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Homebrew Equipment A place to show, design and discuss the weird and wonderful electronic creations from the hands of individual members. |
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11th Apr 2021, 8:28 pm | #21 |
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Re: DIY FM tuner with 6CW4 nuvistor and ECC85
By Jove, he's got it! (Rex Harrison, wasn't it?)
DAvid
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11th Apr 2021, 8:37 pm | #22 |
Heptode
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Re: DIY FM tuner with 6CW4 nuvistor and ECC85
I was not yet born when that film was released. A few years later, I soiled my nappies...
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26th Jul 2021, 6:20 pm | #23 |
Heptode
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Re: DIY FM tuner with 6CW4 nuvistor and ECC85
Just a quick update: I have come back to the FM valve tuner project after messing around with HF SSB receivers for a few months.
I built an oscillator test rig to test the performance and stability of the VHF valves:ECC85, 12DT8, 6BS8 and EC81 (Russian 6C11) as an oscillator. They all work fine on the test rig. The EC81/ 6C11 is an excellent UHF triode rated up to 750MHz. It gives completely flat oscillator output across the full range of frequencies when it was measured with a RF probe. Both the 6CW4 RF amp and ECC85 mixer/Rf AMP work well. In addition, the air gang tracks perfectly. However, I cannot isolate the root cause of ECC85 not oscillating well in the tuner, in spite of hours and hours of debugging. Therefore I started from a clean slate by building a minimalist 4-valve FM tuner with the line-up: RF amp and mixer: 6N3P (USSR valve commonly used in FM and TV, the double triode has low noise and high transconductance.) Oscillator: EC81 (6C11) 1st IF amp: 6BA6 2nd 1st amp/limiter: 6BA6 detector: ratio detector with diodes The tuner works very well with good selectivity and sensitivity using an balanced ribbon indoor antenna. The IF transformers and ratio coils were stripped from a broken japanese FM radio. The dual air variable tuning gang were obtained from ebay. I think it was British made and was designed to track between 88MHz-100MHz. I was able to make it track perfectly from 88MHz to 108MHz after solving the compatibility mathematical conditions for the matched trimmers. Overall I am happy its performance as a mono tuner. At the moment, I am working on my next project; a stereo tuner with the line-up: Cascode RF AMP: EC88CC Oscillator Ec81 (6C11) mixer: 6CB6A 1st IF: 6BA6 2nd IF: 6BA6 1st limiter/3rd IF: 6AU6 2nd limiter: 6AU6 I will use a discriminator coil and broadband IF transformers to maximise the stereo audio fidelity. I omit AFC because it is not required in open chassis in which the temperature change is small. The 6C11 oscillator frequency is very stable. |
27th Jul 2021, 12:11 am | #24 |
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Re: DIY FM tuner with 6CW4 nuvistor and ECC85
ECC85 have a nasty reputation for not lasting long and their gain falling. Usually the first thing to go is the ability of oscillators to start.
On the other hand, the number of double triodes suited for cascode use because of heater-cathode insulation for the running voltages of the upper triode, and also the need to keep cathode-heater capacitance down to prevent horrible coupling problems, is rather limited. I stopped using valves for any new work a long time ago. The only valves being made revolve around hifi people and guitarists with fat wallets. Radio-type valves aren't being made and I see them as a dwindling resource, so I choose not to use any of them up and leave the remaining ones for servicing period equipment. Sometimes transistors are the right answer. They aren't products of the devil, they aren't harder to use. They're just different. You need more of them than you'd need valves to do the same job, but they're small, cheap and light on power so you can use as many as it takes. Like valves, they have limitations, so you have to design ways of evading them. The limitations are different to the limitations of valves, so there is culture-shock involved, but once you see the patterns in the different limitations of different devices it all suddenly becomes easier to undertand and easier to handle. It's all a lot simpler than it looks, once you get up the learning curve.y As a piece of, um, shall we say micturition-artistry, I designed an audio power amplifier using as many transistors as possible but subject to the requirement that I could show that each and every one of them was doing something useful. It worked rather well and was a good joke with a friend who was designing a minimalist one. Bill's amp turned into a product at Linn (of Sondek fame) after he moved there. A small run of mine were built at HP. My prototype has been sitting in the corner of my lounge for 40 years. It's the size of a good picnic hamper. David Oh, should have said, I have two FM tuners I'm playing with, one is one of the last Sony 'ES' series, the other is more interesting, a Revox B261, the manual is available on-line. This takes very different approaches. LC filtering and no ceramic filters. separate stages of limiting in the IF. An analogue pulse-count discriminator, and stereo decoding using several general-purpose ICs, not the usual one-chip wonders.
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27th Jul 2021, 12:46 pm | #25 | ||||
Heptode
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Re: DIY FM tuner with 6CW4 nuvistor and ECC85
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In my case, the ECC85 is oscillating happily in my test rig but its output voltage vs frequency response is non-uniform. The components layout of FM tuner is very critical and an art form itself. i had a number of theories why is not oscillating in my first tuner but all of them have been proven wrong. The ECC85 would only oscillate if I connect a crocodile clip with a lead to the anode which is very puzzling for me. Quote:
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27th Jul 2021, 5:27 pm | #26 | |
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Re: DIY FM tuner with 6CW4 nuvistor and ECC85
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27th Jul 2021, 6:07 pm | #27 |
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Re: DIY FM tuner with 6CW4 nuvistor and ECC85
27 plus a couple of 3-terminal regs and four power supply rails. Sure I could have squeezed a few more in, but then some wouldn't have been quite so easy to say had necessary purposes. That's per power amp, of course.
David
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30th Jul 2021, 5:59 pm | #28 |
Hexode
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Re: DIY FM tuner with 6CW4 nuvistor and ECC85
I have been reading this with a great deal of interest. Back in the early 60s I came across an article in the American Magazine "Radio Electronics April 1953" for a Long Distance FM tuner. having nothing better to do I decided to have a go at building this (fools rush in etc..what did I know about FM radio construction) My boss at the time, Bob, reckoned that we could do it together. It took about 6 months as we had to import a couple of valves from the US.
After much faffing about getting it stable (lotta gain built in) we finally had it running very satisfactorily. Its performance was quite startling, no good for FM stereo as IF bandwidth was quite narrow, but with the proverbial piece of wet string as an aerial, any signal passing was sucked in. When connected to a serious 8 element FM aerial long distance continental FM was eminently possible especially with Sporadic E conditions. I, in particular, learnt a great deal about RF earthing, component, positioning, screens across valve bases which has stood me in good stead in my later life, especially as I am now getting quite good at repairing/aligning Quad FM tuners. A bit of a preamble for "regenfreaks" comment re an IF strip etc. I would think that a Quad valve FM IF strip would fit his purpose very well as it is complete strip with the mixer IFT mounted on it ready. It should be possible to find a valve FMT to butcher, and I think I may have one somewhere, will have a look and post back here If I find it. Next thought I am pretty sure that the article will be in the American Radio Magazine website. John Last edited by John Caswell; 30th Jul 2021 at 6:07 pm. |
30th Jul 2021, 6:05 pm | #29 |
Hexode
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Re: DIY FM tuner with 6CW4 nuvistor and ECC85
Just had a look at the site, now called "worldradiohistory.com" yes it is there April 1953 and I also found the Quad FMT so if you want it "regenfreaks" PM me.
John |
30th Jul 2021, 7:54 pm | #30 |
Dekatron
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Re: DIY FM tuner with 6CW4 nuvistor and ECC85
As far as front-ends are concerned, it could be worth looking at the circuits used on early Pye mobile-radio stuff [Valve-receiver Vanguards/Cambridges from the very-early-60s]. These had to be RF-tough - particularly the 'police' versions operating around 96-101MHz when there were nasty broadcast-stations running 100Kw or so less than 10MHz away.
I built various 144MHz converters back in the early-70s; a 6CW4 Nuvistor - neutralised - was the go-to front-end back then. One thing we learned was that maximum RF gain and lowest noise-figure did not coincide: HT voltage was significant - the 6CW4 seemed to be happiest with a mere 80 Volts anode voltage. I also used the A2599 - http://www.r-type.org/exhib/abn0002.htm - and A2521 - http://www.r-type.org/exhib/abn0004.htm - with success - though it has to be said the appearance of Texas Instruments' GM0290 germanium VHF/UHF transistors [and the TV-tuner-oriented AF139/239] made life a bit easier. Last edited by G6Tanuki; 30th Jul 2021 at 7:59 pm. |
31st Jul 2021, 11:44 am | #31 | ||||
Heptode
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Location: London SW16, UK.
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Re: DIY FM tuner with 6CW4 nuvistor and ECC85
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In the USA, 6BS8, 6BZ7 were used in some high ended tuners. For british design, the Troughline Stereo III used ECC88 in casode. Quote:
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Last edited by regenfreak; 31st Jul 2021 at 12:04 pm. |
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31st Jul 2021, 2:38 pm | #32 | |
Heptode
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Re: DIY FM tuner with 6CW4 nuvistor and ECC85
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https://worldradiohistory.com/Archiv...cs-1953-04.pdf It is a badass project that would require a 5 gang variable capacitor, 6BZ7 cascode with untuned, broadband front end, a rather unusual 6J6 push-up tuned RF amp, 6AH6 tuned RF amp, 6C4 oscillator and 6AB4 mixer. The extra 6AH6 pentode would add lots of gain for DX in expense of more noise. Then there are six IF transformers (back-to-back capacitive-coupled double-tuned IFs) and a cascaded 6AU6 limiter!!! Last edited by regenfreak; 31st Jul 2021 at 2:50 pm. |
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31st Jul 2021, 5:06 pm | #33 |
Hexode
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Re: DIY FM tuner with 6CW4 nuvistor and ECC85
Well yes, I agree with all you say, but I wouldn't call it a badass project, at the time it was pretty hot tuner which was really what we were interested in.
Took some effort to tame it, poor construction techniques on my part, I was just an apprentice, but soon got my act in gear, after much ear bashing both figuratively and literally, those were the days!! I didn't offer the info for you to build it, so much as ideas. For instance we made the tuning gang out of 5 individual ex gov variable caps with flexible/isolating couplers between each stage to avoid common coupling through the shafts, all sorts of filter chokes for the heaters, IFT were easily available, as was pretty well everything else. For me a very worthwhile exercise in techniques and construction. John |
31st Jul 2021, 5:51 pm | #34 |
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Re: DIY FM tuner with 6CW4 nuvistor and ECC85
Amateur radio publications used to talk of 'VHF' construction techniques and UHF ones as distinct from normal radio construction as practised at HF.
But they failed to explain that the construction technique was NOT a function of what frequency you wanted it to work at. Rather it was a matter of what frequency range did you want to ensure it didn't oscillate in. An unstable stage has the remarkable super-power of mucking up your wanted reception no matter what frequency it takes off at. So the choice of construction style comes down to what frequency ranges does the active device have the ability to self-oscillate in? In a conversation with Ian, G3ROO, a very experienced and excellent home constructor, he was discussing buying a spectrum analyser. He'd said that it didn;t need to go much above 30MHz, because he didn't build things for any higher. I explained that the purpose of such an instrument is to confirm that something is only creating the intended signals, and that the analyser therefore must cover enough frequency range that you can be comfortable that there is unlikely to be unwanted stuff above its capability. Even HF stuff can take off at hundreds of MHz, so an analyser covering to 1GHz or so is sensible. Your circuitry must not only succeed in doing the things it is supposed to do, it must also fail to do all the things it must not do. This latter is the bigger deal. Almost infinite unless you can justify some limitations. DeMorgan doesn't quite fit RF logical thinking David
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31st Jul 2021, 6:17 pm | #35 | |
Heptode
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Re: DIY FM tuner with 6CW4 nuvistor and ECC85
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31st Jul 2021, 7:59 pm | #36 |
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Re: DIY FM tuner with 6CW4 nuvistor and ECC85
Even split into many stages, you still wind up needing the same amount of gain in the IF.
Multi-superhets can evade the problems associated by leakage around high-gain strips by choosing to have some of the gain at one intermediate frequency, and the rest at the other. Any feedback by leakage from the overall output to the overall input is on the wrong frequency and causes no trouble. It's like the frequency-shifting howl-round suppressors used on PA systems, though there the shift is rather small. In designing a receiver, gain is cheap, dirt cheap nowadays. Selectivity is expensive and screening is very expensive. Designing a receiver to receive a chosen signal isn't particularly difficult. It's designing it to not also receive everything else at once is where it gets difficult. David
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31st Jul 2021, 9:04 pm | #37 | |
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Re: DIY FM tuner with 6CW4 nuvistor and ECC85
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1st IF: 55.845 MHz 2nd IF: 10.7 MHz 3rd IF: 45 kHz 4th IF: 128 kHz My Tecsun R-9700DX has dual conversion and is one of the remaining few analogue multiple -band receivers that is still in production and it is considered to be a strong performer in term of FM selectivity and sensitivity |
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31st Jul 2021, 11:54 pm | #38 |
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Re: DIY FM tuner with 6CW4 nuvistor and ECC85
A lot of receivers nowadays use I/Q mixdown from the incoming signal frequency into a pair of 1-pole lowpass filters, a bit of variable gain and then a pair of ADCs. ADI and all the usual semiconductor firms have had products out for some time. Last I looked, the Lime chips seemed amongst the best.
BUT The dynamic range is rather limited. The synthesisers with on-chip VCOs and fairly basic Frac-N technology are noisy. Quite a bit of incoming bandwidth hits the ADCs. Also the cancellation of racing I and Q pthes against each other is limited. Fancy calibration helps, but not enough. The HF transceiver currently set up in my radio shack up-converts to an IF in the 60MHz region (carefully more than twice 30MHz) where it has three sets of crystal filters (AM/FM, SSB, CW) then it mixes down to just above audio for its single ADC. Things do NOT go I/Q in the analogue sections, that happens only in nice, precise, digital processing. The synth is a multiloop one. There is a preselector with switched inductors and a switched C, 2C, 4C, 16C etc network. All switched by relays! David
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Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done |
31st Jul 2021, 11:57 pm | #39 |
Heptode
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Re: DIY FM tuner with 6CW4 nuvistor and ECC85
Just come across the schematic of MARANTZ 10B stereo FM tuner:
http://zilla.li/Resources/PDF/Marant..._Schematic.pdf It has EC88 in cascode, six sets of three-pole Butterworth filters to replace conventional double tuned IF filters (see attachment) and CRT scope, Wow!!!! The center LC element of the Butterworth has a shunt resistor to widen the bandwidth for FM stereo, I guess. |
1st Aug 2021, 12:13 am | #40 | |
Heptode
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Re: DIY FM tuner with 6CW4 nuvistor and ECC85
Quote:
https://www.arrl.org/files/file/Tech...0708qex013.pdf |
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