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Designing & Building a Vintage Style 3-valve TRF

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Re: Designing & Building a Vintage Style 3-valve TRF

Post by Ed Dinning » Wed May 18, 2016 7:23 pm

Hi Bob, the RFC is interesting, does the steel tube give any problems, and is the choke better or worse if the steel is earthed?

Ed

 
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Re: Designing & Building a Vintage Style 3-valve TRF

Post by Sparks » Wed May 18, 2016 7:32 pm

I haven't tried it yet in the steel tube. But the small RFC was OK on its own. Don't suppose the tube will make much difference though!
Been rather occupied with the valveholders today. The KT2 is a snug fit in the B5!

Bob

 
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Re: Designing & Building a Vintage Style 3-valve TRF

Post by Sparks » Fri May 20, 2016 11:16 am

Friday, 20th May, 2016
Coils.
I have now completed the coils. These are a bit of "overkill," and are mainly for show! The actual RF cokes are mounted on the black acrylic bases. C3 in included inside the can with L3, L4, L5. The two leads coming out of the top will go to RF choke, L6, and the top cap of V1. They are interchangeable, so do not need to be labelled! The coils cans are old stainless steel salt sellers. Normally quite expensive items, but I purchased a box full of them on a car boot sale. All the nuts and bolts came from a car boot sale as well, at minimum cost
I still have the two B4 valve holders to complete.
Attachments
Coils  Complete.jpg
Coils complete

 
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Re: Designing & Building a Vintage Style 3-valve TRF

Post by Hartley118 » Fri May 20, 2016 12:29 pm

You've certainly set me thinking about modern interpretations of those vintage tuning coils and intervalve transformers. Can we satisfactorily substitute a modern miniature 'commodity component' choke for those big tuning coils which we would painstakingly hand-wind - say 80 turns on a one-inch paxolin former? Looks like you've made the modern chokes work! I wonder what the trade-off is in terms of Q and self-capacitance because your modern approach looks a whole lot easier, particularly using your posh salt cellar screening cans :)

I've also pondered the design of those original 1920s intervalve transformers. Typically, they seemed to use lots of thin wire to give a very high primary inductance so that their shunt impedance at low frequencies was high compared with battery valve internal resistance. The consequential high leakage inductance must have caused HF loss and phase shift, but with no NFB in the circuit, I guess that didn't matter too much. The problem is that we don't normally wind transformers like that any more!

Martin

 
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Re: Designing & Building a Vintage Style 3-valve TRF

Post by Sparks » Fri May 20, 2016 1:04 pm

I have been using these small chokes for years! No good for HF, but 47uH is my favourite because they cover MW and a bit of IF as well. I haven't done anything to make them work, they just do! :aad The cans are only for show, not even earthed! When I have earthed them in the past, it has not made any difference! I never bother about "Q" or self-capacitance, or anything like that. I try it, and if it works, fine, if it doesn't, try something else. I haven't begun to devise a coupling between V2 and V3 yet that uses modern components. In the past, I have used matching transformers, or even small mains transformers and they seem to work OK. I have made superhets using only RF chokes as the aerial and oscillator coils - no problem.
Bob

 
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Re: Designing & Building a Vintage Style 3-valve TRF

Post by Sparks » Fri May 20, 2016 3:12 pm

Here is the superhet!
Bob
Attachments
4 Valve superhet (Medium).JPG
4 Valve Superhet

 
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Re: Designing & Building a Vintage Style 3-valve TRF

Post by crackle » Fri May 20, 2016 3:15 pm

They look very smart.

Mike

 
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Re: Designing & Building a Vintage Style 3-valve TRF

Post by Sparks » Sat May 21, 2016 12:45 pm

Saturday, 21st May, 2016
I have now completed the grid bias battery holder. To change the battery, the four nuts on top are removed, and the top lifted off. The battery itself is a CR2032 computer battery. It is just lifted out. The solder tag on the left is the positive side, and will go to earth. The central tag, that will be connected via a short piece of flexible wire, is the negative side, going to the grid circuit.
Bob
Attachments
Completed battery (Medium).JPG
Grid bias battery assembly

 
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Re: Designing & Building a Vintage Style 3-valve TRF

Post by Katie Bush » Sat May 21, 2016 3:44 pm

That's just brilliant!

Your ingenuity knows no bounds, and to disguise and otherwise 'out-of-place' modern device like this is just brilliant.. I am wondering though, if anyone were to ask what, if any, contemporary device it represents, what would it be?

Marion

 
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Re: Designing & Building a Vintage Style 3-valve TRF

Post by Sparks » Sat May 21, 2016 4:29 pm

I suppose the nearest you could come to it would be the BIOS battery in a computer (which is what the 3 Volt CR2032 is often used for). Hidden away for years, just doing its job. But it would make life a lot easier if computer designers put their BIOS batteries in accessible places with easily removable tops like this one. The grid bias battery lasts ages because there is no current drain at all, so it doesn't need to be switched on and off. With the negative connected to the grid, no current can flow, it is just a "bias" to get the valve working at the correct spot on its characteristic curve!
Bob

 
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Re: Designing & Building a Vintage Style 3-valve TRF

Post by crackle » Sat May 21, 2016 4:56 pm

Bob, I do like your creations, they look wonderful.

Mike

 
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Re: Designing & Building a Vintage Style 3-valve TRF

Post by Sparks » Sat May 21, 2016 5:52 pm

Thanks,
I have now completed the 2nd valveholder and the valve fits like a glove! :thumb The third one is almost complete, just got to put the nuts & bolts & solder tags in it. I have also put the first few coats of French Polish on the baseboard.
Bob

 
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Re: Designing & Building a Vintage Style 3-valve TRF

Post by Sparks » Sun May 22, 2016 11:57 am

Sunday, 22nd May 2016.
I have now sorted out the alternative coupling between V2 and V3 for anyone who does not have an intervalve transformer. I picked a small PCB mains transformer from my junk box that had an input of 250 Volts and two secondary windings of 10 Volts each. The ten volt windings are of no importance, because they are not used! Any small mains transformer will be OK as we only require the 250 Volt primary winding. Initially, I put this in the anode circuit of V2 in place of the intervalve transformer. A 0.1uF capacitor was taken from the anode of V2 to the control grid of V3. A 1 megohm resistor was connected to the control grid of V,3 and the other end to the negative of the grid bias battery. The performance was just as good as with the intervalve transformer, but the anode current of V2 had risen above 2mA, which is the maximum specified for the HL2. I took a 22K resistor and inserted it between the top of T1 and R3. The performance remained exactly the same, but the anode current fell to a satisfactory 1.7mA! This configuration is know as "choke capacity coupling!" The selection of the value of R4 was purely "guesswork" on my part! (It sometimes helps to have a mind "unclouded by fact!"). :aaj
I also put the replica RF choke in the anode circuit of V1. Earthing the can had no effect on performance. At present, I am French Polishing the wood base. The third valveholder is now complete. I will also need to make T1 look like a vintage component. Also, to make mountings for the capacitors and resistors.
Bob
Attachments
New V2 V3 Coupling (Medium).jpg
Alternative V2 V3 coupling

 
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Re: Designing & Building a Vintage Style 3-valve TRF

Post by Sparks » Sun May 22, 2016 1:33 pm

The high impedance half of an LT44 (Maplin Electronics or Ebay) output transformer also works just as well!
Bob

 
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Re: Designing & Building a Vintage Style 3-valve TRF

Post by crackle » Sun May 22, 2016 3:23 pm

Hi Bob
How does this stage now connect in with the first stage?
Mike

 
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Re: Designing & Building a Vintage Style 3-valve TRF

Post by Sparks » Sun May 22, 2016 4:09 pm

Exactly the same as it was before. See diagram Monday May 16th. In the above, today, I have just altered the anode wiring of V2 and the grid wiring of V3 by the removal of the intervalve transformer and substituting the primary with half of a small mains transformer. Adding two resistors and one capacitor.
Bob

 
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Re: Designing & Building a Vintage Style 3-valve TRF

Post by Sparks » Sun May 22, 2016 5:06 pm

I have now made the housing for the LF choke. I didn't waste much time on this. Looking around, I found the top part in the form of half a plastic cocktail stick box. The base was cut from 5mm black acrylic. It contains a miniature LT44 transformer, the high impedance windings connected to the solder tags on either side. The brass screws go into a small block of wood inside, to hold the top on!
Bob
Attachments
Choke (Half of LT44) (Medium).JPG
Choke replacement for intervalve transformer.

 
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Re: Designing & Building a Vintage Style 3-valve TRF

Post by Ed Dinning » Sun May 22, 2016 7:39 pm

Hi Bob, you can probably make a small and compact choke from a pot core (gapped type) and as many turns as you can fit using say, 0.1mm wire.
Saves using up a mains transformer.

Ed

 
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Re: Designing & Building a Vintage Style 3-valve TRF

Post by Sparks » Sun May 22, 2016 9:52 pm

Hi Ed,
Thanks. I don't even now what a "pot core" is! :ccg I have quite a few of the little PCB transformers that I got off Ebay some time ago for £1 each. But after further experimentation, I settled for an LT44. Got a box full of them (unused and still in packets) a while back on a car boot sale for 10p each! Another excellent source of small mains transformers is the small tape recorder PSUs that appear on car boot sales in very large quantities, usually between 50p and a pound!
Bob

 
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Re: Designing & Building a Vintage Style 3-valve TRF

Post by Sparks » Tue May 24, 2016 7:02 pm

I looked up "Pot Cores," and of course I have seen them hundreds of times, just didn't know what they were called! They seem to be far more expensive than normal transformers though! Searched car boot sale today for a container for the output transformer, but couldn't find anything suitable. So assembled a small wooden box that will be made up to look like an old transformer and spray painted black. I will be containing the resistors inside old 20mm glass fuses, mounted in standard fuseholders. The capacitors will be housed in acrylic oblongs in the manner of vintage types and screwed to the base. The base board is now polished.
Bob

 
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Re: Designing & Building a Vintage Style 3-valve TRF

Post by Ed Dinning » Tue May 24, 2016 8:29 pm

Hi Bob, pot cores are often seen at Hamfests and junk sales and go for pennies. Easily rewound to high inductance values in a compact space. Quite a bit less lossy with higher frequencies than a laminated transformer, but you set does not seem short of Oomph!

Ed

 
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Re: Designing & Building a Vintage Style 3-valve TRF

Post by Sparks » Wed May 25, 2016 5:30 pm

Thanks Ed, I might have one lying about in an old computer that I am about to "fillet!" :)
I have now made all the capacitor and resistor mountings. The resistors are inside old 20mm glass fuses. The small capacitors are underneath small oblongs of black acrylic, set into countersunk holes. The two larger 0.1uF capacitors will be mounted on top of the two lower mountings. At the moment, they are drying. They were the yellow ones, but I have just spray painted them black!
Bob
Attachments
Capacitor & Resistor Mounts (Medium).JPG
Capacitor & Resistor Mounts

 
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Re: Designing & Building a Vintage Style 3-valve TRF

Post by Sparks » Thu May 26, 2016 4:01 pm

All the "built up" components are now complete, and here they are. Valveholders, coils, chokes, capacitors, resistors, grid bias battery, output transformer. I now need to find a couple of fairly clean variable capacitors. The board is already French Polished, so the "real" construction will soon begin! :aad
Bob
Attachments
Components  (Medium).JPG
Components

 
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Re: Designing & Building a Vintage Style 3-valve TRF

Post by Katie Bush » Thu May 26, 2016 10:16 pm

Hi Bob,

I was just wondering..... You weren't in a WW2 German POW camp in a past life by any chance? - I'm given to believe those chaps built some pretty impressive sets too! :aad

Marion

 
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Re: Designing & Building a Vintage Style 3-valve TRF

Post by Sparks » Fri May 27, 2016 6:55 am

Not that old! :aaj Great fan of old Macgyver TV series though (1970s/80s TV). He improvises on practically anything technical using common materials. Here iis part of last night's episode (currently running on Freeview TV).
https://youtu.be/Wc4vA5clOWE
Bob

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