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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 22nd Dec 2007, 12:04 am   #1
Sideband
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Default Anode Bend Detectors.

Hi Guys.

We must be a really dedicated bunch trying to keep this old technology alive and well!

There is very liitle information about regarding these detectors and I suspect that they did not have a long run as they are mainly confined to TRF designs.

There were several designs using triodes, tetrodes and pentodes but apart from a vague description of 'an anode bend detector is basically an audio amplifier without bias' I can't really find any information of how they actualy work.

By nature of the fact that they are detectors, they have to be non-linear. I am asuming the valve is operated towards cut-off and possibly the audio envelope biases the valve on so the valve 'chops' the top off the envelope. This would imply that the audio level is quite critical and too much or too little signal will cause distortion. Not a particularly good idea and probably why they were not popular for very long.


Does anyone have information on these detectors?

There have been a couple of questions on these recently concerning distortion on certain transmissions and I reckon it's all to do with modulation levels but it helps to know how these things work so any information would be great.

Rich.
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Old 22nd Dec 2007, 12:20 am   #2
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Default Re: Anode Bend Detectors.

Hi Richard,
Anode bend detection is as you surmise. Its biggest drawback is that the way the valve is operated there is a gradual non linearity ( slow rate of curve) in the charactersitic which means that with low level signals there is a lot of distortion introduced.

I always understood their popularity in amateur shortwave radios was due to the fact that distortion could be tolerated but the big advantage of anode detection is the exceptional high impedance which allows easier or simpler design to maintain sensitivity and selectivity.

They are operated with a high value cathode resistor to bias the valve to cut off and bypassed with a large electrolytic to pass Rf and Audio

mike

Last edited by MichaelR; 22nd Dec 2007 at 12:23 am. Reason: added mor information
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Old 22nd Dec 2007, 12:27 am   #3
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Default Re: Anode Bend Detectors.

American rather than English. They could perform exceedingly well, but component value changes tended to be rather more problematic in my view.

The distortion on quiet passages can be a problem, it all depends on the anode curve you use. They take a higher input level, but put less load on the tuning circuit (TRF Remember) giving improved Selectivity and Sensetivity.

The pages below come from Chas E. Miller's little book 'Rapid Radio Repair - American Midget Receivers'. These little books are in my view worth having!!

Cheers,

Steve P
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Old 22nd Dec 2007, 8:25 pm   #4
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Default Re: Anode Bend Detectors.

Thanks for that info Steve. It's pretty much as I suspected. The one in my Philips set is probably OK but it'll still be worth checking the resistors around it.



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