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PostPosted: Sep Sat 02, 2006 6:44 am 

Joined: Jan Thu 01, 1970 1:00 am
Posts: 11100
Location: Vieques, PR, USA
johnamery wrote:
There is a third capacitor from the second RF amp plate back to the low side of the antenna input tuning coil. Don't know how to adjust it. Still have oscillation at full RF gain, with full fillament voltage on.

May be your REGEN control. May not serve on a strong station but will on weaker ones. Back off that filament and see how it behaves.

 Post subject:
PostPosted: Sep Sat 02, 2006 8:02 pm 

Joined: Jun Fri 02, 2006 9:58 pm
Posts: 97
Location: Elsberry, Missouri, USA
It's not supposed to be a regeneratie receiver.


I have other NEURODYNE radios that do NOT have that 3rd capacitor

This Gilfillan is the first antique I have restored. Perhaps good enough for my first.

John Amery
1251 Deer Run Road
Elsberry, MO 63343-4011

PostPosted: Nov Mon 06, 2017 3:06 pm 
New Member

Joined: Nov Mon 06, 2017 2:35 pm
Posts: 2
How to calculate and choose grid leak detector resistor and condenser values:

1. The grid condenser (capacitor) performs two functions in the grid leak detector. First, the grid condenser couples the radio frequency (RF) signal from the tuning circuit to the tube grid. Second, the grid condenser permits development of dc voltage on the grid. Calculate the capacitance of the grid capacitor by 10 x (Cgp + Cgf + Cs) = condenser capacitance. This makes the capacitance ten times the total input capacitance of the tube grid circuit (grid-to-plate plus grid-to-cathode plus grid wiring) so the grid condenser impedance at radio frequency is small in comparison with that of the grid so as to couple most of the RF voltage to the grid. Values of 100 picofarads (pF) to 250 pF are typical.

2. The grid leak resistor performs two functions. The first function of the grid leak resistor is to set up the grid bias, which is important for small RF input signal amplitudes. The second function, of importance primarily for large input signal levels, is discharging the grid condenser.

Changing the resistance of the grid leak changes the grid bias.

Indirectly heated cathode tubes will develop negative bias due to electron flow from cathode to grid and then through the grid leak (initial velocity bias). The grid bias voltage is the product of this grid current and the leak resistance.

Directly heated cathode tubes with the grid leak connected to the positive end of the cathode, either in parallel with the condenser or directly to the positive side of the cathode, will have the grid at a positive fixed bias with grid leaks of less than around 6 megohms (the bias is always measured relative to the negative end of the cathode).

3. Grid leak and condenser effect on frequency response of the grid leak detector:

A. At small RF input signal levels, grid current occurs during 360 degrees of the RF cycle. The parabolic grid voltage versus grid current curve causes the detection (rectification). The audio voltage developed at the grid is in series with the effective grid resistance, Rg, which is typically in the range of 50,000 to 250,000 ohms (Everitt, pp. 419-420). Rg is determined by the grid bias, which is usually determined by the resistance of the grid leak resistor. The grid resistance Rg and the grid condenser form a low pass filter. As the frequency of the demodulated audio at the grid increases, the condenser reactance drops while the grid resistance Rg stays the same. For example, if the condenser is 250 picofarads and the grid resistance Rg is 100,000 ohms, the -3 dB frequency response will be 6400 Hz. Increasing the resistance of the grid leak increases Rg, which reduces the -3 dB frequency for a given grid condenser but increases the total impedance of the grid condenser grid leak resistor combination at lower audio frequencies, which increases the demodulated signal voltage at lower frequencies.

In small signal operation of the grid leak detector, the highest resistance grid leak that results in the desired intelligibility of the demodulated audio will provide the greatest practical detection sensitivity. Generally, higher resistance grid leaks can be used for radiotelegraphy reception where limiting the audio bandwidth to around 2500 Hz or less is acceptable. Most simple one or two tube radio receivers and even receivers having two or three stages of low amplification factor triode RF amplification operate the grid leak detector in the small signal region. For triode grid leak detectors, around 2 millivolts of modulated RF at the grid will produce sufficient audio output from the detector to be audible with sensitive headphones.

B. At large RF input signal level (amplitude enough to make cathode-grid conduction stop during part of the RF cycle, typically around 500 mV to a volt), the condenser will charge to a percentage of the peak voltage of the input signal. The grid leak resistor must discharge the condenser quickly enough that the dc voltage of the grid can follow the amplitude modulation envelope. Large signal operation of the grid leak detector is known as power grid detection. For large signal operation, grid leak resistor values of around 250,000 to 500,000 ohms are typical. The grid leak resistance for large signal operation of the grid leak detector can be calculated using R = 1/(6.28xFxC), where F is the highest modulating frequency to be reproduced and C is the grid condenser (K .R. Sturley, part 1, pp. 371-372). The bias won't be at maximum sensitivity, but that is not a concern because of the large RF input signal amplitude. For a directly heated cathode tube as a power grid detector, the grid leak dc return is to the negative end of the filament. Large RF input to the grid leak detector is usually encountered when it is preceded by two or three stages of screen grid or pentode RF amplification.

Bibliography: Cruft Electronics Staff, Electronic Circuits and Tubes, 1947; W. L. Everitt, Communication Engineering, 2nd edition, 1937; K. R. Sturley, Radio Receiver Design part 1.


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