October 1935 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.
As I have written in a couple articles recently, there was a huge push in the mid 1930s to adopt the revolutionary new metal-encased vacuum tubes over established glass-encased tubes. Today, interest in vacuum tube amplifiers is building among audio enthusiasts both from a nostalgic perspective and from a long-perpetuated belief that sound from vacuum tube circuits have a distinctively richer quality than that of solid state devices. I post this stuff for the benefit of those who otherwise might not be able to find the information otherwise. Your patience indulgence is appreciated.
Metal tubes and all-metal construction are the features of this new deluxe model receiver. Range - 4½ to 2,000 meters!!
W. A. Smith*
The new Midwest Deluxe 18-tube radio is designed around the new metal tubes and uses these new metal tubes throughout, entirely eliminating glass tubes in all sockets with a resulting improvement in selectivity, sensitivity and a marvelous increase in fidelity of tone, on account of the more rigid construction and greater uniformity of these new tubes. Also it has been found possible to extend the tuning range to less than five meters without sacrificing any other portion of the tuning range all the way to 2,000 meters with the exception of a small gap for I.F. amplification as is customary.
These new tubes appear to be entirely justified in spite of the large selection of tubes already available and in spite of their slightly higher cost. They are much more uniform, permitting the set to be designed closer to the proper operating point of the tubes without fear of variations in the tube constants causing trouble.
The tube complement is made up of five power pentode output tubes bearing the number 6F6. Four of these are used in the output stage in push-pull parallel giving over 20 W. of undistorted power and necessitating a specially designed loudspeaker capable of accepting this output. The fifth of these power pentodes is used as a driver for the preceding audio stage.
The new 6K7 is a very worthy replacement for the old 6D6. This R.F. pentode is very stable in operation and very uniform in results. Five of these little iron men are used in the R.F. and I.F. amplifiers as well as in the mixer stage.
Outstanding among these metal tubes is the 6C5, which is a detector-amplifier triode that is a radical departure from past practice. Heretofore all triodes have been designed as R.F. amplifiers with the requirement that the inter-electrode capacities should be low. This resulted in poor operation in audio circuits. This new 6C5 is specially designed for audio work and makes a remarkable low noise level audio amplifier.
Four of these metal triodes are used. One as a resistance-coupled audio amplifier preceding the 6F6 driver. One is used in the tunalite circuit controlling the dimming action of the pilot light as an indicator that the set is tuned to a station. One of these is used in the beat-frequency oscillator circuit which is provided as an aid in tuning in the weaker high-frequency stations. One of these 6C5s is used as the main heterodyne oscillator, tuning below five meters.
Two of the 5Z4s are used as full-wave (push-pull) rectifiers. These new metal rectifiers are very quiet in operation.
The last and smallest of these tubes, but not the least in importance is a new development being a duo-diode called the 6H6. This little tube is only about 1 in. high but is by far the best R.F. rectifier that has ever been developed. Two of these are used. One in the A.F. generator circuit (the second-detector) and one is used as a separate A.V.C. bias generator for giving more perfect volume control action.
* Engineering Dept. Midwest Radio Corp.
Posted November 3, 2015