Path: ...!eternal-september.org!feeder2.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail From: brian 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 Lines: 59 Message-ID: References: <1r2rj8l.msi28f14weovyN%liz@poppyrecords.invalid.invalid> Reply-To: brian MIME-Version: 1.0 Content-Type: text/plain;charset=us-ascii;format=flowed Injection-Date: Mon, 11 Nov 2024 17:12:29 +0100 (CET) Injection-Info: dont-email.me; posting-host="b61a51c8cf1c5f7e0871b20a256b019c"; logging-data="1133111"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX1+lBzdOjwwOtLFmEzvMPhuM" User-Agent: Turnpike/6.07-S () Cancel-Lock: sha1:BW2/ZqBbj5BHwmLVXwfZcPjbpXA= Bytes: 3779 In message <1r2rj8l.msi28f14weovyN%liz@poppyrecords.invalid.invalid>, Liz Tuddenham 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