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Old 25th May 2022, 10:07 pm   #21
Osvaldo LW1DSE
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Default Re: An all tubes FM tuner with permeability tuning device.

New permeability tuned IF transformers. The bottom coil is the primary and is fixed in place. Top one is the secondary and is moovable in order to adjust M and k for proper BW. Tunning capacitor is 22pF @ 500V silver mica. The plate cap isn't directly in parallel to the coil, it returns to ground using a dedicated pin.

The screen decoupling cap is only 910pF mica plata. This make the screen alive for RF of about 2.7VAC and is out of phase respect to the plate signal, neutralizing the coupling trough anode-grid capacitance. It is fully stable irrespective the high gm of the tube. Anode and screen voltages are about 140VDC.

This idea isn't mine and I learned it reading the article "Neutralization of Screen-Grid Tubes to Improve Stability of Intermediate-Frequency Amplifiers" by C. A. Hultberg in the Proceedings of the IRE December 1943 available free in the web (worldradiohistory.com).
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Old 30th May 2022, 12:25 pm   #22
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Default Re: An all tubes FM tuner with permeability tuning device.

This is the schematic of the IF stage as it is until this moment. Tube is a double remote cutoff pentode 6AR11 but it may change. Note the similitude with the schematic proposed by Hultberg. This isn't casual :=)

Grid leak resistor isn't necessary, but it is there to measure the point in which the tube starts to drive grid current or is driving positive, that's the same. If the stage runs properly, the bottom of it will be removed from ground and receive the AGC bias if any. Otherwise it will be erased together with the decoupling capacitor.

The grid inductor very possibly, will be a π tank or a link coupling from the front end. At this moment, it is a link with 2 turns over the main tuned inductor, consisting in 24turns 0.22mm varnish copper wire, used also in transformers and motors. It has been wound in two layers of 12T each, using cyanocrilate to fix the first layer to bobbin, when dryed, the second layer over it and again fixed same way. Plate winding is exactly the same as it. The still unused secondary, is done equally but not fixed to bobbin, it is fixed to a short piece of thin mylar paper in order that it may slide to or from the primary to adjust coupling.
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Old 3rd Jun 2022, 6:58 pm   #23
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Default Re: An all tubes FM tuner with permeability tuning device.

Yestwrday I made a modification in order to try a 6BN11 (double sharp cutoff pentode), but it was very unstable and although neutralized, it oscillates uncontrolledly and no way to stabilize it. So I returned to 6AR11 again.
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Old 11th Jun 2022, 8:36 pm   #24
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Default Re: An all tubes FM tuner with permeability tuning device.

This morning I tryed the 6BW11 disimilar dual sharp cutoff pentode 12pin compactron tube with very good results. The high gain section (gm ~12mS) is the first stage and the lower gain (gm~8.5mS) the second. Once neutralized both stages indepently, no global oscilation was found, sincerely unknown if still persis some amount of re/de-generation. It is critical respect to oscillo tip placement, and the gain seems to be very high. Only crossing the generator cable to the input pin causes second stage to drive into limitation. Ebb is in the order of 140DCV, the cathode current of the first pentode is about 15mA with 220R as bias resistor, the second is about 8mA with 82R.

Seems that this layout is satisfactory. Decoupling from global Ebb is 470R common to screen and plate, and the former is live for RF in order to neutralization of Miller effect. Soonly, a schematic will be posted here.



Certainly, what is LNA ​in the above reply?.



Also I listen about pros and cons from the performance point of view, about ratio detector vs phase discriminator. Possibly I shall use a diode limiter at the second grid stage as depicted in the patents already added here.
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Old 12th Jun 2022, 7:14 am   #25
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Default Re: An all tubes FM tuner with permeability tuning device.

LNA = Low Noise Amplifier

But be careful, there is a lot of confusion being created by people using that name for any RF amplifier stage whether it is low noise, high noise or whatever! Also, some other people use that name for MMIC amplifier devices.

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Old 12th Jun 2022, 1:34 pm   #26
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Default Re: An all tubes FM tuner with permeability tuning device.

In my case, discard the word low and let the noise amplier alone
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Old 13th Jun 2022, 10:42 pm   #27
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Default Re: An all tubes FM tuner with permeability tuning device.

This's the schematic as it actually is. Omitted deliberately the "boxing" of the transformer cans for simplicity.
Recently, I tryed some other 6BW11 and the circuit continues performing as desired.
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Old 16th Jun 2022, 1:38 pm   #28
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Default Re: An all tubes FM tuner with permeability tuning device.

I tried the limiter, and appear to do its job, more or less. Below, some pics. They are illustrating the first attemp to limit the second grid to ground, and to a cathode voltage. It will be best understand with the schematic, it resembles the idea from Chauvin @ post #103 but it isn't exactly like it. But wait for it some hours, please. From first to fourth, the lower trace is the input directly measured at the link input coupling means, and the upper is the output at the cuasi-decoupling capacitor of the second stage, screen grid live in order for neutralizing it. All pics with same oscilloscope sensitivity and @12MHz. If not perfect, limitation is easily seen as a ten-fold input only reflects a twice ouput signal in the 3rd and 4th pics when limiting appears to commence.
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Old 16th Jun 2022, 1:40 pm   #29
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Default Re: An all tubes FM tuner with permeability tuning device.

Here is the physical layout. Compactron (fat) tube is still 6BW11 and novar (thin) is a quad diode series conected 6JU8A. One section still remains unused.
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Old 17th Jun 2022, 3:05 pm   #30
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Default Re: An all tubes FM tuner with permeability tuning device.

Few minutes ago I tryed the second variant of a limiter, this one as a soft clamping to a RC network. Below are two pics in comparable conditions to the previous. Now, a 5-times increase in the signal level at the input reflects as a 2.5-3 times at the output. Rectified voltage at the clamping net increases from 100mV (contact potential) to over 2.2V when limiter action takes place. Coil adjust has changed slightly but tubes, passive elements excepting those belong to the clamping, are all the same; together with the instrumental used. EICO 368 generator a Kenwood CS5140 oscilloscope and GDM8040 bench multimeter.
Soonly both schematics, please be patient.
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Old 19th Jun 2022, 12:25 pm   #31
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Default Re: An all tubes FM tuner with permeability tuning device.

Finally, I made the desired schematics. The former is the corresponding to post #28 while the latter to #30.
Observe in the first, clamping to ground and to a twice cathode voltage, but grid returns to middle point, thus grid signal is symmetrically clamped to the single cathode bias. In the second, clamping level depends on signal strength. A 8.2K and a 10µF 63V film capacitor is the time constant (as a try only). 10nF plate is in parallel to proper RF bypass.
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Old 25th Jun 2022, 2:12 pm   #32
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Default Re: An all tubes FM tuner with permeability tuning device.

This is an old debt. The schematic of the front end. RF Chokes aren't measured yet. Some are linear and other, toroids hand coiled. Tubes and passive components may change without notice.
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Old 29th Jun 2022, 1:42 pm   #33
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Default Re: An all tubes FM tuner with permeability tuning device.

As all in the Earth changes, my project too.
Yesterday l unespectedly could own a 3 section gang variable capacitor very similar to the used in my AM Tuner Project but with a couple of sheets less. It has about 450pF each section. So I dissasembled it and removed sheets to get a wonderful triple 25pF per section gang with brass plates. Thus, added to the difficulty for me for proper mechanical makes, l am almost sure to discard the original permeability tuning means that gave the tittle to the thread and migrate to a more common capacitance tuning.
Here pics of the before and the after.

The removing of plates from a variable condenser to get a smaller value unit has been used by ham radio for several decades here because of the availability of such stuff was difficult.
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Old 5th Jul 2022, 4:52 pm   #34
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Default Re: An all tubes FM tuner with permeability tuning device.

This is a crude idea of the front end of my project. Some components, RF amp and LO are more or less defined, but the mixer is still uncertain. Particularly, a small inductor placed in the plate of the mixer anode that it is said to be useful for neutralizing it. Some tuners of the tube era used it at this place or at the screen of the pentode mixer if this kind is used, particularly with 6CG8 and similar devices.

Langford-Smith in his Radiotron suggests that the first IF tuning capacitor return to ground or cathode in place of being paralleled to the IF transformer primary. This is so to ground properly undesired signals as a result from the mixing process. Who knows...
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Old 20th Jul 2022, 10:35 pm   #35
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Default Re: An all tubes FM tuner with permeability tuning device.

More (newer) pics.
I moved the IF stage to the same protoboard as the gang and the oscillator. The front end is temporarily suspended until the IF section be fully tested and adjusted.

The IF stage now is composed by 3 pentode amplifiers coupled by doubly tuned transformers although they aren't still adjusted as the detector isn't still made. The first stage is the pentode unit inside 6BD11 followed by the high gm unit of the 6BW11 and finally the lower gm unit of it.

Trafos are probably largely undercoupled still, but in any case very high amplification is obtained free of oscillations thanks to the neutralization of each individual step using screen neutralization above explained. Refering to pics, the RF output at the last screen is displayed in the oscillogram, but consider the 10x att of the tip plus a voltage attenuation in ratio of the tunning capacitance (22pF mica silvered) to the cuasi-screen-decoupling (of 820pF mica silver) giving an extra attenuation of 820 to 22 or abut 40 times. So the level at the plate is very large. In the other hand, see the very poor coupling between the generator to the first grid with about 6mm of separation. Pics talks by their own.
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Old 21st Jul 2022, 9:46 pm   #36
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Default Re: An all tubes FM tuner with permeability tuning device.

This is a prototype of the ratio detector transformer. Upper (far from the base) is the secodary moovable to and from the primary, close to the bottom. Secondary will feed the 6AL5 double diode while the primary will be feed by the unit 2 (lower Gm according to the datasheet) of 6BD11.
Tuning cap of the primary is wired but seconary no, in order to show the transformer better.

Primary is 24T of 0.22mm enamel Cu wire while the untuned secondary is wound directly over it. Tuning capacity is 22pF mica. Tuned secondary is the same (# of turns and diameter), centre taped and wired in series with untuned secondary. Remember (once more) that my IF is 12MHz in place of the 10.7MHz took as standard.
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