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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 19:36:09 +0100
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Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

> On 2025-04-07 13:22, Liz Tuddenham wrote:
> > 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.
> > 
> 
> You tap it Colpitts-style when necessary.  That isn't as good as tapping
> a tightly-coupled coil such as a gapped toroid, because the impedance
> change comes only from the tank Q, rather than tank Q plus transformer
> action.

If you use an 'open' transmission line, such as a trough line, then you
do get a transformer effect.   Most UHF television tuners used trough
lines with tappings to match the aerial impedance to the transistors and
still retain a reasonable 'Q' for tuning. 

 Leak even did it at VHF in their FM tuner but I don't know whether it
gave any advantages (I believe some of their designs were actually made
under licence from the BBC and weren't designed by H.J. Leak at all.)


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