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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 15th Apr 2020, 12:45 pm   #1
AdrianH
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Default Please explain an EBC41 Valve

I have been pondering over the circuit for my Pye P76 and I am confused on the operation of the the diode sections of the EBC41.

Are they anodes, or cathodes?

In the circuit the main cathode is at ground potential so if anodes the can only go positive voltage, if they are cathodes they could potentially move negative with respect to ground and drive the agc, but I find it all very un-clear.

Can some body please try to explain the action to me?

Adrian
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Old 15th Apr 2020, 1:14 pm   #2
Radio Wrangler
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Default Re: Please explain an EBC41 Valve

If you make the diode anodes positive, the cathode (common to both diodes and the triode) is grounded, so the diodes turn on, take current and clamp whatever is driving the diode anode at a small positive voltage on the peak.

This charges up coupling capacitors. When the signal reverses, the diode anode goes below ground and the diode is firmly off. So the signal becomes offset in a negative direction. Filter off the DC and the IF components and you have audio to feed to the triode grid and thence the audio output stage.

Filter off everything except the DC and you have a nice negative going voltage to apply as agc to the RF and IF amplifiers.

Negative voltage is hard to come by in a system with only positive power supplies, and this circuit not only does it as almost a freebie, it also suits a convenient common cathode structure for the EBC41 and it does your AM demodulation at the same time.

Elegant simplicity or what?

If you've not come across the trick of using a diode to ground and looking at the DC build-up on the AC coupling capacitor feeding it, it'll mystify you the first time.

David
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Old 15th Apr 2020, 2:00 pm   #3
AdrianH
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Default Re: Please explain an EBC41 Valve

Yes thank you, I think I can see it. For some reason I kept thinking that the diode anode must be positive in voltage, not that being positive it would draw current and clamp it to ground. I will re-read Scroggie, there is a section on it, it may make more sense to me now.

Adrian
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Old 15th Apr 2020, 2:19 pm   #4
turretslug
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Default Re: Please explain an EBC41 Valve

Up until about the mid '30s, it wasn't unusual to find that early superhets had a separate double-diode valve for detection and AGC, which makes the circuit easier to unravel and understand. It was soon realised that incorporating two diode anodes in an existing triode structure was almost trivial in manufacturing terms and saved a valve envelope and socket, especially as they only need to be low-current types- if you look at the structure of a typical double-diode-triode, the diode anodes are usually a couple of tiny plates or folded cylinders at one end of the triode's cathode cylinder. That step meant that the circuit became a little more convoluted, especially when features such as AGC delay or applying a little standing bias to the detector diode for lower distortion also come in to a circuit.

Colin.
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Old 15th Apr 2020, 3:40 pm   #5
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Default Re: Please explain an EBC41 Valve

The 'combined double-diode-triode' setup is a good example of efficient manufacturing reducing the component-count (and so the cost) of a radio - it does however have a slight disadvantage in that - at least in some versions - stray capacitive coupling between the diode anodes and other electrodes along with less-than-perfect decoupling of the 'shared' cathode can mean that - even with the volume-control set to zero - a very weak (and often rather distorted) audio signal can still be heard.

This is why the higher-class receivers of the 'communications' class often still used a separate diode-detector/AGC valve such as the Octal 6H6 or the B7G 6AL5/EB91
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Old 15th Apr 2020, 8:56 pm   #6
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Default Re: Please explain an EBC41 Valve

Quote:
Originally Posted by G6Tanuki View Post
it does however have a slight disadvantage in that - at least in some versions - stray capacitive coupling between the diode anodes and other electrodes along with less-than-perfect decoupling of the 'shared' cathode can mean that - even with the volume-control set to zero - a very weak (and often rather distorted) audio signal can still be heard.
You can get a similar effect with the EABC80 triple diode triode. The effect can be different per valve. I tried three before I found one that was silent at minimum volume.
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Old 15th Apr 2020, 9:35 pm   #7
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Default Re: Please explain an EBC41 Valve

The 6AT6 is also notorious for it- the distinctive U-shaped diode plates running parallel to the triode anode made for simple, inexpensive stampings and jigs but also a significant amount of capacitive coupling to the anode. Thus, the output valve gets a whisper of toppy AF at its grid even at minimum volume setting. The Eddystone 750 here exhibits the effect, I've occasionally been tempted to fit a Rimlock EBC41 to keep the 2x ECH42 company but in the end you just live with it.
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