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From: legg <legg@nospam.magma.ca>
Newsgroups: sci.electronics.design
Subject: Re: Valve frequency multipliers
Date: Sat, 01 Feb 2025 08:51:30 -0500
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On Fri, 31 Jan 2025 13:51:25 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:

>I am trying to use a 15 Mc/s crystal oscillator to generate a 150 Mc/s
>signal, the obvious multiplication ratios are x5 and x2.   The 150 Mc/s
>has to be distributed to two other units by a 120-ohm screened cable.
>
>The whole thing must be done with the minimum number of valves and no
>semiconductors.  The Colpitts-derived xtal oscillator is an EF91 and the
>multiplier stage(s) can be either another EF91 or an ECC91.  
>
>I have tried picking the x5 signal (75 Mc/s) off the anode of the
>oscillator with a tuned circuit but can only get a couple of volts
>pk/pk.  This isn't enough to drive the ECC91, which I  had hoped could
>be used as a 'push-push' doubler, it also won't drive an EF91 over
>enough of the curved portion of its characteristic to give sufficient
>frequency-doubled signal.
>
>Alternatively, I have tried using a parallel-tuned circuit at 15 Mc/s in
>the anode of the xtal oscillator to drive one of the triodes of the
>ECC91 which can then act as the multiplier.  There is a whopping great
>15 Mc/s signal going into the grid of the triode (about 25v pk/pk) and,
>with the cathode earthed, this develops enough grid-leak bias that the
>valve is conducting anode pulses of over 20 mA about 10% of the time.
>
>I would have thought that under those conditions the triode would have
>given a large signal at 75 Mc/s in an anode circuit tuned to that
>frequency - but it doesn't appear to.  I can't use the triodes as
>straight earthed-cathode amplifiers at those frequencies because of the
>Miller capacitance effect, but they should be perfectly satisfactory as
>multipliers where the grid and anode circuits are  tuned to different
>frequencies.
>
>Does anyone know how to determine the optimum conditions for generating
>the 5th and 2nd harmionics in valves?

I'd be more selective of the crystal frequency. Two triplers might get
you there with a lot less grief.

You can do that with something like a 6CA8 - pentode side doing xtal 
osc and 3x with the triode another x3. See ARRL handbook 1969 
(46th Ed)CH17 p239. That was for 220MHz (they stuck a P-P tripler 
on the output, so 3x triplers)

RL