Advice needed: Reaction coil for home-brew 3-valve TRF receiver

[Enthusiast]

Greetings....

I'd be grateful for advice on winding tuning and reaction coils for a home-brew 3-valve TRF receiver based on a circuit from Practical Wireless published in December 1962. The article is re-produced at:

http://www.pasttimesradio.co.uk/build/ef91radcomp.pdf

I want to make five or six five plug-in coils to cover medium wave and several of the main SW broadcast bands (31m (5.950-9.750MHz); 41m 7.100-7.300MHz); 49m (5.950-6.200MHz); 60m 4.750-5.060MHz); 75m (3.900-4.000MHz) using Octal valve bases and matching "plugs" (all available in my junk-box).

(I plan to use a slow-motion drive for VC2 on the circuit (500pf air dielectric tuning cap) and add a 30pF variable cap across it for bandspread).

Based on using 1 and two eights inch diameter plastic formers (super-glued onto Octal valve plugs) and 22swg enamelled copper wire, how many turns will be required for the reaction winding (connected top end to anode V2 (an EF91); bottom end to 300pF variable 'reaction' cap)?

And how will the number of turns of the rection coil need to vary between tuning coils for different bands?

Ideally, based on 22 swg wire and a one and two eighth inch formers, could I have advice on number of turns for a reaction coil plus turns required for tuning coils to cover:

Tuning coil and reaction coil turns required for each of these ranges?

31m band
41m band
49m
60m
75m

And medium wave (526.5 - 1606.5 KHz).

Finally, any hints on building Octal valve base mounted plug-in coils using junk-box/household material for formers?

The project is only a bit of fun for dark winter evening and as a test-bed for experimenting with home-brew plug in coils (I'm not expecting great sensitivity/selectivity re reception....)....but all suggestions/advice would be much appreciated.

Best wishes,

David

[kalee20]

To calculate turns for tuning coil you need to know the required inductance.

Since f = 1 / (2 x pi x sqrt (L x C)), knowing C you can calculate L. Use for C the max value of tuning capacitor; use for f the minimum frequency you want to tune down to.

Then, knowing L, calculate turns using L = a² N² / (9a + 10l), where a = coil radius in inches; l = coil length in inches, and N = number of turns. You'll have to make a guess at length, and then refine after the first iteration, as length is itself dependent on number of turns (unless you get clever and derive a quadratic equation for N).

This formula is Wheeler's formula, and is fairly accurate for single layer coils with an air core.

Finally, a fair rule of thumb is to use about 1/3 of the turns for the reaction coil.

[Sideband]

You may also wish to check this out http://vintageradio.me.uk/radconnav/constructinfo.htm

Rich.

[Enthusiast]

Many thanks Kalee 20 (especially for guidance saying "a fair rule of thumb is to use about 1/3 of the turns for the reaction coil").....And to Rich (Sideband) - your link provides an excellent practical guide to winding coils.

Best wishes,

David

[Phil G4SPZ]

As an alternative to home-wound coils, you may like to consider using Denco plug-in coils. These were very popular, and many designs used them. You might save a good deal of trial-and-error by getting your circuit working using a Denco coil for one band, then moving on to home-brewing the coils for other bands. The 'green' range of coils were for TRF circuits and had a reaction winding.

The earlier ones were Octal-based but later ones were smaller with B9A bases. I have a few lying around. There is a really good source of information here and you may even be able to glean some data to help with the construction. The formers, as far as I know, all had adjustable dust-iron slugs within them.

Phil

[Enthusiast]

Thanks, Phil - using a "GREEN" range Denco plug-in coil would make sense as a starting point to test the completed TRF circuit ....I've been fighting shy of a Denco approach seeing the prices they fetch at online auction ...and have not seen any on the 'junk" stalls at radio rallies (looked for them most recently at Kempton.) But I'll continue to keep an eye open....

One aim of the TRF project is to be as inventive (and economical) as possible by "making-do-and-mend" re sourcing as many components as possible from my junk-box, winding my own coils etc (Although, I still haven't quite worked out the most practical way of mounting them on an Octal plug for use with an Octal socket - suggestions gratefully received...)

Thanks to the coil winding chart via the link supplied by you and Rich (Sideband) plus the "rule of thumb" re reaction turns from Kalee 20, I feel confident enough to (at least initially) go down the home-wound plug-in coil route.

(It's probably me, but Wheeler's formular made my head spin, so there will be an element of trial and error re coils.....but experimentation is part of the hobby and long winter nights will allow plenty of time for getting coverage of the SW bands I want)

David

[Phil G4SPZ]

Originally Posted by Enthusiast

...Although, I still haven't quite worked out the most practical way of mounting them on an Octal plug for use with an Octal socket - suggestions gratefully received...

Hi David,

The octal base from a broken valve would be good. You could Araldite a plastic tube (the former) of suitable diameter into it. Take care however to select a plastic material that is not lossy at RF. Check by heating a small sample in a microwave oven, alongside a glass of water. If the sample gets warm, it's lossy and won't be suitable.

Others may be able to suggest suitable materials. I've heard of some types of plastic plumbers' pipe being suitable, but again, test a few off-cuts before buying.

Good luck!

Phil

[Enthusiast]

Thanks the for advice on coil-formers, Phil.

One slighly bizarre thought I had was using one or more 25g plastic pepper pots (glued end-to-end for the longer medium wave plus reaction coil) as these (with a bit of packing) fit neatly onto an Octal "plug".

However, not having access to a microwave oven, I can't test the plastic pots for RF losses ....can anyone comment on their suitablility for formers?

I've access to lengths of plastic water-pipe (as used in bathroom plumbing etc) - have any Forum members experience re use as coil formers?

Deep waters indeed.....

D

[Herald1360]

Nothing wrong with bog roll cores for coil formers- especially if nice and dry and treated to a coat of varnish! Easy to make holes in for terminating the windings as well. You might even find some that are a good match for the inside or outside diameter of your old valve bases.......

[val33vo]

You dont need many turns for the reaction tapping I would say that perhaps for a MW receiver 5 or 6 would be ok, the simple answer is just to experiment with the coil to get a smooth jump into positive feedback

regards

val33vo

[Enthusiast]

Thanks Herald 1360 - yes, a BR has many uses - with varnish a useful suggestion.

And thanks also to val33vo. I'll begin with 5 or 6 turns for medium wave reaction and see what results.

I expect - and am looking forward to - a lot of "try it and see". It's probably the best way to learn.

Has anyone on the Forum built this particular TRF circuit? (See link to it in my first post in this thread.

D

[Skywave]

By and large, looks like a sensible cct. design to me. The only change I would try would be to replace the EF91 detector with a 6BR7, since a 6BR7 has a very low slope (gm) of about 1.25 mA/V as opposed to 7.5 mA/V for an EF91, and it's been my experience that the lower the gm in a reacting detector, the easier it is to obtain smooth reaction. Note that the pin-outs for an EF91 and a 6BR7 are different, although they are both on a B9A pin-out. The differences in the other parameters are not significant in this application (but the 6BR7 does draw substantially less HT current for a grid bias of about -2.5 volts).

Al.

[Enthusiast]

Wise words (as always), Al.

All noted re substituting a 6BR7 as the detector and thanks for the warning re the pin-out differences.

Best wishes,

David

[Skywave]

Following on from Phil's suggestion (Post #7), re: testing the suitability of a material by using a microwave oven, I have just tried that on a piece of circular plastic downpipe that is used with plastic guttering systems: it passed the microwave test.
This pipe has an outside diameter of 7 cm., which I realize is larger than the plastic plumbing pipe as referred to above, but I would suggest that this downpipe material would probably make for a better coil since the ratio of diameter to length of the resultant coil will be greater. It's my understanding that this will produce a higher Q. It's noticable that the early designs of crystal sets always favoured coils with relatively large diameters - similar to what this pipe provides.

Just a thought.

Al.

[Skywave]

This has recently arrived:

https://www.vintage-radio.net/forum/...046#post484046

Note the diameter of the coil.

It is relevant to this Thread.

Al.

[Enthusiast]

More useful info., Al - many thanks.

The Forum link you supplied provides fascinating insights into coils and I take your earlier point about ratio of diameter to length re formers.

If nothing else, the TRF circuit might provide a useful test-bed for home-wound tuning and reaction coils....

David

[Skywave]

David,
In any "R.F. project", the design and choice of the coils is - and has always been - the biggest hurdle for the home constructor to overcome. And that's why companies like Eddystone (in the early years) and Denco (later) - there are many others - did so well in this market. For the home-constructor, there are number of choices. (1) Buy a ready-made coil; (2) design your own using standard formulae and test equipment; (3) the 'cut-and-try' D-I-Y approach. This last route is effectively what you appear to be suggesting by the comment in your last post ~ and that's just fine ~ provided you have plenty of time and plenty of wire. Personally, I prefer option (2), but that does assume the necessary degree of comfort with the mathematics and the availability of the necessary test equipment - such as an R.F. bridge, to measure the inductance of the resultant coil. Now although this route may involve an expense in time (getting to grips with the maths, etc.) and cost (possibly buying equipment), if you feel that your interest in 'D-I-Y Radio Projects' is more than a passing flirtation, that is the route I would recommend. In the long-term, it's quicker, cheaper, you gain a proper understanding of 'coils' and can proceed to design more advanced projects with confidence.
Please do not mis-understand me: I'm not criticising your implied approach - merely trying to give you the benefit of my experience - and probably that of others, too.

Whatever route you take, I wish you good luck with your coil experiments.

Al.

[Neil Purling]

Enthusiast: Did you get anywhere with this project?
I tried this radio with a Repanco DRR2.
I think that the article was written around using a very common valve you could get ex-equipment.
Tuning was like standing on the fulcrum of a see-saw and trying to keep the beam absolutely level.
The little sod was also rather unstable, the idea of using a different valve sounds like a very good one. I think potentiometer reaction control to be far better.

[AC/HL]

Unfortunately, the OP hasn't logged on for over two years.

[space charge]

in the current edition of pw there is a multiband regen based on an f grayer original with improvements by stef niewiadomski. the coil details are all there in exquisite detail .regards EI7KA.