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Designing & Building a Vintage Style 3-valve TRF

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Designing & Building a Vintage Style 3-valve TRF

Post by Sparks » Fri May 13, 2016 2:32 pm

Three-valve Vintage Reproduction TRF Design Progress

I aim to design, and build a 3 valve TRF using an SG215 SG Valve, an HL2 detector and a KT2 output valve. The first task was to fit all the main components on the board. The initial construction will be quite rough, with no pretences at neatness. All I require of it is that it will work. Once I have got it going to my satisfaction, I will re-build it on a polished wood base in the time-honoured fashion.

I have now got the detector stage working. All three valve-holders have their filaments wired up to save time later. The aerial coil is two 47uH RF chokes contained inside old glass fuses, the central junction being the aerial feed-in. The reaction choke is 100uH, mounted on a double tag-board next to the lower aerial choke. The receiver has good volume when working at 100 Volts HT and 2 Volts LT (obtained from a 2V sealed lead acid battery). Several stations can be received in the daytime, and more at night. Initially, the reaction did not work. All I had to do was to pop the lower 47uH choke out of its fuse-holder and put it back the other way round!

I am not looking for advice or suggestions as to how to proceed, but feel free to say what "you" would do if you were building it. I am building it using a bit of simple theory, but mainly trial and error and experimentation. Be as critical as you like, it doesn't bother me! :qq1 I am just building it to amuse myself and don't aim to prove anything with it!

Bob

One valve (Medium).JPG
Detector stage

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

Post by Michael Watterson » Fri May 13, 2016 3:35 pm

Sparks wrote: Be as critical as you like, it doesn't bother me!

I'm sure you don't mean that.
The first task was to fit all the main components on the board.

No, absolutely not.
The first task is to design it and produce a schematic.
Then Publish that and review it. Then people might learn something.

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

Post by Sparks » Fri May 13, 2016 3:48 pm

Of course I mean it :) But it will be built the way I want it, whatever is said! Just trying to promote some interest in members building things for themselves. There is no need to have a fantastic grasp of theory to make something like this! The HT current at 100V HT is 3mA, by the way. Considering I just picked the HL2 out of a big box of ancient valves, I am surprised it works so well, as I must have quite a lot of low emmissions amongs them, but I don't have the heart to throw them away, even the ones that I know to be defunct!
Once it is working with all three valves, I will be looking to replacing the old intervalve transformer with something rather more easily (and cheaply) obtained. I also intend to make valve-holder adaptors, so that I can plug in small battery valves, 1T4, 3S4 etc, as not everyone has access to these ancient types. Some of them, I obtained when I was at school and have had them for over 62 years!
Bob

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

Post by Sparks » Fri May 13, 2016 4:05 pm

I do draw a rough circuit before I start, but it is all very basic and subject to change. In my younger days, when I tried to be clever, and design something on paper, it seldom worked first time. That is why I am doing this in stages, first the detector, then the output stage, and finally the RF amp. That to be followed by modifications, and finally re-building it properly. Here I am 43 years ago, repairing the main radar aboard the passenger liner RMS Windsor Castle, with only an old AVO to help me! Photographer happened to be making publicity photographs at the time! :aaj

Bob

RMS Windsor Castle 1973.JPG
Windsor Castle, 1973

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

Post by Sparks » Fri May 13, 2016 5:06 pm

Coils
The aerial coils are made up from 47uH RF chokes fitted inside old 1 1/4 inch glass fuses. A simple technique is used for making them. First of all, a simple aluminium jig is made for removing the cap from one end. It is a piece of aluminium angle that has had a hole drilled in it that is the diameter of the glass part of the fuse. A lead is cut into the hole using a jeweller's saw, or junior hacksaw. The jig is placed in the vice, and the fuse slotted into it. Pull on the fuse below the jig, whilst applying heat from a small soldering iron to the cap. This melts the glue, and the cap comes off easily. The glass is a poor heat conductor, so as long as you are quick, you will not get burned! You only need to remove one cap. Hold the metal cap in a small pair of pliers, and drill through it with a fine drill. I use a 12 Volt hand-held battery drill for this. Always hold the cap to be drilled in a small pair of pliers, unless you are not bothered about accidentally drilling a hole in your finger! :ccf Drill the cap you have taken off. Insert the RF choke in the fuse, and solder the end. Replace the cap on the other end and solder it. Cut off the protruding choke leads. No need to glue the cap back on, of course, as the choke holds it in position.

The advantage to choke coils when building TRFs is that you can put different values in to experiment with different wavebands. They are fine on medium waves, but not effective for short waves. You can also turn one round if the reaction control does not work, as the coils are held in standard fuse holders and may be "popped" out quite easily.
The recation coil is a 100uH RF choke, but this should not be inside a fuse. I mounted mine on a double tagboard screwed to the base. The choke rests as close to the glass as you can get it.
Choke fuses may bring "screams of anguish" from experts, but I find them cheap and effective!

Bob

1 Jig to remove fuse cap (Medium).JPG
1 The jig

2 Removing the fuse cap (Medium).JPG
2 Remove the cap from one end

3 drilling the cap (Medium).JPG
Drlll the caps

4 The completed coil (Medium).JPG
The completed coil

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

Post by crustytv » Fri May 13, 2016 6:30 pm

Hi Bob,

fantastic idea for a thread and hopefully it may encourage others who have long thought about it, to have a go.

I hope you don't mind I've moved this to the radio section and made the thread a sticky. Your new thread will make a perfect partner to the Minimalist Superhet thread here that ran a couple of years ago.

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

Post by Sparks » Fri May 13, 2016 6:47 pm

Spotted it immediately - thanks.
Bob

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

Post by Sparks » Sat May 14, 2016 7:54 am

14th May, 2016
I am off to the local Model Shipbuilding club today, so the TRF will have to take a "back seat!"
http://www.forum.radios-tv.co.uk/viewtopic.php?f=20&t=13151 I have, however, added the KT2 output stage, and it is working fine with moderate loudspeaker volume.

The baseboard measures 12 1/2 by 7 1/2 inches. The grid bias potentiometer is not giving me the range of bias that I require, so that will have to be modified. At the HT voltages that I have tried it on, the maximum anode current of the KT2 has not been exceeded, so no big deal at the moment.

To digress a little, and give you something to read, the following three images show where I "learnt my trade!" so to speak. RMS Wray Castle (Radio Maritime School), on the side of Lake Windermere, 1959 - 1962. In the group photograph, 1960, I am the third from the right of the picture on the front row, aged 16. I qualified on the 27th October of the same year. I had a head start on the others. Most of them were aged between 17 and 30, with loads "A" levels and grammar school backgrounds. I didn't do very well at school, and left at 15 without gaining even a single "O" level, but had the advantage of possessing a mind "unclouded by fact!" :ccg My only interest was radio and the sea, consequently, I had a head start on the rest of them !

Here is a U Tube video of the place:



I will not pretend that it was like that when I was there. Back in 1959, it was pretty basic, and stretched my brain to its utmost. I doubt if I could even have qualified when things had advanced to the state of the video, but by then, it didn't matter because I had picked up all the modern stuff as the years progressed, and my original qualifications were still accepted. The "dreaded" transistor was not even in the course in 1959!

Bob

Wray Castle.JPG
Wray Castle

Wray Castle Group Photograph 1960 (Medium) copy (Medium) (Small).jpg
Group Photograph 1960

Wray Castle Gear room 1959.JPG
The Gear Room 1959

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

Post by sideband » Sat May 14, 2016 9:20 am

I love this basic breadboarding stuff! Takes me back to when I started nearly 50 years ago and using bits of hardboard and 16SWG copper wire as bussbars. First successful valve radio I built was using a 6Q7 running on batteries. Worked well.

Sparks wrote:I am building it using a bit of simple theory, but mainly trial and error and experimentation.


The best way to do it! You know roughly what you want....throw something together, get it working then do the fine tuning afterwards..... :aad



I shall follow this thread with great interest.

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

Post by Sparks » Sat May 14, 2016 9:49 am

I agree. I have built lots of battery sets using mains valves such as 6Q7, EF39, 6V6 etc. Once I have something working, I can experiment to imptove it, or decide how it works, safe in the knowledge that it does. Invariably if I try and work out the scehmatic beforehand in one big piece, it invariably does not work. I am sure most of us have also discovered that a beautiful, and correctly wired, radio will not work, where the "scruffy" prototype was perfect in performance! :ccb
Off out in a few minutes, but glad to see members are starting to show interest in what my old captain (See photo) said was "your different style approach" to things in general. I am on the right :qq1

Bob

RMS St Helena, Octobert 1992 (Medium).jpg
RMS St Helena, Falmouth, October 1992

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

Post by Sparks » Sat May 14, 2016 4:39 pm

Detector Stage:

This is V2 in the completed receiver, but it will function as it is as a one valve set I will start by saying this is nothing special, in fact it was probably one of the most common designs for a grid leak detector one-valve set in times gone by.
Most of you will be familiar with valves, but if anything is not clear, just ask. I have already described how to make the three coils from old fuses and RF chokes. The chokes may be obtained from Ebay. HL2 valves are often to be found on Ebay, but the advantage of this circuit is that it will work with practically any valve, as long as you use the correct voltage for the LT. The HL2 required 2 Volts. I obtain it from a 2 Volt sealed lead acid rechargeable battery. HT can be anything from about 45 Volts to 120 Volts. A few small 9 Volt batteries clipped back to back will provide the HT.

L3 lies alongside the lower 47uH choke, L2. As L1 and L2 are in glass fuse cases in standard fuse-holders, there is no need to worry about connecting L2 up the wrong way round. If the reaction does not work, simply flip L3 out of its holder and put it back the other way round! VR1 in this case is a 500 Ohm variable resistor. It plays no part in this circuit, but will form the bias circuit for the output valve, when that is added. In this circuit, it should be turned down to zero resistance. The variable capacitors, VC1 and VC2 are standard tuning capacitors taken from old MW/LW radios, either mains or battery sets. Also available on Ebay. VC1 is the tuning capacitor and VC2 is the reaction. The reaction control couples RF back into the grid circuit via L3 and L2 increasing the gain (volume). My VC2 had a much lower capacity (fewer vanes) than VC1, but seems to work OK

Transformer T1 matches the valve to standard low resistance headphones. This is not included in my receiver, but is contained in a separate small box that plugs into the receiver. The headphones plug into the box. You can obtain a suitable transformer from an old valve radio receiver. They are often bolted onto the speaker itself. If you can't find one, use a small PCB mains transformer, (250 Volt to 6 or 12 Volts) with the 250 Volt mains winding in the anode circuit (the left hand side of T1 in the diagram). The 6 or 12 Volt winding go to the headphones. These small transformers are quite common on Ebay or even obtained from small tape recorder power supplies that seem to be present in great numbers on car boot sales for as low as 50 pence!

To operate the receiver, connect to a good aerial and earth. Connect the batteries. Put the reaction control VC2 at about mid way, and sweep VC1 to find a station. Advance VC2 slowly and the volume should increase until the set breaks into oscillation. Then back off slightly. If the set does not oscillate, pop L2 out of its holder and put it in the other way round!

Bob
PS
Forgot to add that in the circuit, the black dots indicate wires connected. The wire from the valve anode to the top of L3 crosses the wire between the top of L1 and C2, but is NOT joined to it, because there is no black dot!


Detector Stage (Large).jpg
Detecor Stage

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

Post by crackle » Sat May 14, 2016 5:34 pm

I enjoyed watching the video of Wray Castle, I think I could have enjoyed the radio operators/electronics course course and a job on a ship.
But my working career has been enjoyable as it was, and I would not have my wife and son if I had chosen that career.
The circuit diagram is nice and clear.
Mike

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

Post by Sparks » Sat May 14, 2016 6:12 pm

Mike
Thanks,
Glad you enjoyed the movie. We weren't radio operators though, they only had them in the Royal Navy. :aaj Our correct job tile was "radio officer" Our duties, in addition to operating, also covered the repair and maintenance of all the radio equipment, radar sets, navigational systems, even the auto pilot. Onboard computers and videos, public address systems, the list was endless. As you could see from the Wray Castle publicity, the training was lengthy and intense. :ccf Eventually, satellite communications became so easy that they dispensed with radio officers altogether, and issued radio operators certificates to captains and deck officers who had completed a ten-day course on using satellite communications and they did their own communications - no extra pay, or course! :cch Our old technical duties were passed off onto the electrical officers who were renamed Electro-Technical Officers. One of the "perks" of the job was that I could take my wife along free of charge whenever she wished to travel. Here we are on barbeque night aboard the old RMS St. Helena in the late 1980s. Our voyages were about two months duration from the UK to Cape Town and back. Followed by the same length of time on fully paid leave!
Bob
Attachments
Barbeque night.JPG
Barbeque on deck, RMS St. Helena, 1987

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

Post by Katie Bush » Sat May 14, 2016 11:02 pm

Sparks wrote:Once I have something working, I can experiment to improve it, or decide how it works, safe in the knowledge that it does...... I am sure most of us have also discovered that a beautiful, and correctly wired, radio will not work, where the "scruffy" prototype was perfect in performance!

Bob


Hi Bob,

That sums up my earliest efforts to build a small audio amp - meant to be a triode/pentode design, I cobbled together a PCL82 and associated components onto a small piece of scrap PCB (stripped of all copper tracks) pulled from a derelict TV set.. Once assembled it looked like rat built it on a night off, but worked beautifully - bits of insulated wire standing in for copper tracks, and components connected "point to point".. I thought I'd be a clever and move on to using an ECC82 and EL84 on a 'bespoke' (folded tin plate) chassis - I could not for the life of me get that thing to work as well as the junkbox special.

Marion

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

Post by Sparks » Sun May 15, 2016 7:09 am

:aaj True enough, I am sure it has happened to most of us :ccb I wonder how I will fare with this one?
Bob

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

Post by Sparks » Sun May 15, 2016 11:36 am

Further experimentation on the two valve version.

I have a number of small wire-ended CV122 triodes, and decided to try one of these in place of the HL2. They require 1.5V filament at 150mA. I took on old B4 valve base, and soldered the triode into it, inserting three ten Ohm resistors in parallel in the filament positive lead. These had a combined resistance of 3.33 Ohms, and dropped 0.5 Volts across them, leaving the required 1.5 Volts on the filament from the 2 Volt supply to the receiver. The valve performed almost as well as the HL2 with plenty of volume. But I was not able to advance the reaction control enough to cause oscillation, although it did increase the volume greatly.

To correct this, I added a 10pF capacitor across VC2 and that sorted out the problem. If any of you are interested in these useful little wire-ended triodes, they are currently available on Ebay at £12.45 for five of them. This is NOT my listing, but it has been there for some time now, so it would appear that the seller has a lot of them! Find them by simply putting CV122 in the search. The green spot marks the anode, the next is filament + followed by filament - and finally the grid! I did try the CV122 in place of the KT2 and it did work, but much reduced volume, and some distortion!

Bob

Two Valve (Large).JPG
Output valve KT2 added

CV122 (Large).JPG
CV122 wire-ended triode

CV122 in base (Large).jpg
CV122 in B4 base

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

Post by sideband » Sun May 15, 2016 3:05 pm

Experimentation at its finest!! :thumb

Takes me back to my first restore when I discovered I needed a 12Q7 due to an OC heater. Being a Saturday evening I decided that rather than waiting until the Monday to buy a new one, a little improvisation was required. I took the base off the old valve, wired in a B9A holder to it and fitted an ECC83 (only one triode wired) with series heater so it was 12.6V 150mA...exactly the same as a 12Q7 and an OA81 to replace the diode in the 12Q7. It worked and got the set going until I could fit the correct replacement.

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

Post by Sparks » Sun May 15, 2016 5:44 pm

Yes, I found that quite often, any old valve would do if you made sure its heater voltage was OK. I have done it on a number of ocxcasions at sea when we did not have a spare of the correct number!
---

3-Valve TRF Progress
With the output stage working, I turned my attention to the grid bias. The anode current of a KT2 should not exceed 7.5mA. I was well below that with an HT of 45 Volts, but if I turned it up to 100 Volts, I got a lot more volume, but exceeded the maximum anode current. The grid bias potentiometer did not work! The amount of anode current flowing through it for both valves was insufficient to provide any useful level of bias. As a result, I reverted to the time-honoured method of using a grid bias battery.

In vintage receivers, this consisted of six 1.5 Volt cells in series with tapping sockets every 1.5 Volts. The recommended grid bias for a KT2 is -4.5 Volts on the maximum HT of 150 Volts. I did not want to go higher than 100 Volts, so I decided that three 1.5 Volt cells would be ample. I mounted them (very untidily) in three short sections of square plastic cable conduit, with spring brass connections at the ends. I found that using negative 3 Volts on the grid, I could reduce the anode current to 4mA, thus leaving a good safety margin. The screen grid current should not exceed 1.7mA, so I was pleased to find that it was only 0.9mA. The HL2 is taking 2mA and that is the maximum specified amount, so that is OK.

The receiver will work without grid bias, but do not be tempted to do this. The maximum anode current will be exceeded, and the life of the valve will be shortened as the emission fails. Considering these valves are now in excess of 70 years of age, it is important to run them at the correct levels.

I have not drawn out the two-valve circuit yet, because it has already changed with the grid bias experiments!

Bob

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

Post by Sparks » Mon May 16, 2016 8:29 am

I added the SG215 RF amplifier stage, and the set then produced excellent results with adequate volume for a normal sized room. I then noticed that I had only used one 46uH RF choke in the grid circuit of the SG215, when I should have put two in series. I added the second choke and the volume increased considerably, in fact far greater than expected. :aad I really thought the reason for this was that the RF tuned circuit inductance and the detector tuned circuit inductance were now both the same, and not 47uH out on the RF amplifier because of the missing choke. But it was not that simple. I noticed that I had wired the second choke in incorrectly. On wiring it correctly, the performace was much reduced, but still a lot better than the two-valve version. I changed it back to what I "thought" was incorrect, and regained the fantastic performance, more like what I would expect from four valves. It is now time for me to sort out the circuit diagram, and then the more learned amongst you may come up with an explantion as to how an apparently incorrectly wired tuned circuit has altered the performace from good to excellent! :aaj
Never was up to much on theory :ccg but "hands on" practical wok has always been a different matter! :thumb
Bob

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

Post by Sparks » Mon May 16, 2016 3:26 pm

Three Valve TRF. 16th May, 2016.

The receiver is now complete, and working extremely well with more than adequate volume, and receiving a number of stations with good selectivity. The circuit is complete with all the component values. It is a very conventional arrangement, apart from the rather unusual configuration of L1, L2, and VC1a that, as I said, was discovered by accident!

The total HT current drain at 100 Volts is about 10mA. 2mA for V1, 2mA for V2 and 6mA for V3.
The valves may often be found on Ebay, but other similar types will work just as well! Of the components, the rarest are L6, RF choke in the anode circuit of V1 and the intervalve transformer, T1.. I used a vintage components, but I did try a modern 2.2mH RF choke in place of L6, that performs just as well. The intervalve transformer, T1 will probably be rather harder to find, and can be quite expensive these days, but I hope to experiment with a more modern substitute shortly. The terminals of this transformer are labelled HT, P, G and GB, standing for High Tension, Plate, Grid and Grid Bias. In times long gone, the anode was referred to as the "plate," hence the P! Newer intervalve transformers are labelled IP, OP, IS, OS. The equivalent designations are:

HT = OP
P = IP
G = OS
GB = IS

The next task is to rebuild it, but in a much neater manner. (This one looks a mess!) This will probably take some time, because I will need to either find, or make, components that fit in with the 1930s era.

VC1a and VC1b are a double gang tuning capacitor. VC2 is a separate reaction capacitor.

In the prototype, the loudspeaker matching transformer T2, was not mounted on the board, but on the separate loudspeaker. When I build the set properly, I will put it on the board!

In the circuit, bear in mind that the black dots indicate where wires are connected. If there is no dot where lines cross, they are not connected at that point! Note that the polarity of the grid bias battery is very important, the positive must go to the earth line, and the negative to GB on the transformer.

I will be pleased to answer any questions (may not know the answers though).

Incidentally, I drew the circuit myself, it is not copied from anywhere! :thumb

Bob

Circuit.jpg
Circuit

1 (Medium).JPG
1

2 (Medium).JPG
2

3 (Medium).JPG
3

4 (Medium).JPG
4

5 (Medium).JPG
5

6 (Medium).JPG
6

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

Post by Ed Dinning » Mon May 16, 2016 9:06 pm

Hi Bob, thoughts on the I/V transformer.
These are quite expensive these days, even the RS ones; however, there are 100v line transformers available that are designed to match local speakers to 100v line PA systems that have various taps for power levels and speaker impedance. They might have a low enough ratio to be useful.
Bear in mind that a ratio of 3 to 1 is about right, greater than that and it starts loading the preceding circuit, 5 to 1 is about max. The real transformers had a very high inductance as well to help with frequency response; they were also air-gapped due to Dc flowing in them (could always parafeed with a cap and use another of your nice miniature chokes).
The range of pulse transformers available will probably not work due to lack of an air gap and inductance being too low.

It trust many folk will have a go at your design and not be put off by the availability of the I/V transformer.

Ed

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

Post by Sparks » Mon May 16, 2016 9:28 pm

I have a number of ideas for the coupling between V2 and V3. I do see these old intervalve transformers from time to time on car boot sales, and take a chance on whether they are OK or not, but generally stallholders don't know what they are, so they are usually cheap enough :thumb But I am aware that if you specifically want one, they are very expensive from vintage radio suppliers!
What about the V1 tuning circuit though? I was surprised at its performance, never having seen that arrangement before! I think my mind was in neutral when I did it. Initially I only had one 47uH choke across VC1a, and when I added the second (omitted in error), was thinking in terms of the aerial going to the junction between L1 and L2, as in the detecor coils L3 and L4 when I had intended to take it to the top of L2. When I corrected it, the performance fell dramatically, so I put it back as it was, supposedly incorrect!
Bob

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

Post by Sparks » Wed May 18, 2016 12:10 pm

I have now made a brand new "reproduction! RF choke for the anode circuit of V1. The top and botton are black acrylic, whilst the casing is a steel tube, cut from a length obtained from a DIY store. I may spray paint the casing black later. I wonder if anyone else undertakes this type of project? Electronically, it is pretty basic, but my main pleasure is turning it into a beautiful piece of vintage wireless "furniture!"
Bob
Attachments
Completed choke (Medium).JPG
Reproduction RF Choke

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

Post by Sparks » Wed May 18, 2016 4:34 pm

Ready for metalwork. Temporarily bolted together whilst glue dries!
Bob
Attachments
15 Ready for metalwork - Copy (Medium).JPG
Three valveholders

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

Post by Sparks » Wed May 18, 2016 5:45 pm

I have now completed the B5 valveholder for the KT2, but am getting quite "fed up" with the sight of baseboard valveholders at the moment. Will take a day off tomorrow, and go out! :aad
Bob
Attachments
B5 Completed.JPG
B5 Complete

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