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Old 8th Feb 2022, 3:24 pm   #1
Osvaldo LW1DSE
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Default An all tubes FM tuner with permeability tuning device.

My apologies because this pics had already showed in another thread but now I want to make a new own one. Mods fell free to delete the former if needed.

The idea is to design an FM tuner with home made permeability tuning mechanism. I read in thr Radiotron Book from Langford Smith that the parasitic coupling inbthe inductances and resistances in usual gang condensers, and in any instances I lack one of them

So i though in use copper tubbing insidecthe RF coils . Some help was found accidentaly in an old Tele Tech magazine at wordradiohystory.com.
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Old 8th Feb 2022, 3:29 pm   #2
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Default Re: An all tubes FM tuner with permeability tuning device.

More pics and the addition of the first FI coil.
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Old 8th Feb 2022, 3:37 pm   #3
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Default Re: An all tubes FM tuner with permeability tuning device.

This is the idea borrowed, but in my case the coils will be in line for mixer grid coupling, except the antenna coil.

The RF amplifier will be a ECC88 or ECC189 cascoded and the mixer & oscillator, a ECF801. FI will be 12MHz and the tubes selected for it are EF184's. Detector is a 6AL5 but unsure the configuration (Foster Seeley or ratio detector).

Please note that its ugly appearance is obvious, but will be corrected after prototyping.
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Last edited by Osvaldo LW1DSE; 8th Feb 2022 at 3:41 pm. Reason: Typos :-(
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Old 13th Feb 2022, 12:58 pm   #4
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Default Re: An all tubes FM tuner with permeability tuning device.

Small progress. The ECC189 RF cascode amplifier. Final tube will be ECC88 with higher gm, but as it is expensive, during test the former will be used. Base connections are the same.

Below, at first intance, the coils, the screwed core for padder inductance, the copper core re-coppered and the main tuning inductor.
Next, the amplifier mounted in the protoboard, the sweeped waveform showing the carrier beating with the oscillator and the sweep generator dial showing the frequency. Consider that the dial doesn't coincide with actual frequency when measured the output with a digital frequency meter. Note also an undesired 50Hz signal that makes the sweep baseline non straight. I couldn't find where it enters in the signal path but momentarily it doesn't care too much.
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Old 13th Feb 2022, 2:27 pm   #5
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Default Re: An all tubes FM tuner with permeability tuning device.

It looks like you are enjoying the adventure!

50 Hz modulation is likely pick-up from the heater circuit, if your HT is filtered well. Is there any wiring common to both the heater circuit and the oscillator ground connections.

Or, 50Hz field from heater current can modulate the permeability of your cores a little, but enough to inject FM hum.

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Old 13th Feb 2022, 3:40 pm   #6
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Default Re: An all tubes FM tuner with permeability tuning device.

I'm interested in this! Any chance of the circuit?
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Old 13th Feb 2022, 8:11 pm   #7
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Default Re: An all tubes FM tuner with permeability tuning device.

Yes. Of course, but let me finish the project, almost the front end.

Update: I found a breackage in the connection of one anode of the EZ81 and the traffo secondary causing lots of ripple at the output of the ECL82 voltage stabilizer. Then I shall see tomorrow if the baseline of the sweep osillogram is straight as it must be.
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Old 14th Feb 2022, 6:16 pm   #8
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Default Re: An all tubes FM tuner with permeability tuning device.

This is the antenna (or aerial?) to cascode's grid coupling resonator, similar to the other but axis rotated 90 deg and shielded inside a 25mm copper tube from plumbing tubing. It is mechanically mounted but still not electrically wired. Momentarily the copper core monement will be independent because of mechanical complexity.
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Old 20th Feb 2022, 8:58 pm   #9
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Default Re: An all tubes FM tuner with permeability tuning device.

UPDATE: the grid input tank is wired and connected but not tested yet.

Below is partial schematic with voltages for if anyone is interested.

Also some 1 & 10 nF @ 500V for the final project
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Old 20th Feb 2022, 9:43 pm   #10
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Default Re: An all tubes FM tuner with permeability tuning device.

Thanks!

Cascode RF amp; Colpitts oscillator; both feeding into the grid of the pentode as an additive mixer?

Looks reminiscent of TV Band I tuners, rather better than your average VHF radio front-end!

Watching with interest! (What's oscillator radiation like from the aerial - I'd hope the cascode would give plenty of reverse isolation?)
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Old 21st Feb 2022, 9:40 am   #11
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Default Re: An all tubes FM tuner with permeability tuning device.

Yeah. Good observer. My inspiration is effectively old TV tuners. I like the idea of magnetic coupling between stages over the more classic capacitive one. I saw several of them in a web page that depicts their evolution. Let's see what happen.
Thanks for your repply and comments.
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Old 21st Feb 2022, 11:41 am   #12
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Default Re: An all tubes FM tuner with permeability tuning device.

The mixer responding in the high frequency side of the band, copper slugs inside the coils. No FI amplification.
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Old 21st Feb 2022, 11:43 am   #13
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Default Re: An all tubes FM tuner with permeability tuning device.

The same as above but in the lower side of the band.
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Old 22nd Feb 2022, 11:18 am   #14
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Default Re: An all tubes FM tuner with permeability tuning device.

https://www.maximus-randd.com/tv-tuner-history-pt1.html

Here is where I found valuable info for my development.
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Old 26th Feb 2022, 8:14 pm   #15
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Default Re: An all tubes FM tuner with permeability tuning device.

Does anyone know the name of this simple FM detector?
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Old 7th Mar 2022, 10:25 am   #16
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Default Re: An all tubes FM tuner with permeability tuning device.

This is the prototype for one of the coils for the home made 12MHz IF transformer. This is a single coil for pi coupling from mixer anode load to first IF grid by means of a short piece of coaxial line (say 10cm).
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Old 13th Mar 2022, 4:04 pm   #17
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Default Re: An all tubes FM tuner with permeability tuning device.

This is the first attempt to make the IF strip. The tube is a dual pentode 6AR11 remote cutoff biased with 82ohm ad 10nf in cathode and about 140VDC at plate and screen. Independent pin for suppressor is grounded. The coil is about 6ųH tuned with about 30pF. 20T copper wire 0.3mm. Former is 15mm diameter. Anode load temporarily is a choke.
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Old 21st Mar 2022, 10:30 am   #18
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Default Re: An all tubes FM tuner with permeability tuning device.

These are the first IF transformer. The low winding is the primary and is fixed in place. The upper is the secondary and is wound over a short piece of mylar tape and fixed to it but can slide over the bobbin in order to adjust it relative position to the primary and thus adjust k and M.
The other pic shows the above in its site on the protoboard and the neutralization trimmer and the tunning trimmers (the biggest one).
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Old 25th Mar 2022, 2:49 am   #19
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Default Re: An all tubes FM tuner with permeability tuning device.

Quote:
Originally Posted by Osvaldo LW1DSE View Post
Does anyone know the name of this simple FM detector?
Given the single coil, it might be a form of slope detector. Abandoned very early on because of its poor performance, it periodically resurfaced, only to be buried again fairly soon when its defects were rediscovered.

That said, RCA’s “differential peak” detector of 1970 got around some of the inherent non-linearity by using the slopes on both sides of the peak, and measuring their difference.


Cheers,
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Old 31st Mar 2022, 11:11 pm   #20
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Default Re: An all tubes FM tuner with permeability tuning device.

Here in the U.S. permeability tuning was used in some table radios I have worked on as well as most car radios from the tube era, (And some of the xistor car radios) with some exceptions back in the 20's & 30's.
If you could obtain one of the car radios and use the tuning assy out of it, it might be easier.
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