padding condenser cannot be placed PLAT.E at the customary position of the low side of the oscillator coil because a 6L7 d.c. path must be provided for the PENTAGR ID CONVERTER cathode return to "B-." It must be SUPPRESSOR placed in series with the oscillator GRID 05 tuning . Various OSC. INJECTION SCREEN GRID other oscillator arrange- GRID 03 02.04 ments are possible, but the use of a H Hartley type oscillator has advantages SIGNAL GRID GI when the receiver is to be designed for multi -band reception, since it simpli- fies the band switching arrangement.

CATHODE Advantages of 6SA7 Pentagrid Converter Tube CATHODE (A) ;6) Because of their improved construc- tion the 6SA7 and similar modified Fig. 5. Schematic representation and actual structural ar- types of pentagrid converter tubes rangement of electrodes in 6L7 type pentagrid mixer tube. give better performance than do the older pentagrids of the 6A8 type.. The ing a Hartley type oscillator using cathode current. Consequently, var- gain at AM broadcast band frequen- cathode -circuit feedback. Notice that iations in a.v.c. voltage applied to the cies is higher due to the combination one end of the tank coil is grounded, signal grid do not change the cathode of fairly high conversion conductance and the cathode is connected to a tap current appreciably, so the oscillator (about 450) and high plate resistance on this coil. This causes the cathode frequency is almost independent of (about 1,000,000 ohms), and continues to operate at high r.f. potential. Also, a.v.c. bias voltage. Variation in screen high up to approximately 6 mc. (50 the cathode circuit (and current) re- voltage produces a shift in oscillator meters). The efficiency falls off at turns through a portion of the oscil- frequency in the opposite direction, frequencies higher than this owing to lator tank coil. This circuit arrange- and the two effects practically cancel. the difficulty of maintaining optimum ment adds to the stability of the 6SA7 Consequently, excellent frequency sta- oscillation strength. circuit. The feedback is obtained with bility is obtained. At very high frequencies, above the total cathode r.f. current. Because The position of the cathode tap on approximately 6 mc. it becomes diffi- of the shielding effect of the inner the oscillator tank coil is extremely cult to maintain sufficiently strong screen, the electrostatic field of the important when operating on the and uniform oscillation when the signal grid has little effect on the higher frequencies for, if the tapis 6SA7 type tube is used as'a converter. . The collector plates too far from the grounded end of the One way of effecting a considerable and the inner screen serve to isolate coil, the cathode r.f. voltage will be improvement on frequencies higher the signal grid from the cathode space too high, causing an overbiasing effect than this is to employ a separate charge. The result is that a change which reduces the gain. If too low, oscillator tube, in which case the in signal -grid voltage produces little only weak oscillations are produced, 6SA7 serves excellently as a mixer. change in the main electron stream which reduces the gain because of A separate oscillator can generally be emitted by the cathode, i.e., in the insufficient modulation. Note that the built with better operating chacter- istics than a built -in oscillator. Also, Fig. 6. Conventional frequency converter circuit employing a 6L7 pentagrid a tube that has to perform mixing mixer tube and separate 6C5 oscillator. This type of circuit (with only can generally be made to operate suitable waveband switching) is widely,used in larger all -wave receivers better than one that must also in- where the necessity for the separate oscillator tube is no disadvantage. corporate an oscillator function. With this arrangement, a.v.c. may

MIXER CIRCUIT F CIRCUI'. be applied at all frequencies, and does not result in any serious shift in oscil- 6L7 MIXER I / lator frequency. The oscillator fre-

I I SIGNAL 1 CIRCUIT I IF. quency is also reasonably stable with I AMP. variations in supply voltage. How- ever, there is the disadvantage that two tubes must be used in order to SIG I accomplish the frequency conversion. INPUT I

500 0.1 A typical circuit for separately - n excited operation, using a separate 50ík 6SJ7 tube operated as a triode oscil- lator, is .illustrated in Fig. 4. The oscillator is coupled to the oscillator grid of the mixer by means of cou- CA. CI VOLTAGE DROPPING pling capacitor With this separate r excitation, there is no oscillating volt- OSC. CIRCUIT I age on the cathode of the 6SA7 tube. I ósc. The amplitude of oscillation, there- TANS 6C5 OSC. I CIRCUIT fore can well be made higher than the I amplitude used in self -excitation. As a result, somewhat higher conversion transconductance can be obtained

g 50 with separate excitation than with self- excitation, and increased gain is TO A V C. obtained. BIAS VOLTAGE~ PAD. VOLTAGE When separate excitation is used, T ICR LER DROPPING RESISTOR it may be desirable to neutralize the tiMMNe B* 250 V. 0.11! 20, 000n effects of space-charge coupling by L J connecting the small neutralizing capacitance C. between the oscillator 54 RADIO ?EWS

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