Valve FM Receiver - Tuning Range

[E93AFAN]

I am thinking of building the FM tuner shown below, I have Denco IFTs, RDT and coil formers purchased years ago with the intention of building the receiver and after many years in storage, waiting until family demands allowed me time to use them, their time has come.

However, on carefully reading the article in PW of January 1960 it dawned upon me that this receiver was designed to cover the 3 original BBC stations Light, Third and Home grouped around 88 - 95 Mhz, whilst in 2023 coverage needs to be 88-108 Mhz.

I have calculated the inductance of L2 and L3 using an online calculator, see coil winding data and L3 calculation attached. However, the calculator deals only with coils without slugs, so I cannot establish a sensible value for L1 and the usual rule of thumb that applies at 10 Mhz of "knocking off a turn" won't wash.

Entering 0.0686 uH for L3 into the Professor Coyle website (the only source I know) translates to 121.535 Mhz with C10 at 25pf and 192.163 Mhz at 10pf, clearly the calculation is incorrect or my understanding of how the circuit is affected by R5 and C8 is wrong (probably the latter !) So, four questions arise:

1. Do the coils as specified have latitude up to 108 Mhz (I'm guessing not) ?

2. What steps can I take to accurately alter the coils such that they will provide extended coverage assuming the answer to 1. above is "no" ?

3. Do I need to change the value of C9 and C10 to accommodate the wider frequency range and will this affect the values of R5 and C8?

4. Given that in the alignment procedure L1 is simply peaked at the centre of the 3 station group covered in ones reception area should I tune the grid of V1 by using a three gang capacitor to replace C9 and 10 ?

I'm sorry if these seem stupid questions, I have successfully built > 15 am valve and solid state superhets in the last 50 years but this is my first foray into VHF and in spite of "doing my homework" to gain a working knowledge of FM principles I remain daunted. Clearly the principles of operation and working tolerances at 100 Mhz are considerably different than amplitude modulation at 10 metres, which thus far has been the highest frequency at which I have operated.

Thank you as always for your patience and I would welcome any input you might have to offer.

[Radio Wrangler]

Originally Posted by E93AFAN

I'm sorry if these seem stupid questions, I have successfully built > 15 am valve and solid state superhets in the last 50 years but this is my first foray into VHF and in spite of "doing my homework" to gain a working knowledge of FM principles I remain daunted. Clearly the principles of operation and working tolerances at 100 Mhz are considerably different than amplitude modulation at 10 metres, which thus far has been the highest frequency at which I have operated.

Very few questions are stupid. Not asking is always a worse option. Some answers can be stupid, but asking questions is safe

Doing an FM tuner with valves is quite possible, but getting good performance is not easy. The EF91 will be significantly down on gain at that frequency, and also rather noisy. Noise usually impels the use of triodes. To get decent gain out of a triode stage means either grounded grid mode for moderate gain, or neutralisation for higher gain.

You'll be used to halves having a gain-per-stage advantage over transistors, but at these frequencies, it's gone unless you use elaborate techniques which negate the expected simplicity.

If you want to dip your toe into VHF construction, then maybe starting with a transistor-based design would be easier and result in something more sensitive and lower noise. The small size and close connections minimise strays.

There's the classic Nelson-Jones design, and the later one in PW. Dual gate MOSFETs help make things easier and are still available, though semiconductor makers have been dropping many types.

Another advantage to solid state is cool running and lower LO drift as a result. Thermal drift isn't bad at medium wave, becomes a problem at shortwave and is a serious concern at VHF.

If I had to do one with valves, I'd probably go for a cascode triode front end with a TV tuner valve and a long-tailed pair as mixer. IF is easier IFTs will need swept adjustment as will the discriminator.

You're entering a new world. It's not harder, but it's different and you need different approaches.

David

[chriswood1900]

If I were to tackle an FM tuner, I think I would approach it in stages. Build the valve IF strip and use an off the shelf tuner unit like the Mullard LP1186 or a similar unit borrowed/salvaged from an FM radio. This would allow you to get a good working IF strip without having to worry about if the tuner was working and then try different tuner designs along the line of Davids suggestions. Whatever you do I’m sure it will be a fun and educational experience.

[Silicon]

The Nelson-Jones FM Stereo tuner seems to have first appeared in the April 1971 issue of Wireless World. That was a 60th anniversary issue with several articles on radio history.

A modified version with varicap tuning appeared in the June 1973 issue of Wireless World.

[Sideband]

To be honest and it it was me, I would look at a more up-to-date circuit. I've just looked at the entire article courtesy of the American Radio History web site. It was probably a great little set in its day (63 years ago)! It would have been fun to build and had I been older (I was 8 in 1960) I would probably have done so myself. There were only three stations generously separated by 2.2Mhz. Selectivity wasn't really a problem. Nowadays with much improved circuitry, ceramic filters and high Q coils etc local stations (at least around here) are separated by as little as 400Khz and 500Khz (104.9, 105.4 and 105.8 are used by three locals). Even if you could easily modify the existing circuit to 108Mhz, you will never be able to improve the selectivity sufficiently to cope with 400Khz separation. I've got a couple of valve sets that tune to 108 and they all have problems with the close-spaced locals including the Leak Troughline that I use regularly.

It's up to you of course and as it stands it will be fine but rather limited.

[E93AFAN]

Thank you for your patience and insight, maybe before I spend considerable time and effort following my desire to build an FM receiver I should stop and think again about my approach.

My rationale for clinging to valves was that I am familiar with their vagaries, plus I have boxes of them doing nothing, a set of IFs and a RDT. But clearly I have only known the am HF world and I had for instance not stopped to think about selectivity, even though as Sideband points out separation is now as little as 4/500 khz, I can clearly understand that would be a challenge for a 63 year old design.

I am starting to appreciate that a solid state approach might be a better route to travel. Somewhere I have some 40673s, MPF102s and a number of BF***s, I think my best approach in the short term is to build some simple receivers, looking quickly there are several in RC and PW from the late 70's and where ICs are used largely they remain available. This route will enable me to gain confidence in my abilities before tackling something more complex. In the longer term the idea of an LP1186 appeals, I shall look out for a suitable donor and if I can find one give this solution a shot in due course.

You are never to late to learn and since returning to the hobby during lockdown at age 75 after a 30 year absence I have (re)gained much knowledge, so I shall welcome the challenge. Many thanks again for your support.

[Sideband]

I have a Stella valve radio that tunes to 108Mhz but it really struggles with some of the locals especially with 400Khz spacing. It's even worse with the Leak tuner that has an AFC button.....press that and you might find it selects the next station only 400Khz away.... Fortunately the stations I mainly listen to are well away from each other.

[Cruisin Marine]

If you do a search online you will find some nice articles on HQ FM tuners to build (all transistorised) in old copies of the Ambit Catalogues/ Magazines.
They may also have been covered in R&EW.

They were fond of dual gate mosfets.
The IF was called a 7020 from memory and featured a CA3089/3189.

Found a link https://worldradiohistory.com/UK/Mis...Ambit-Catalog/

[kalee20]

It's also on my own to-do list sometime.

If you have IFT's and RDT, plus appropriate valves, you can at least get yourself an IF amplifier and discriminator. That's probably the easy bit.

Where you get your 10.7MHz from is another question... as a starter, you could plunder a 1970's or 80's transistor radio and tap off the IF from there, at least as a start.

Making the front end is likely to be the hardest bit. I'd try and copy a VHF TV tuner, using similar valve pair (cascode double triode plus triode-pentode frequency changer). I don't think I'd try a single triode of the PC900 variety, but it's worth considering.

Getting good performance yourself is only part of the problem - as a good citizen you'll also be interested in minimising local oscillator radiation from your aerial, and that's something else to watch for!

[Ed_Dinning]

Hi, there was an extensive posting by Regenfreak a few months ago on FM front ends. You may find some inspiration in there.
With your IF stage you could then (as previously suggested) look at a hybrid design with a silicone front end

Ed

[Radio Wrangler]

Originally Posted by kalee20

Getting good performance yourself is only part of the problem - as a good citizen you'll also be interested in minimising local oscillator radiation from your aerial, and that's something else to watch for!

It's usual for superhets to run their LO on the high side of the received station frequency.

As the FM band is now populated above 100 MHz, receiver LOs will often go above 108MHz

Guess what uses frequencies from 108 to 118 MHz?

ILS; Instrument landing systems and automatic landing systems at airports. So now you know why there was a gentleman's agreement to not use the upper 10MHz of the FM band. Unfortunately, once the government had learned that megaHertz equals megabucks, money forced the issue of new licences.

Let's say that it's one of the few areas enjoying fast service from ofcom when any interference is noticed. 100% of reports get investigated.

Avoiding stray emissions is a very good idea.

David

[ms660]

Originally Posted by Radio Wrangler

It's usual for superhets to run their LO on the high side of the received station frequency.

It wasn't always the case, some VHF receiver manufactures (eg: Bush) had the LO below the carrier frequency, to prevent harmonics upsetting some band III users.

Lawrence.

[Sideband]

Originally Posted by Radio Wrangler

Guess what uses frequencies from 108 to 118 MHz?

ILS; Instrument landing systems and automatic landing systems at airports. So now you know why there was a gentleman's agreement to not use the upper 10MHz of the FM band.

Avoiding stray emissions is a very good idea.

David

That's interesting! Straying off the original subject slightly (sorry) I also wondered about the upper end of FM and the aircraft band. I understood that there was a 200Khz guard band at the beginning and end of an FM carrier making a minimum 400Khz 'gap' between adjacent carriers. In our area there is a local station on 107.8 which I thought would be the absolute allowable maximum frequency for broadcasting allowing for the 200Khz guard band which would extend to 108Mhz. I was very surprised when I was looking at used FM frequencies to find one local up north somewhere on 107.9 admittedly on low power but surely this 'bends' the rule slightly?

[Radio Wrangler]

You would not believe the difficulty and testing needed to get an aviation NAV receiver design to adequately reject 107.9 while trying to receive a localiser signal on 108.1. You've only got 200kHz and you need to work with signals in the -90dBm region with good enough freedom from interference to meet navigational accuracy requirements.

One high up the FM band transmitter has a mast close to the coast, with the main approach to a large airport going right past it. Turning onto and acquiring localiser takes them almost over it. So yes, the worst case scenario does exist.

Doing a straight forward FM broadcast receiver looks quite inviting after that.

David

[Jez1234]

I have a couple of Rogers valve FM tuners which have a 3 way switch marked Home, Light and Third... There is an "IFT" near the front panel which has a knurled plastic knob sticking out the top and is the actual tuning control. IIRC all valves are ECF80's.

I would still go the valved route personally but using considerably more advanced circuitry than the 1960 magazine article (if wanting a simple but novel valve FM tuner though then check out various "pulse count detector" units by M G Scroggie and others in Wireless World late 50's).

The best of the valve tuners were very good indeed and up there with the best of today's offerings... but non were British (no UK made valve FM tuner was good enough for stereo unless in an exceptionally good reception area IMHO. And that means lack of multipath as much as good signal strength). USA companies Marantz, Scott and Macintosh showed how it should be done, but would all have been in the "if you have to ask how much it costs..." price range!

I have an interesting hybrid FM tuner made by Kenwood/Trio which uses a valved front end (Nuvistor and a double triode) but the rest is germanium transistors. RF performance is good but unfortunately like so many tuners of the time the crude stereo decoder rather spoils things by making it all sound a bit mushy and lacking top end when switched in (not as bad as the Mullard one fitted to the Troughline stereo which makes FM sound like AM!).

[Radio Wrangler]

It wasn't a case of British FM tuner designers being incompetent, their produce was as good as it needed to be and they kept prices down to levels where they dare publish them.

In the UK, the broadcast band was heavily managed, sparsely populated and planned to keep RF selectivity and overload requirements modest

America was much more of a free-for-all. Toners had to have great dynamic range for DXing stations in other cities while withstanding the power of local ones.

The UK had all the stations serving your area on the same site so signal levels were roughly equal, and any distant stations were mostly dupes of what you already got locally.

Europe was more like America, and so the Revox tuners for example do quite well.

That ws then but this is now and the UK has a heavily [populated band, radiated from transmitters in many more locations. We now need American style performance. 3 IFTs won't cut it for selectivity. Simple front ends lack dynamic range.

David

[Boulevardier]

Originally Posted by Jez1234

if wanting a simple but novel valve FM tuner though then check out various "pulse count detector" units by M G Scroggie and others in Wireless World late 50's).

It's always puzzled me that those very simple pulse-counting tuners didn't seem to have much take-up amongst amateur constructors...

Mike

[Radio Wrangler]

They weren't taken up by constructors, but they do appear in a few respected commercial models where they give a good showing.

Perhaps the ease of use of the CA3089 and its offspring overshadowed them?

One of the commercially-made tuner sitting has a very elegant pulse-count discriminator where the pulse width is timed by a length of coax as a delay line.

It seems that the ratio detector was seen as good enough and the quadrature similarly but easier and that the benefit of more advanced discriminators was not that obvious to most of the market. They would only appeal to those who either knew that they were there and what they did, or to those with the gear to measure performance beyond what was immediately obvious to the general public listening to programme material.

If I wanted to design an FM radio, I'd be inclined towards something not available as an existing model. Both my tuners ar design classics which I've fixed/restored. I couldn't afford them back in their day. But that's not what the original poster is looking for. He said he wants to cut his teeth on FM radio and have a go at building one to learn from.

This is where the standard 10.7MHz shines. He can build something modular. A few lumps of PCB with coax cables linking them. Try several front-ends, several filter/IF boards, several discriminators and several stereo decoders.

Build one type of each function, get a radio going and then play around building alternatives for each function as limitations are found.

There is a lot that can be learned from this and approaching it for the first time can be fun. The front ends will teach 'VHF construction technique" and then you're into finding the limitations of ceramic filters and into filter design techniques and the importance of linear phase ( = constant group delay) characteristics for handling FM signals. Then limiting IF amplifiers are quite unlike anything suitable for AM. Discriminators have already been mentioned.

Circuits for Quad FM3 as an archetypal classic British design, and a Revox B261 are easily found and show the differences between basic but competent design and something a bit special. The signal handling stages of either can be made with currently available parts, but the synthesiser in the B261 is something I wouldn't duplicate, things have moved on and its devices aren't so available.

With a modular approach, some section could be done with valves to explore their different techniques and limitations. Learning ought to be fun.

The end goal of all the effort is knowledge, won first-hand and unforgettable, not a tuner. If the goal was just a tuner, fixing something from someone's round tuit pile would be far more efficient and avoid a lot of learning.

I think we are still in a window of opportunity as far as availability of things like dual gate MOSFETs and ceramic filters, but they are vanishing from catalogues. A really high performance design would probably use neither but they're needed to build something teaching the trade-offs of classic FM techniques.

David

[Wellington]

Originally Posted by E93AFAN

I am thinking of building the FM tuner shown below, I have Denco IFTs, RDT and coil formers purchased years ago with the intention of building the receiver and after many years in storage, waiting until family demands allowed me time to use them, their time has come.

As this project has been waiting for a long time, perhaps build it as specified, accept its limitations, enjoy it, and then move on…

At least then you will have the satisfaction of fulfilling a long-held goal, leaving your way clear to learn about/construct the more modern designs.

[kalee20]

Originally Posted by Radio Wrangler

As the FM band is now populated above 100 MHz, receiver LOs will often go above 108MHz

Guess what uses frequencies from 108 to 118 MHz?

ILS; Instrument landing systems and automatic landing systems at airports.

Let's say that it's one of the few areas enjoying fast service from ofcom when any interference is noticed. 100% of reports get investigated.

Useful info, David!

I put up my post because a few superhet FM receivers don't have an RF amplifier - the aerial goes straight into the oscillating triode frequency changer. The Ever Ready Sky Monarch is one (it's actually a triode-connected pentode but that's just a detail). So there's no isolating buffer stage.

The alignment instructions specify the need for an RF voltmeter, connected to the aerial at one point while a neutralising capacitor is tweaked for minimum reading of the LO.

It's a single-point adjustment, and the tuning range only goes to 100MHz anyway. But for anyone making an FM front end having a larger range, the 20MHz from 88-108MHz I'll bet that neutralisation would fall off rather more towards the edges of the band! The lower end doesn't matter much, except perhaps to another Band II listener, but the higher end, as you point out, could be much more serious.

Even the 'classic' ECC85 front-end circuit, with self-oscillating triode preceded by a grounded-grid triode, has neutralising, albeit usually fixed - I guess it was enough of an issue that manufacturers found there was enough LO going backwards through the RF amplifier and to the aerial, to guard against it.