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Old 22nd Nov 2009, 1:23 pm   #1
YT2095UK
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Question I.F demodulator help/advice please.

I`v been doing a lot of reading about I.F stages for my modular super-het radio project, I`v chosen a valve line-up for it using what seems to be the best choice for each stage out of the valve assortment I have.
for the demodulator/detector I`ll be using the EBC90 as I have 2 of these.
I`v been studying many cct diagrams for this stage from a variety of different valve radios and most all of them seem to similar to this:

http://www.radioremembered.org/images/detector.gif
taken from: (http://www.radioremembered.org/images/detector.gif)



but I don`t understand the role of the diode(s) in the position they are in?
the secondary of the transformer would be served equally well if the diodes weren`t there and it was taken straight to the cathode surely?so secondary one goes to deck, secondary 2 goes through a diode to the grid and then gets amplified etc...

or do the diodes being in the same envelope impart some action over the triode? maybe in the space charge or something?

Thanks
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Old 22nd Nov 2009, 1:43 pm   #2
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Default Re: I.F demodulator help/advice please.

Yes, you're right. It's good you're starting to understand these things. By adding one electrode to the existing triode (which is separate and has no effect over it) it save using a separate detector diode. Also as the cathode is earthed, you dont get any hum from the heater circuit coupled into the audio path.
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Old 22nd Nov 2009, 1:53 pm   #3
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Default Re: I.F demodulator help/advice please.

In this circuit the 6SQ7 performs the functions of detector, agc voltage generator and first AF Amp.

The RF current in the secondary of the IF transformer is recitified by the diodes and appears across the volume control. It is then applied to the grid for amplification.

C28 and R28 have a suitable time constant for deriving the agc voltage. ie the agc voltage depends on the average signal level rather than the audio signal.
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Old 22nd Nov 2009, 2:26 pm   #4
YT2095UK
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Default Re: I.F demodulator help/advice please.

so if from the top of L11 (or pins 4&5) there was a simple Diode to ground instead, the same effect would be had, yes?

and if we ignore the AGC and Volume control for a moment, would something like This work? :
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Old 22nd Nov 2009, 2:44 pm   #5
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Default Re: I.F demodulator help/advice please.

Almost, but you need a DC blocking capacitor between the diode and the triode grid. The diode produces an output that is notionally a pulsed DC signal i.e. its ouput always swings positive. In a practical circuit you'd also need a low value capacitor in parallel with the diode load resistor to bypass the RF part of the demodulated signal to ground. Without this capacitor, the triode would amplify both the wanted audio signal and the unwanted IF. This would cause very strange behaviour in the output stage and probably instability throughout the set.

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Old 22nd Nov 2009, 2:52 pm   #6
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Default Re: I.F demodulator help/advice please.

that`s great! thanks guys
it really IS starting to make quite a bit more sense now.

one last question and I think I`ll be done with this section, I have the 2 I.F transformers from an old cossor that I`ll be using, I marked them IF1 and IF2.
they also have a factory color code, one is Red the other Yellow, now I don`t exactly remember marking them, but I think I would have marked IF2 (the yellow one) because it was closest to the OP valve but can`t be 100% certain, so is the Yellow code for the Demodulator?
and does it matter which way around it is (pri/sec)?
as all windings on both transformers read as 14 ohms.
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Old 22nd Nov 2009, 2:59 pm   #7
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Default Re: I.F demodulator help/advice please.

Quote:
so if from the top of L11 (or pins 4&5) there was a simple Diode to ground instead, the same effect would be had, yes?
Yes, as that is precisely what the circuit has - a simple diode to ground.

Quote:
would something like This work? :
As jjl said, you need a DC blocking capacitor and an RF bypass. You would also need a diode load resistor of about 1M, as the 10M would be on the other side of the DC block.

Going back to your original circuit, I don't know whether the EBC90 will be happy with grid leak bias - try it and see, or check the data sheet. You may instead have to use cathode bias, and ensure that there is a DC connection between the bottom end of the IFT secondary and the cathode. This makes the circuit a bit more complicated, but gives the possibility of introducing delayed AGC by separating the audio and AGC detector diodes (which is why the valve has two separate diodes).
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Old 22nd Nov 2009, 3:09 pm   #8
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Default Re: I.F demodulator help/advice please.

cool, I was wondering whether some sort of Cathode injection was taking place as seen in some Mixer stage circuits, and my trying to visualise the electron path was oversimplified, seems not

the valves I chose: the ECH81 for my mixer (I have several brand new ones bought by mistake), the EF91 (I have 6 and like the look of them), and also the EBC90 (Nice and small and the Diode plates look cool), I`m not following any particular circuit for this though, my Aim is to understand it well enough to design my Own circuit using these valves that I`v chosen and get a basic working knowledge of the Super Het, that`s why I`m making it Modular for now.

maybe one day I`ll be able to design/build one all in one go, but that day is not Today
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Old 22nd Nov 2009, 4:08 pm   #9
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Default Re: I.F demodulator help/advice please.

The EF91 does not handle AGC too well. Try EF92 or EF93 instead, as they are designed for this purpose. Choosing components for their external appearance is not usually a helpful technique in electronics, although audiophools seem to do it all the time.
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Old 22nd Nov 2009, 4:20 pm   #10
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Default Re: I.F demodulator help/advice please.

well, it wasn`t based upon that Entirely
the Valve Museum said that the EBC90 was used in Super-hets (not my idea, theirs).
and I have to go with what I have here as well, These are the design constraints.
I`m also sure I`m not the only one that`s seen a valve and thought Wow, or That`s cute etc... based soley upon its looks, maybe I`m just more Honest about it?
the fact that they Look so different to "conventional" parts and Glow in the dark IS part of their charm IMO, I don`t know, Maybe I am guilty?
however having only half a working ear, Audio-Anything would be pushing credibility a bit

I simply don`t have those valves you suggest, and as such find myself in a position whereby I Must learn these principals well in order to use what I DO have.
I beleive it should be perfectly possible to build a super-het with these And have it look reasonable at the same time.

anyway, Cossor Transformers... is the Yellow for the demod and the red for the Mixer to IF amp?
and is there a defined Primary/Secondary on these, and a way to tell them apart if there is?
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Old 22nd Nov 2009, 5:16 pm   #11
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Default Re: I.F demodulator help/advice please.

ok Scratch the above...

I`v just spent a few mins in the workshop and knocked up a circuit using the IF transformer from the cossor.

Have I got it right When, I input a modulated (1KHz) signal on a 470KHz carrier into the Primary, and receive the sound perfectly in my amp speaker, with No RF present at the anode?

as that is what I currently have here
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