Singlestage Regeneration

Single-stage regeneration in amplifiers is usually the least understood type of regeneration trouble and frequently has baffled many service men and radio experimenters. It is peculiar in that no amount of isolation and filtering applied to screen, cathode, plus "B" or grid return seems to make any improvement.

The feedback actually occurs inside of the tube in the stage that is giving trouble. The coupling exists between grid and plate through the inter-electrode capacity of the tube or any additional stray capacity between these two points. To some, this may seem unreasonable when the inter-electrode capacity is as low as .01 mmfd. or less, but it is an actual fact that is easily proven.

When single-stage oscillation is suspected, raising the grid bias will stop the regeneration, but so will this change stop over-all regeneration. The true test for this phenomena is to connect a milliammeter (properly bypassed) in the plate circuit of the suspected tube as shown in Fig. 22. Remove the preceding and following tubes, and then place an intermittent short-circuit on either the grid or plate circuit of the suspected stage while watching for changes in the plate current in that tube as an indication of the starting and stopping of oscillation. The tube and associated tuned

Wov* Trap

Figure 21 Dual Wave Trap

Wov* Trap

Figure 21 Dual Wave Trap

Figure 22 Test for Single - stoge Oscillation

circuits form a tuned-grid-tuned-plate oscillator similar to a transmitting circuit that was very popular in the early days of vacuum tube transmitters.

The standard cures for this trouble are:

(1) I'se a tube with lower inter-electrode capacity.

(2) Neutralize the inter-electrode capacity.

(3) Reduce the gain of the circuit by raising the tube bias.

(4) Reduce the value of the resonant impedance in either grid or plate circuits, or possibly both. This may be done by using coils of lower Q, or coils of the same Q but lower inductance, or by tapping one or both of the tuned circuits so that only a portion of the resonant impedance is introduced into the grid or plate circuit.

This type of oscillation is not confined to intermediate-frequency amplifiers but is encountered in RF amplifiers as well, if the primary is closely coupled to the secondary (as most shortwave RF primaries are) and has too many turns in an attempt to obtain high RF stage gain. In such cases it is necessary to reduce the number of primary turns until single-stage oscillation stops.

A peculiar effect may have been observed by some experimenters that they have never been able to explain, which can be understood when considered in the light of single-stage regeneration. This phenomenon is that a given amplifier stage may be stable when lined up properly, but will he unstable and oscillate when one or two of the associated circuits are misaligned. This phenomenon may have been observed accidentally and has not been reproduceable because the reasons were not understood. Referring to Fig. 22 it will be seen that the resonant circuits are lettered for easy identification. Consider that the middle tube is the offender, but with all circuits aligned it refuses to oscillate or give any other evidence of misbehavior. If circuit A is progressively misaligned in one direction and circuit I) progressively misaligned in the opposite direction, single-stage oscillation will soon result. The same results can be accomplished on variable coupling IF circuits, that are mechanically variable, if the coupling is progressively reduced. The explanation is the same in both cases, but is accomplished by a different agency. In both cases the impedance of circuits C and I) rises until single-stage oscillation occurs through the inter-electrode capacity of the tube. The explanation for this statement is given here below.

When a single circuit is resonant it presents a definite resonant impedance that is a direct function of its "Q" and its reactance. If another similar circuit, similarly resonant to the same frequency, is coupled to the first circuit, and set near "Critical Coupling" the resonant impedance of the combination approaches half the impedance of either circuit separately. It is this loading effect that keeps the impedance down when all circuits are aligned, and the absence of which, when circuits A and I) are detuned, that permits the impedance of circuits B and C to climb high enough to cause single-stage oscillation.

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