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Old 11th Sep 2016, 7:37 am   #1
Variometer
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Default Three-valve TRF

This is a 3-valve receiver that I designed and built a few weeks ago. Most of the components are fairly recent, but disguised as "vintage." The valve holders are home-made. The valves are ancient originals that I have had for over 50 years - SG215 - HL2 and a KT2. The set works at full loudspeaker strength and it quite selective.
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Old 11th Sep 2016, 8:28 am   #2
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Default Re: Three-valve TRF

Very nice.

Lawrence.
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Old 11th Sep 2016, 8:48 am   #3
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Default Re: Three-valve TRF

I like the "PCB"!
 
Old 11th Sep 2016, 9:44 am   #4
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Default Re: Three-valve TRF

I suppose that is how PCBs evolved. In my schooldays in the 1950s, someone gave me an old baseboard set wired with brass strips and brass bridges, and I never forgot it. This set evolved by trial and error from a very (very) rough design that progressed through many changes and modifications until it ended up in the receiver shown. It works at full loudspeaker volume, and is very selective with a good tone. The RF tuning circuit is unusual, and was a result of incorrect wiring. It works beautifully, but when I wired it as I had intended, its performance fell off, so I put the mistake back! The tuning coils are 47uH RF chokes contained in old stainless steel salt-sellers. The feedback choke is 100uH. I have been assured that this is all wrong, and inefficient, but cannot imagine any three-valve set having better performance. The valves are SG215, HL2 and KT2, all between 80 and 90 years old. The valve-holders are home-made. I run the filaments on a 2 Volt sealed lead acid battery, and the HT is 100 Volts obtained from 9 Volt batteries clipped back to back. The grid bias battery is a 3Volt CR2032 (computer bios battery). I don't bother too much with theory these days, Q and all that sort of thing goes in at one ear and out at the other!
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Old 11th Sep 2016, 9:51 am   #5
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Default Re: Three-valve TRF

It's good to see another one of your excellent creations with the emphasis on home construction and the re-cycling of existing components or creation of new ones from odds and ends! Looking at your ship models page, the detailed layout you achieve suddenly makes sense. I recall that you started a site to display your radio projects at one stage [I thought that was a really great idea] but then seemed to abandon it because there wasn't much reponse but I think you only gave it a few days Perhaps you should re-consider that decision?
On the nautical front, I can't help thinking that the Cable Repair Ship I mentioned in last nights thread [of the same name] would be a good subject

Dave W

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Old 11th Sep 2016, 11:47 am   #6
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Default Re: Three-valve TRF

I have tried twice with radio blogs, both dismal failures. The last one, after running for a month, had only attracted about 7 views, and I deleted it a few days ago.
Bob
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Old 11th Sep 2016, 4:34 pm   #7
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Default Re: Three-valve TRF

Very nice job.

I believe the use of a wooden board as a chassis in the 1920s is the origin of the term 'breadboard' in electronics. Experimenters and home constructors would literally use an old breadboard stolen from their mother's or wife's kitchen.
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Old 11th Sep 2016, 4:35 pm   #8
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Default Re: Three-valve TRF

Shouldn't C6 be on the other end of L7?
 
Old 11th Sep 2016, 5:25 pm   #9
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Default Re: Three-valve TRF

I don't know! I assumed that L7 stops the audio going up, so it goes via C6 into V3, and that the combined resistance of R3, R4 makes the RF go back through the reaction circuit, whilst C5 decouples anything not wanted to earth. I did a fair bit of experimenting on that part, and the circuit shown was the only arrangement that allowed the reaction to work properly. L7, incidentally, is the high impedance half of an LT44 transformer, inside the top of a plastic cocktail stick box. I did know all about such things once-upon-a-time, but nowadays, I only bother with getting it to work, and don't worry about theory beyond what I think may work. Long ago, when I tried to design things beforehand, they seldom worked, and eventually I gave up and go soley by experience and the freedom of a mind "unclouded by fact!"
I can't think of any reason why the funny RF tuning configuration works better than the accepted method, but it does! So far, no-one has been able to come up with any answer at all to that
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Old 12th Sep 2016, 11:43 am   #10
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Default Re: Three-valve TRF

That's very nice.
Surface-mount technology, 1920's style!
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Old 12th Sep 2016, 12:09 pm   #11
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Default Re: Three-valve TRF

That's not just a set; it's a work of art.

Paul, I've heard a fancier version of the same etymology, with the experimenter being asked what he was doing with his wife's breadboard. It's certainly plausible.
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Old 12th Sep 2016, 12:45 pm   #12
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Default Re: Three-valve TRF

Agree with Julie, that's a work of art!

How did you make the valve holders make I ask?
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Old 12th Sep 2016, 1:42 pm   #13
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Default Re: Three-valve TRF

Lovely! I'm fond of home built TRF's, perhaps because I've never managed to make a superhet from scratch.
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Old 12th Sep 2016, 1:49 pm   #14
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Default Re: Three-valve TRF

Thanks for replies.
I cut them from coloured acrylic sheet with a round saw. Made three large disks and three small disks, drilling the holes in them before cutting them out. Glued them together with Araldite (Nut & bolt is while glue dries). Fitted brass tubes in holes. Added nuts, bolt and solder tag connections. Wired up underneath.
I got the images a bit mixed up. The last image should be the first in the queue!
ITAM805 I have sent you a PM.
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Old 12th Sep 2016, 8:19 pm   #15
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Default Re: Three-valve TRF

Great job!

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Old 12th Sep 2016, 11:03 pm   #16
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Default Re: Three-valve TRF

I take your point about the Blogs Bob and [of course] it's very much up to you but I didn't know you'd had another go so maybe that applies to your potential audience as well Personally I'd "buy the book" as your style of construction etc isn't really seen anywhere else in the same way and it would be good to access most of your projects at once What do others think I wonder?

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Old 13th Sep 2016, 1:50 am   #17
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Default Re: Three-valve TRF

That is absolutely beautiful!

I think you've captured a really authentic vintage look there.

I did something similar a few years back but its nowhere near your league.

Well done!
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Old 13th Sep 2016, 7:13 am   #18
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Default Re: Three-valve TRF

Thanks. That is a very neat radio. The valve has the look of an SP61? I covered my baseboard with sapele veneer and French Polished it, which added to the vintage look. The brass wiring strips were cut from a sheet of brass shim (using a scalpel and steel ruler) obtained from a model shop and stuck down using double-sided tape.
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Old 13th Sep 2016, 8:00 am   #19
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Default Re: Three-valve TRF

It's a HL41DD. I found it in a junk shop, complete with box and certificate. I bought it, found the heater was okay, so decided to do something with it.

I'm keeping with breadboard tradition in a manner of speaking - my base is a sawn down bread bin! The lid lifts off to reveal PP3 batteries and 'D' cells.

My coil is made from two sizes of downpipe (yeah, I know - lousy 'Q').

It works well but then I'm not far from Droitwich.
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Old 13th Sep 2016, 8:16 am   #20
Variometer
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Default Re: Three-valve TRF

Hello Karen,
I have some HL41DD valves, very similar in appearance to SP61. Mazda Octal bases and 2 Volt filaments. The coil in my Heath Robinson-style one valver is also made from a piece of drainpipe, and like yourself, I couldn't care less about Q, as long as it works OK
Bob
PS
Is that a Mazda Octal base you used, or something homemade, I know they will not go in standard Octal relay valve-holders which is what that appears to be?
PPS
I see that you also put components inside old glass fuses the same as I do. I have never seen anyone else using this method, before. Also wonder if the reaction is a rotating coil inside the main one?

Last edited by Variometer; 13th Sep 2016 at 8:22 am. Reason: Added PS and then PPS
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