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a small plate of copper or tin-can st n-k using Vector turret sockets, and substituted for the old octal socket in the set. Naturally, the receiver must be realigned after any such change, •
These circuits (except the grounded-grid) are also adaptable to being constructed in the form of plug-in units which simply substitute for the existing rf tube. Complete units of this nature are the S-i)er, the Improved S-i)er, and the S-9er Mark II (see bibliography agiiin). The pentode circuit change can also be incorporated into a plug-in adapter.
With noise in the first rf stage minimized* it's time to check elsewhere in your efforts to attain the ultimate in sensitivity. Frequently, the mixer stage of a receiver makes a substantial noise contribution arid masks weak signals. This is especially true if the first rf stage is not doing its job properly, but by now that portion of your set should be at peak performance,
The difference between a quiet and a noisy mixer is largely a matter of tube choice. Mixer tubes designed especially for low-noise service include the (iBA7 and the GSB7-Y (now almost obsolete.) The 6U8 is a good one also hut usually requires circuit changes.
(f you don't mind extensive surgery inside the set, substitution of a 6AC7 operated as a pentode mixer as shown in Fig. (i will yield exceptionally low-noise mixing action. However, you may experience a bit of difficulty in realigning the set after this change due to the drastically different circuit and resulting change in circuit capacities.
Once the proper tube type is substituted, a few changes in circuit constant may increase mixing sensitivity* Lowest noise and greatest freedom from cross-modulation are obtained with a non-grid-leak-biased mixer. However, such mixers are also the most sensitive to variations in oscillator output and so are usually avoided by set designers.
Bias values for the mixer tube of your choice can be obtained from the tube-handbook charts. Either fixed grid bias obtained from the power supply or from a mercury cell, or cathode bias developed across a resistor of the proper size (as shown in Figures 6 and 7), can be used instead of the original grid-leak biasing. The grid resistor can then be reduced in value, or left unchanged.
With any biasing arrangement other than the grid-leak circuit, mixer performance is extremely dependent upon proper oscillator injection voltage. Oscillator output must be adjusted while listening to received signals, for best results. At the right point, you will notice low noise, good gain, and little distortion. Excessive oscillator output will result in reduced gain tint! increased noise, while too little output gives relatively noise-free results but little mixing gain,
A circuit for use of the (5U8 is shown in Fig. 7. Note that no coupling between oscillator and mixer is indicated. All necessary coupling is provided by the proximity of the tube sections within the envelope. This circuit, adapted from the International Crystal Mfg. Co. model
FCV-2 converter, provides exceptional results when preceded by one stage of rf amplification, even at 144 me*
With both the rf stage and the mixer cleaned up, there's little more to do in your search for sensitivity, Itfs a good idea, however, to check AVC action after all modifications have been made. Tube and circuit changes sometimes upset normal functioning of this important circuit, since action of the front end at low bias voltages may be vastly different from its action near tube cutoff.
To check AVC, tune across the broadcast band if your receiver has one. Lacking that, listen to the kilowatt down the block. Examine the signal carefully to see if you can detect distortion, splatter, or other objectionable features (the broadcast station is recommended
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