Simple Valve Rx from 1966 - Page 3 - UK Vintage Radio Repair and Restoration Discussion Forum

kellys_eye · 7th Nov 2022, 4:17 pm

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.

Jon_G4MDC · 7th Nov 2022, 5:01 pm

Are you building the 455kHz or 1.7MHz IF ?

Bazz4CQJ · 7th Nov 2022, 6:05 pm

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

Jon_G4MDC · 7th Nov 2022, 8:27 pm

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?

Bazz4CQJ · 7th Nov 2022, 8:53 pm

Quote:

Originally Posted by Jon_G4MDC

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?

Mmm, I note that none of the three designs added today use high-gain valves for the RF amp, just the EFC82 and 6BAB. If you think the sensitivity is low, does my suggestion of the EF183 (or similar) make sense or is it the case that a high gain amp will "not suit a simple IF strip"?

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

G6Tanuki · 7th Nov 2022, 9:10 pm

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.

Bazz4CQJ · 7th Nov 2022, 9:29 pm

Quote:

Originally Posted by G6Tanuki

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.

Thanks for that insight. But, could a man with a magloop avoid the sharsh (to a large degree) and benefit from the extra gain? Perhaps the magloop, with its own pre-amp, would work well with the ECF82? Of course, if the RF amp valve base is just for the RF amp, one could experiment quite easily.

B

Bazz4CQJ · 9th Nov 2022, 12:37 am

Well, after 47 posts, I think I've frozen my ideas on what to do about the QST receiver.

I'm going to start off keeping the ECF82 pentode as the RF stage, but forget the triode half of that valve.
A new AF stage will use an ECL80
I'm trying to get it on to an area of 6 x 7 inches, using a "Universal Chassis" bought in 1968 from 'Home Radio Mitcham'
I'll add a tube voltage stabiliser and a few polystyrene caps and maybe a few silver micas in the hope of getting acceptable stabilty.

The drawing office is working on it 24/7 (it's the production department where the problems usually crop up).

B

7th Nov 2022, 4:17 pm	#41
kellys_eye Octode Join Date: Jan 2014 Location: Oban, Scotland, UK. Posts: 1,129	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. Attached Thumbnails

7th Nov 2022, 5:01 pm	#42
Jon_G4MDC Nonode Join Date: Nov 2015 Location: Welwyn Garden City, Hertfordshire, UK. Posts: 2,015	Re: Simple Valve Rx from 1966 Are you building the 455kHz or 1.7MHz IF ?

7th Nov 2022, 6:05 pm	#43
Bazz4CQJ Dekatron Join Date: Apr 2009 Location: Oxfordshire, UK. Posts: 4,935	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 __________________ Saturn V had 6 million pounds of fuel. It would take thirty thousand strong men to lift it an inch.

7th Nov 2022, 8:27 pm	#44
Jon_G4MDC Nonode Join Date: Nov 2015 Location: Welwyn Garden City, Hertfordshire, UK. Posts: 2,015	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?

7th Nov 2022, 9:10 pm	#46
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 14,007	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. __________________ I'm the Operator of my Pocket Calculator. -Kraftwerk.

9th Nov 2022, 12:37 am	#48
Bazz4CQJ Dekatron Join Date: Apr 2009 Location: Oxfordshire, UK. Posts: 4,935	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. I'm going to start off keeping the ECF82 pentode as the RF stage, but forget the triode half of that valve. A new AF stage will use an ECL80 I'm trying to get it on to an area of 6 x 7 inches, using a "Universal Chassis" bought in 1968 from 'Home Radio Mitcham' I'll add a tube voltage stabiliser and a few polystyrene caps and maybe a few silver micas in the hope of getting acceptable stabilty. The drawing office is working on it 24/7 (it's the production department where the problems usually crop up). B __________________ Saturn V had 6 million pounds of fuel. It would take thirty thousand strong men to lift it an inch.