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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment. |
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7th Nov 2022, 4:17 pm | #41 |
Octode
Join Date: Jan 2014
Location: Oban, Scotland, UK.
Posts: 1,129
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Re: Simple Valve Rx from 1966
The OP's original post reminds me a lot of two other designs I've seen (one of which I'm currently building) as per the two pdf's attached.
I bought a job lot of 6U8's a few years ago for this purpose but stumbled at the crystal for the filter. I managed to source an FT-243 type but still can't find the 'Miller' transformer to go with it - it's now going to be down to a suck-it-and-see solution. |
7th Nov 2022, 5:01 pm | #42 |
Nonode
Join Date: Nov 2015
Location: Welwyn Garden City, Hertfordshire, UK.
Posts: 2,015
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Re: Simple Valve Rx from 1966
Are you building the 455kHz or 1.7MHz IF ?
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7th Nov 2022, 6:05 pm | #43 |
Dekatron
Join Date: Apr 2009
Location: Oxfordshire, UK.
Posts: 4,935
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Re: Simple Valve Rx from 1966
Well, this is getting interesting. I didn't start out with the intention of building an Rx, but was looking for ideas about alternative crystal filters for the HRO, and in particular, the use of the cheap ceramic filters and that lead me to the QST66 circuit. That then turned in to a "winter project".
The additional 3 circuits are all very interesting and will take a bit of time to compare and contrast. No one has commented on the claimed sensitivity of the QST66 "-114dBm (0.4 uV) / 10 dB S/N with the FT241 crystals in place" which I mentioned in my last post; seemed pretty fair for a simple Rx to me? Similarly, the question raised about the Ge diodes is still open. So, I'm collecting parts, but I'm not sure now what they are for . B
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7th Nov 2022, 8:27 pm | #44 |
Nonode
Join Date: Nov 2015
Location: Welwyn Garden City, Hertfordshire, UK.
Posts: 2,015
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Re: Simple Valve Rx from 1966
0.4uV -114dBm for 10dB S/N on an 80m or 40m receiver might be of some use in a very quiet location with a very short antenna - e.g. maybe a mobile whip?
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7th Nov 2022, 8:53 pm | #45 | |
Dekatron
Join Date: Apr 2009
Location: Oxfordshire, UK.
Posts: 4,935
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Re: Simple Valve Rx from 1966
Quote:
I see that one design has a 6AQ5 (EL90) for the AF amp; seems a bit like overkill? Think the ECL80/82 looks more attractive (extra amplifier and lower HT current). B
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Saturn V had 6 million pounds of fuel. It would take thirty thousand strong men to lift it an inch. |
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7th Nov 2022, 9:10 pm | #46 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 14,007
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Re: Simple Valve Rx from 1966
Historically, lots of pre mixer gain was bad, you just needed enough to overcome the mixer noise contribution. Too much pre mixer gain led to crosdmodulation, a big problem back in the 60s/70s/80s when you were listening for amateur stations running 100 Watts in among Radio Moscow, Radio Tirana, and similar Megawatt broadcast stations in Eastern European COMECON countries.
The big thing back then was to use valves which remained linear (so they don't introduce crosdmodulation) when they were biased back to reduce gain. These days things are easier, you may no longer need to deal with several signals of 100 Microvolts sitting within 25KHz of the signal you want to hear. Alas the propaganda stations have been replaced by sharsh from SMPS, noise from obsolete plasma tellies and other nonsense that raises the noise floor, so ultimate sensitivity is not relevant.
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7th Nov 2022, 9:29 pm | #47 | |
Dekatron
Join Date: Apr 2009
Location: Oxfordshire, UK.
Posts: 4,935
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Re: Simple Valve Rx from 1966
Quote:
B
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Saturn V had 6 million pounds of fuel. It would take thirty thousand strong men to lift it an inch. |
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9th Nov 2022, 12:37 am | #48 |
Dekatron
Join Date: Apr 2009
Location: Oxfordshire, UK.
Posts: 4,935
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Re: Simple Valve Rx from 1966
Well, after 47 posts, I think I've frozen my ideas on what to do about the QST receiver.
B
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Saturn V had 6 million pounds of fuel. It would take thirty thousand strong men to lift it an inch. |