Re: Valve frequency multipliers (followup)

john larkin <jlArbor.com> 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.

All the 150 Mc/s coils are air-cored but the 50 Mc/s ones could have
been either air or ferrite cored - or so I thought - wrongly!

The formers were some NOS ones that had about 40 turns of wire on them,
bfeore I stripped them and wound on 7 turns for 50 Mc/s.  That looks as
though they were originally intended to work somewhere around 1 Mc/s and
presumably the ferrite core would have been optimised for that frequemcy
range.

 I am now beginning to wonder about the other inductors in the crystal
'pulling' circuits, which are running at about 15 Mc/s and come from the
same batch.  The behaviour of the oscillator had been somewhat strange
at times, so the cores could be causing trouble there too.


--
~ Liz Tuddenham ~
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