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Forum Free Registration Closed
Granada Television Brochure, 1970s
Long Gone UK TV Shops
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PYE Australia Circa 1971
Radios-TV VRAT
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Thorn TX10 Chassis
Crusty-TV Museum, Analogue TV Network
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Remove Teletext Lines & VCR Problems
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The one that got away
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1969 Philips G22K511
Memories Of The TV Trade
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Dual Standard and Single Standard CTV’s
Radios-TV on YouTube
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Minimalist superhet project restarted.
Remember the Minimalist Superhet Radio topic on the forum?
Well, today I took delivery of a Bakelite cased TRF radio set, unfortunately although well packed the tuning dial glass is broken. I'm sure something can be done about that, a new piece of glass can be cut locally, so no problem about that.
It'll be an easy task to return the TRF receiver to working order. However, this set is the ideal candidate for completing the minimalist superhet project. The W61 RF amplifier will be replaced by a suitable frequency changer valve, there's plenty types to decide on. For starters all the valves in this set are 6.3volt octal types. As it is now: W61 RF amplifier, 6SJ7 detector and 6V6G output. So it's out with the W61 and fit a 6K8GT. The 6SJ7 low slope AF pentode will be replaced by a 6AC7 high slope RF pentode. The 6V6G will be replaced by a 6V6GT or maybe a higher slope type.
Alternatively, replace the whole lot with miniature B7G or B9A valves. For example: 12AH8, X79 etc.
Get rid of that SenTerCel HT rectifier. Fit valveholder and use 6X5GT or miniature 6X4.
Good project? The prototype receiver works extremely well, that's been proven by many other members of the old forum.
Till Eulenspiegel.
I have just looked at the thread in the old forum, which circuit are you thinking if using, I found a few different ones, well at think they were different.
Looking at the Ninnolo circuit, any views on mixing the U series and E series valves requiring special taps on the mains TX? The diode symbols are the wrong way around but there was a post about old conventions. It's a very long thread so there may be an explanation in there somewhere.
Frank
Frank
Hi Frank,
the development "mule" morphed into an AC/DC mains receiver with delayed AGC, perhaps the design was becoming in the end too sophisticated. See attached circuit diagram.
The link to the old topic: http://www.forum.radios-tv.co.uk/viewtopic.php?f=3&t=6469&start=50
The valves employed were: UCH42 frequency changer. 10F1 anode bend detector. UL84 output and UY85 HT rectifier.
The detector works on the near starvation principle, rather like the EF86 in the Mullard 3-3 amplifier, maybe a bit less than that. The anode bend detector imposes little damping on the preceding tuned circuits making it possible to achieve good selectivity with only two IF tuned circuits. Early Philips "Superinductance" radios employed this type of detector.
Till Eulenspiegel.
OK on the anode bend detector, not used a lot due to requiring the expense of a choke I presume.
Was the OA70 delayed AGC diode an after thought?
Frank
Frank
Till and I worked on this for a few weeks and in the end my set consisted of an ECH81 frequency changer , EF80 anode bend detector and.......an ECC82 as AF amp and output....and it worked extremely well. I was limited by the use of an extremely small mains transformer, the idea being that it should cost only 'junk box' parts. The LT was 12.6 volts so I used a series - parallel arrangement for the heaters, the ECH81 and EF80 being in series and the ECC82 being used in 12.6 volt mode directly across the LT winding. Despite not having an IF amplifier, it worked very well indeed. The ECC82 was overdriven by the preceding AF amp so I strapped it as a diode and used it for AGC instead.....even better and the single triode output stage gave more than enough output for a small room.
I don't have it any more although it could be re-built. I used the transformer for a twin ECC82 output transformerless headphone amplifier.
Sideband's success outlined above makes me wonder if a 2x ECH81 set could be made (given that the ubiquitous triode-heptode must be one of the commonest "pulls" of all time....) using the heptode of the second valve connected as anode-bend detector and the triode as AF output. Or maybe t'other way around! There were a few continental sets that used the ECH21 as IF amp and 1st AF amp, taking advantage of triode grid/heptode modulator grid separation and cutting bottle count by one. I think possibly a few car radios also dual-purposed similar triode-heptodes where compactness was an obvious motivating factor. Junk-box Ge diode if AGC needed?
turretslug said
Sideband's success outlined above makes me wonder if a 2x ECH81 set could be made (given that the ubiquitous triode-heptode must be one of the commonest "pulls" of all time....) using the heptode of the second valve connected as anode-bend detector and the triode as AF output. Or maybe t'other way around! There were a few continental sets that used the ECH21 as IF amp and 1st AF amp, taking advantage of triode grid/heptode modulator grid separation and cutting bottle count by one.
Indeed, 'twas the basis of the Kommiβbrot set; 2x UCH21 and a UBL21. Tesla used that lineup in sets well into the 1950s. If you use a leaky-grid detector you already have a source of AGC potential right there.
Superhets devoid of IF amplifiers were a staple of Cossor; some sets were superhets on SW only (about 1360kHz) and simply fed the output of the frequency changer into the detector.
Good call re. the AGC source there. As this project evidently calls for gain per envelope, I wondered about using the PFL200 for detection/output? The 10-pin base and 16.5V heater are awkward but it's the sort of bottle that lurks in spares boxes and cries out for employment. I imagine that the EFL200 would be a somewhat rarer beast. Did the PFL200 feature much in CTVs or was it more a latecomer to the monochrome scene? Well out of my usual stamping ground, I have to confess....
turretslug said
Did the PFL200 feature much in CTVs or was it more a latecomer to the monochrome scene? Well out of my usual stamping ground, I have to confess....
The PFL200 is to be found in two early British colour TV's, it may have featured in others. In the 1967 Baird M702, the ‘F’ section was sub carrier drive, ‘L’ section luminance output. In the 1967 Philips GK502 (G6) the PFL200 was again used for Luminance Output and also a Sync Separator.
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It would be good to get this topic going again. The original thread is still available on the old VRAT forum and shows how the different designs were developed.....and why!
If people don't want to try their hand at creating a valve receiver perhaps because of power supply requirements, maybe we can think about some simple transistor circuits. For a very simple design, one could think of an amplified crystal set. The main problem with these is that you need a long aerial...probably some 70 feet or so and the performance would leave a lot to be desired. Definitely not something one would consider for normal use.
A much better idea would be a reflex design using two or three transistors with enough power to drive a small loudspeaker using just a ferrite rod aerial. Something like that could be built into a small case and used around the house. There would be no alignment issues other than the possible adjustment of the aerial coil for optimum tuning.
Ideas?
The anode bend detector was chosen over the leaky grid type because the reduced damping on the preceding tuned circuit. With a leaky grid detector you need reaction to counter the damping imposed on the tuned circuit. To answer Frank's question the choke was introduced into the anode load in order to develop sufficient IF component for the AVC diode, later on a degree of delay was introduced. However, the component count is going up and that defeats the whole objective of the project, but you can look at it another way, we're getting amplified and delayed AVC on the cheap. I've often thought about the leaky grid detector as a source of the AVC voltage, something to consider in the future.
The set I'm presently working on uses octal valves. The W61 RF amplifier will be replaced by a 6K8 triode-hexode or the heptode 6A8. The Brimar valve book tells us that the 6J7 is suitable for service as an anode bend detector so for that reason the existing single ended 6SJ7GT will be retained. Our old friend the 6V6 also retained for the output.
Till Eulenspiegel.
The detector valve in this set is in fact a 6SH7GT. The chassis is out of the cabinet and I discovered that the cathode bias components go to pin 3. On a 6SJ7GT pin 3 is the suppressor grid and pin 5 is the cathode. In the 6SH7GT the cathode and suppressor grid is connected internally, the connection goes to pins 3 and 5. The 6SH7 is an RF pentode, gm is a modest 4.9mA/V. Was also used as a video amplifier in some American TV receivers.
Successor valve is the 6AU6, B7G base.
It has been determined that the detector in the set is an anode bend type. Very high value cathode bias resistor, 22Kohms. Screen volts supplied from a potential divider of 1Megohm and 100Kohms, therefore 1/9th of the HT voltage.
From the Radiomuseum: http://www.radiomuseum.org/tubes/tube_6sh7. html
Till Eulenspiegel.
Another neat trick was carried out in the Wartime Civilian Receiver, using the suppressor grid of the EF39 as a diode in order to introduce delayed AVC.
I am not is a position to do any builds or repairs but this thread will be interesting.
Frank
Frank
Cathovisor said
Another neat trick was carried out in the Wartime Civilian Receiver, using the suppressor grid of the EF39 as a diode in order to introduce delayed AVC.
ISWYM, that's ingenious. It's always refreshing when effectiveness is wrung out of simplicity and when "basic" doesn't mean "compromised". Although it's far from "minimalist", Telefunken's E52 uses an RV12P2000 pentode as Det/AGC/triode AF amp with screen as AF amp anode and suppressor and anode as the diode anodes as part of the overlooming urge to minimise valve variants- I wonder if any of those 'fifties hobbyist magazines did anything similar with their "Build a broadcast superhet using only EF91s" -type articles.
Cathovisor said
Another neat trick was carried out in the Wartime Civilian Receiver, using the suppressor grid of the EF39 as a diode in order to introduce delayed AVC.
Also clever in the Wartime Civilian Receiver was the forward bias for the selenium detector diode. A portion of the output valve cathode bias was tapped off to supply the forward bias.
till Eulenspiegel.
Read about the all purpose RV12P2000 here: http://www.radiomuseum.org/tubes/tube_rv12p2000.html
And here: http://www.r-type.org/exhib/aaq0302.htm
Till Eulenspiegel.
The RV12P2000 was advanced in its time but continued the seemingly European predilection for over-complex valve bases and holders which probably added to manufacturing costs and limited ultimate frequency response. To be fair, part of the spec. seems to have been for easy replacement in the field by the relatively unskilled (possibly by jaded, shivering squaddies or aircrew with gloved hands....). The excellent US 6AK5 wasn't so far behind (1942?) and was broadly comparable with its low heater power and HT voltage requirements and somewhat higher gm, reflecting US mastery of precision repeatability.
So, "minimalist superhet" could have a few interpretations- fewest envelopes (2x ECH81?), simple circuit with just one valve type (EF91 being the obvious candidate, though EF80 also comes to mind and is ready-screened), or ingenious circuit design using tricks like the above-mentioned suppressor grid as a diode and so on to cut component count. I found a probably well-known circuit for an AM broadcast receiver that used EF91 mixer with separate EF91 LO, but surely a self-oscillating circuit as used in some early '30's superhets keeps to the "minimalist" theme better- it might be a bit anti-social from the point of view of modern oscillator radiation expectations, though!
I wonder if any US experimenters cooked up a 1-envelope non-reflexing superhet using a Compactron?
I did attempt to build a simple superhet radio around the PFL200. The "F" section was the frequency changer and indeed it did resemble early 1930s practise, no problems at all making that part of the set work. The "L" section was to be the reflexed IF amplifier and output, rather like the Italian radio set mentioned at the beginning of the old VRAT topic. This could not be made to work at all, serious issues with instability, perhaps a result of the very high slope of the L section. It will noted that most AM radios use for the IF amplifier an RF pentode of relatively low slope. Rarely more than 3mA/V, so the valve of choice is the EF89 or EF41. Older octal valves like the 6K7 and EF39.
Getting back to the radio I'm going to use for the project. The set was constructed sometime in the late 40s or early 50s and the rule is, "use only the tech of those times" so the ECH81 is ruled out.
Till Eulenspiegel.
sideband said
If people don't want to try their hand at creating a valve receiver perhaps because of power supply requirements, maybe we can think about some simple transistor circuits.A reflex design using two or three transistors with enough power to drive a small loudspeaker using just a ferrite rod aerial. Something like that could be built into a small case and used around the house. There would be no alignment issues other than the possible adjustment of the aerial coil for optimum tuning.
Ideas?
Hi Rich,
Not only does it build upon and compliments the great work you and others invested in the original VRAT project, it further extends the project scope to folk who as you say, might be more comfortable with the lower voltage requirements of transistors.
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