schaub: W; Alpha-Standard |Radiomuseum.org

? schaub: W; Alpha-Standard

24.Mar.11 17:41

57

Dirk Bladt (B)
Articles: 31
Count of Thanks: 10

Can anyone explain how this radio does the AM detection ? Grid of REN804 ?

thanks for your attention,

Dirk Bladt

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Plate bend detector

25.Mar.11 13:54

57 from 4727

Dietmar Rudolph † 6.1.22 (D)
Articles: 2492
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Two (or more) circuits TRF sets used an anode bend detector. A description of this type of detectors may be found at "Ghriardi, A.A.: Receiver Circuitry and Operation, Rinehart, 4th printing, 1955".

A bias or „square-law“ (weak signal) plate detector is shown in the schematic circuit diagram, Fig. 3-13 (A). It employs a tube which is operated exactly like a Class A amplifier except that the grid is biased sufficiently negative to make the operating range fall on the curved "bend" of the grid-voltage plate-current characteristic curve, as shown at (B) of Fig. 3-13. Bias resistor R₁is shunted by by-pass capacitor C₁ to prevent degenerative effects.

When a signal is applied to the grid of such a detector, the positive half-cycle causes much greater plate-current changes than do the negative half-cycles. This makes the plate-current pulses lopsided or distorted as shown.

The r-f components in the plate current are by-passed by capacitor C₂, which is of the order of 1,000 μμf (pf). The demodulated audio component flows through plate load R₂, which is of the order of 50,000 ohms, causing an amplified voltage drop to appear across it. This is the a-f voltage that is fed to the audio amplifier through coupling capacitor C₄. Observe that the demodulation takes place in the plate circuit - hence the name "plate detection."

Notice that in this detector, as in the grid-leak type, amplification of the signal takes place. The amplification applies mainly to the r-f wave (modulated carrier) because the demodulation does not take place until the signal has passed into the plate circuit.

The principal disadvantage of this type detector lies in the fact that the tube characteristic departs widely from linearity unless the plate current flowing through it is large. At small input-signal levels, operation is over the portion of the characteristic where the plate current is approximately proportional to the square of the grid voltage, with the result that distortion of the a-f modulation results. Hence the name "square law" plate detector is applied to this detector when weak signals are being detected.

Regards,

Dietmar

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anode bend detection

25.Mar.11 14:27

66 from 4727

Dirk Bladt (B)
Articles: 31
Count of Thanks: 9

A very big thank you for this cristal clear explanantion to you, Mr Rudolph.

Dirk Bladt

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schaub: W; Alpha-Standard

05.Jun.22 07:57

3636 from 4727

Rolf Rudi (N)
Articles: 5
Count of Thanks: 2

I am restoring an Alpha-Standard that was in a miserable state, especially the scale wheel. I am wondering if there was a kind of pointer in front of the scale to see the exact frequency? Otherwise the reading of the scale must have been very coarse. And was there a window (cellophane) to protect the scale?

Attachments:

Scale wheel (261 KB)

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