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From: brian <nospam@b-howie.co.uk>
Newsgroups: sci.electronics.design
Subject: Re: Grounded grid VHF front-end
Date: Mon, 11 Nov 2024 14:14:39 +0000
Organization: A noiseless patient Spider
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In message <1r2rj8l.msi28f14weovyN%liz@poppyrecords.invalid.invalid>, 
Liz Tuddenham <liz@poppyrecords.invalid.invalid> writes
>My current receiving aerial system is very inefficient at 2 metres (144
>Mc/s) and I have thought about making a sleeve dipole for that band.  My
>VHF receiver is an Eddystone 770R, which covers the band but only in a
>small portion of the whole scale.  While I am improvomg the aerial
>system, I could also make a crystal-controlled down-converter, that
>would allow me to use an HF communications receiver or the lower ranges
>of the 770R, so that the  band 2 Mc/s wide would cover a much greater
>scale length.
>
>It's been a few years since I designed anything with valves, so I
>thought I might have a go at making a down-converter using valves - but
>not necessarily the expensive 'cult' ones which everyone seems to regard
>as having magical powers.  The EF91 is plentiful and cheap as New Old
>Stock, so that seems like a good valve to start playing about with.
>
>The EF91 was used as an RF amplifier in the input stages of television
>sets working at about 45 Mc/s, so it can't have too bad a noise figure
>(although Mullard don't quote one in their data sheet).  If I
>triode-strapped it and ran it in grounded grid mode, that would reduce
>the noise and increase the maximum frequency it could usefully amplify.
>From the data sheet, with 200v on anode and grid 2 and an anode current
>of 6mA, the gm is about 6mA/V, which gives an input impedance at the
>cathode of  160 ohms.  A 75-ohm feeder could be matched to this with a
>Pi tank or by tapping the L or the C of an input tumed circuit.
>
>The voltage gain may not be as high in this configuration as in grounded
>cathode mode, but it allows the valve to be triode strapped for low
>noise without instability problems or the dependence on neutralising
>that a cascode stage would have (especially the need for correct
>neutralising to obtain the best noise figure).  If I also use an EF91 as
>a mixer, I might need one more stage of RF gain to get the signal up to
>a level where the mixer noise is negligible - but this isn't such a bad
>thing because it would allow extra tuned circuits to give better image
>rejection and allow a lower output frquency if I wanted one.
>
>Anyone with experience of doing something like this with valves?
>
>

I built a 2metre down converter in 1971. It used an E88CC ( gold pins!) 
cascode grounded grid  front end followed  by an ECC81 mixer. Another 
ECC81 was used as xtal Local Oscillator/multiplier.

I still have it.

The design was in UK publication Practical Wireless. I had quick look on 
World Radio History to see if I could find it , but no joy.  Similar 
designs might be available.

Brian





-- 
Brian Howie