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June 1940 Radio-Craft[Table of Contents]
People old and young enjoy waxing nostalgic about and learning some of the history of early electronics. Radio-Craft was published from 1929 through 1953. All copyrights are hereby acknowledged. See all articles from Radio-Craft.
(Dual reproducers, class B push-push A.F. power output fed by a class A push-pull driver stage, meter tuning, A.V.C., tone control, static control.)
The Crosley Chief, 12-tube superheterodyne console model radio receiving set, is the most recent addition to the line. This receiver incorporates the model 132-1 chassis. Although incorporating. a large number .of tubes, the power line current consumption is held to a minimum by use of the new tubes which consume much less current than the older types.
Resistors: R1A; R1B, 1.5 megs. (per section); R2,- 0.4-meg.; R3, 80,000 ohms; R4, 0.15-meg.; R5, R6, 60,000 ohms; R7, 2,000 ohms; R8, 1 meg.; R9, 7,000 ohms; R10, 40 ohms; R11, 750 ohms; R12, R13, R24, 0.5-meg.; R-14, R23, 0.3-meg.; R15, 450 ohms; R16, 3 megs.; R17, 30,000 ohms; R18, 20 ohms center-tapped; R19, 3;500 ohms; R20, 6,000 ohms; R21, 10,000 ohms; R22, 5 megs.
Condensers C1 to C4, tuning units; C5 to C8, I. F. trimmers; C9, C25, C26, 0.02-mf.; C10, C11, C13, C14, C16, C30, C31, C32, 0.1-mf.; C17, C15, 4 mf.; C18, 150 mmf.; C19, 100 mmf.; C20, C23, 0.006-mf.; C21, C22, C28, 8 mf.; C24, 0.05-mf.; C27, 0.003-mf.; C29, 12 mf.
With a line potential of 117.5 V. the above figures may be taken as average readings; for "220 V." sets a line voltage of 235 is taken as standard. Bias (unless otherwise stated), screen-grid, and plate readings are taken between these tube contacts and the emitter; bias for V3, V5, V7 to V11, cathode to chassis.
Late chasses of this model have a 1,400-ohm resistor shunted across the visual tuning meter. Also, these later chasses may have two 1 meg. resistors (total) in the manual volume control circuit, connected from the moving arms to the ground ends of R1A and R1B.
The tuning meter of this receiver has a resistance of approximately 440 ohms and the deflection is approximately 10 ma.
The A. F. output of V6 actuates tube V11 which. in turn, controls the bias on the amplifier tubes for A.V.C. operation. By manual adjustment of R2 the degree of background noise is controlled to suit individual preference of sensitivity.
Note that the transformer secondary supplying tubes V1, V2, V3 is bypassed by a dual-section condenser, C30-C31. Another unusual circuit arrangement is the use of a double choke coil arrangement comprising Ch1-Ch2. and field coils 1 and 2. The first two choke coils are connected in the positive high-voltage lead in the usual manner, while the second two chokes, the field coils of the dynamic reproducers, are connected as an "inductive voltage divider," one terminal of the two coils in series being connected to the positive output of the regular filter system, and the other end being grounded to the chassis; the center-tap of these field coils supplies voltage to the screen-grids of V1, V2, V 4, and to the plate of V5.
The tone correction furnished by C25-C26 applies equally to V9 and V10, the center-tap of these two condensers being grounded to the chassis. If any portion of the tone control R3, C24, becomes grounded the A. F. portion of the receiver will become inoperative as the entire tone control operates at the potential of the plates of V7, V8.
The manual volume control operates in the input circuit of the push-pull driver stage, V7-V8. The two sections of this volume control, R1A, R1B, operate simultaneously as a dual unit.
The first A.F. stage, V7, V8, is driven by a so-called split-diode circuit furnishing A.F. voltage from both the cathode and plate circuits, which are out of phase (in push-pull relationship) .
The input circuit of V1 is preceded by a band-selector which must be carefully aligned in order to maintain the required degree of selectivity at all points in the tuning band.
The A.V.C. action is obtained through the voltage drop across resistor R4. There is an initial current flowing through this unit and the 5 meg. grid leak, R22, furnishing a normal bias for the R.F. and I.F. stages. The A.V.C. tube V11 is delayed by means of a positive potential on the cathode of about 60 V. When an R.F. signal of sufficient intensity is applied to make this A.V.C. tube (D.C. amplifier) draw plate current, its plate current also flows through R4 and furnishes an additional A.V.C. bias.
The "static" control is also connected to change the current flowing through R4, thus changing the initial bias on the controlled tubes, so that the overall sensitivity of the set is reduced.
Posted March 24, 2015