/s output was utterly feeble.
> >>
> >> Eventually I decided I was never going to get it to work in the space
> >> available, which was only just big enough for one valve, so it would
> >> have to be split, with the first tripler in the oscillator box and the
> >> second tripler in another box.  I decided to use a pentode (EF91) for
> >> the first tripler as it could be biassed to give a lot of distortion and
> >> a large anode voltage swing.  Because there was now room available and a
> >> trimming capacitor to spare (which had previously been used to tune the
> >> 150 Mc/s coil), I abandoned the ferrite slug-tuned 50 Mc/s coil and
> >> wound an air-cored one instead.
> >>
> >> The circuit gives a *huge* output, far more than ever before (and it is
> >> not due to self-oscillation or any other vice like that).  I think the
> >> cause of the previous low output must have been the ferrite tuning slug,
> >> which probably wasn't rated for 50 Mc/s and was damping the circuit or
> >> saturating to give lots of unwanted harmonics.
> >>
> >> There's plenty of work still to do, but at least one link in the chain
> >> is now working and I have an explanation of the probable reason why it
> >> didn't work before.
> > 
> > Air core inductors, simple coils, would have best Q at your
> > frequencies.
> > 
> > 
> Decent coax is pretty good too--to make an inductor at 150 MHz takes 
> only a few inches of coax with a short at the other end.
> 
> I like to tune coaxial stubs using thumbtacks--you stick it through the
> shield and the center conductor.  It survives very well, so you can do

That's an excellent idea, I hadn't thought of doing it that way.  It
might be a bit tricky 'tapping' such an inductor for various loads.

-- 
~ Liz Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk